I love how Clive explains things about rather common electronics and how/what they do. I already knew how the ionization detectors worked, but his explanation was a 1000% better than any I've heard about or read.
I did a little research and it seems that zener diodes conduct a nearly constant current of less than 1 micro amp in the reverse direction until near the breakdown voltage. They may have used it as a constant current device. At higher currents, shorted FETs are used for constant current regulation.
According to the data sheet, the sensitivity pin is at 1/2 VDD unloaded. This would keep the FET turned on given there's 18M between the gate and ground which would keep the Zener shunted in normal operation. When timer/hush is triggered, the timer out pin is pulled down to VSS, both lowering the sensitivity via the 3.3M resistor and sensitivity pin and causing the FET to turn off. This means the Zener is now in series with the sensor divider and the reverse leakage limits the current within that string effectively lowering the sensors sensitivity.
It isn't unusual for the thermistor used in a heat sensor to be a high value - current savings of the high values gives you longer battery life. I work with thermistors in the 1-2M ohm region.
The zener may be used as some sort of stabilizer. Back in the 1980s the signal diodes used in the production of smoke alarms were oven stabilized as to hold calibration longer than a few months. This was back when manual calibration was done on each smoke alarm.(what a pain) Your unsoldering of the diode might have changed that calibration. Check the polarity of the diode in the circuit to make sure it's not just being used as a signal diode.
Heat alarm with dead battery? It was in the dark green Connex box at the top of the stack on the container ship crossing the ocean in August. Further questions? ;-)
according to the datasheet the 1N4740A has a leakage current of 10 µA at 7.6 V - which is a factor of 10 higher than the leakage current of the 2N7000. It might very well be used here as an extremely low constant current source for the resistive divider.
@@power-max yes, through most diodes it is a rather constant current, described by the term Is in the diode equation Id(V)=Is(exp(qV/kT)-1). Very often there is a parallel shunt conductance which will lead to an increase in current for higher reverse voltages. Also when reaching breakdown the current will rapidly increase. Fig 3 shows the reverse current over a logarithmic voltage scale, on a linear scale it will be much flatter. www.vishay.com/docs/81857/1n4148.pdf www.vishay.com/docs/84064/anphyexp.pdf
@@Gunzee That is a clever way to do it. However, I've had another look at the datasheet of this chip just now to see if that was the case, and it doesn't appear to have that functionality. It does have an I/O pin to connect it to other alarms but it goes off to the part of the PCB that isn't populated.
Commercial units are usually 'rate of rise of heat' which uses 2 sensors, one is more insulated than the other, Doing a quick search for 'zener diode as a temp sensor' i find this in the results "The sensor is essentially a zener diode whose reverse breakdown voltage is proportional to absolute temperature." this is referring to the LM335. I already knew that diodes can be temp sensors, but first time i knew about zeners, though i guess any junction could be.
Lead "burnishing" (bent over on the solder side) is actually a very common auto placement feature as it stops components moving too much during wave soldering.
@bigclivedotcom - Zeners don’t clamp as sharply as you may think, they act like a resistor and partially conduct before the clamp voltage is reached, then they clamp hard at their rated voltage, a curve tracer shows the characteristics.
I suspect that the zener leakage current varies with temp in such a way as to counteract sensor current. If both diode and sensor heated together (e.g. on a hot day) alarm trigger point may be much hotter. A fire would heat the sensor first, the diode would take longer to heat up due to the boards thermal inertia, thus alarm triggered at lower sensor temp. I tried to get data on zener leakage vs temp, didn't find anything succinct so all this is just conjecture.
Depends if it's a Zener or avalanche diode. Zener is below 5.5V (ish). Zener leakage has a negative temp coefficient, avalanche has a positive temp coefficient. So (unless I've had a brain fart) a 10 V avalanche diode will compensate (a little) for ambient temp changes. However, everything I've just dug out says that while maximum leakage at a specific temp (or temps) is specified and controlled, the temperature coefficient of leakage is not specified or controlled. What you get depends on the manufacturer and the batch. And the phase of the moon. If it *is* to compensate for ambient temp (why not just use another thermistor?) that might explain Clive seeing it trigger at what appeared to be an absolute temp. Because, electrically, that's what it does if you heat only the thermistor. If you heat the thermistor and diode equally it won't see much change. So it's the time it takes for the heat to get to the diode that allows it to trigger on rapid changes. That might also explain the difference in trigger temp after unsoldering/resoldering the diode. It could have altered the leakage slightly. But it's also possible Clive was holding the heat gun at a slightly different angle and the amount of heat reaching the diode was different.
Excellent observation. I was thinking the same as well. But if you look at the 2N7000 datasheet, it already has a built in ESD protection diode. Maybe they chose the 10v to protect the IC and not just the FET?
That was also my guess, because the 2N7000 is the only component I destroyed repeatedly by handling it without proper ESD precautions. But as The Dollar Guy states, and as I verified in the ON Semi and ST Micro data sheets, the data sheet in fact shows a Z Diode as body diode between source and drain. Most likely, I destroyed those chips with ESD into the gate. On the other hand, they have an absolute maximum rating on 60V Vgd, and specify the minimum breakdown voltage of that Z Diode as "above 60V", so I am unsure whether relying on the internal diode for ESD protection is within spec.
ehsnils I thought of that too. Even if a mosfet has integrated ESD protection it‘s common practice to add an external (zener) diode for protection. And as mentioned before, there is the IC too.
I think it’s just there for over-voltage protection of the FET used on some other circuit by the designer of this circuit, so he/she habitually always puts one in. Even when the supply voltage is known.
My thoughts on why the Zener changed its tripping threshold is due in part to the solder used. The solder used originally had a slightly different resistance value and that's why it was spec'd out during production and the solder you used is just different enough to produce the different triggering amounts.
in the theater where i worked a while ago they had Carbon monoxide detectors, i think. Apparently the heat detectors caused problems when the old school lights gave off so much heat that the air under the ceiling got hot enough to trigger them.
In our old historic theater(1926), the detectors are sensitive as hell. Any time we use haze or fog the city turns them off and puts someone on fire watch. Our new theater built in the 70's doesn't have any and when it gets remodeled this summer we expect to have to do the same.
@@nickbird7742 I was there to fix the DMX routing and install some of my multiplexer boxes so I had nothing to do with them. They just told me that, when I asked about the detectors as the looked strange, so i dont know if it is true.
Bigclivedotcom The zener is acting as path to ground *and* as a constant current source (~10uA) in the reduced sensitivity mode when the FET is disabled. Without the zener, the sensor voltage divider would be floating, and disable the sensor entirely (100% reduced sensitivity). This simply raises the ground potential slightly, and shifts the sensitivity. Try reading voltage across the zener in normal vs. mute modes.
For some reason, I find the photo of the white circuit board especially gorgeous. Kudos to your photography, Clive, and to your printer. The zoomed, high-quality photos really help to see what's going on. BTW, I only discovered your channel relatively recently, but I think I've watched at least 50 videos. Very addicting. Besides the electronics, as an American, it's nice to learn about the Isle of Man, the UK, Scotland, and even your quirky friends. Keep up the good work, sir. --pred
Huh. Heat detectors have come a long way from the old Simplex style with the finned metal button that snaps into a hardwired base. (As a child I "helped" my father install several Simplex commercial fire alarm systems in the 1980s, it amazed me how modular the systems were and how customizable they were for the time.) It blew my mind when my old man explained that the entire system was essentially loops of wire with NO or NC contacts - whether it be heat detector, smoke detector, manual pull station - and being so simple made it a robust system, where system failures were typically from a detector itself (or some other work crew taking a saw through two 5/8" rigid steel conduits and disabling two zones' detectors, pull stations, and the horns/strobes as well). We re-pulled a lot of 18 AWG stranded wire to fix that! LOL
I did lights and sound for my high school theater several years ago when I was under 18. I've since become the local "expert" electrician and designed and run several shows. They call on me to design and plan every show. I design and run every show manually. I never leaned to program a cue or put a timestamp on a scene. I have an ETC Express 48/96 and I do everything manually every show. I know all the channels and all the groups by heart. Our catwalk is 30 ellipsoidals and the rest of the lights are above the stage on three pipes. Roughly 10 par cans per pipe. Anyway, I was trying to say that my theater didn't have any smoke detectors. I could do anything I want without a worry about smoke or heat. Smoke bombs and pyro were no problem.
I venture the guess that the NP junction of the Zener is being used to set the ambient room temperature to be compared to that of the exposed thermistor. If the room slowly changes temperature then they both change and there isn't the discrepancy between them to trigger the device. It explains why after you soldered the leads of the Zener, the thermistor needed to be heated to a higher level before the triggering happened. When I worked on thermal meters we were required to wait over 24 hours after we soldered any component in the sense circuit before we could align and check calibration.
Shorting out that diode, if anything, will increase sensitivity, assuming an NTC thermistor. Assuming it is some sort of zener, when the FET is off it will raise the voltage at the bottom end of the voltage to the zener voltage. Since the chip triggers on a low voltage, this will decrease the sensitivity. When the FET is on, it drags the low end of the divider down to ground and increases the sensitivity. Say it's a 3V zener. The input at 20°C will be 3V + 6V×[470K ÷ (102K + 470K)], or 7.93V. With the FET on, it would be 9V×[470K ÷ (102K + 470K)], or 7.40V. It might be that the diode is some sort of polarity protection and than the sensitivity output is biasing it into the linear range.
Thanks for covering these, I've never heard of them before. An interesting experiment might be to substitute a 9v Zener diode for the 10v one and see what that changes the triggering temperature to. I'm, going to say I agree with the commenters that the diode used might have been intended for running the unit off a 12v power supply. I'm also happy you took the time to explain the uninstalled components, that's always been a thing I wonder about when 'chinese ebay specials' are torn down. You might be interested to know there's a number of cheap smoke detectors available via eBay that only sound if you press the test button, they might be worth investigating in the future.
Regarding the Zener Diode - Because the sensitivity pin is actually a pulse train [smoke sample time], so the external mosfet actually turns on to pull the sensor down so that the actual resistance/temp can be measured by the IN pin. Hence the zener diode is for back emf protection in-case the sensor is inductive. This must be the operating method otherwise the IN pin will float at Battery Voltage defeating the purpose of the sensor. To proof, a oscilloscope can be hook up at the sensitivity pin to see the pulse train and the zener can be removed to proof that the circuit is still functional without it. I suggest a Part 2 is necessary.
Assume that the zener is used in it's most common fashion. It protects the voltage from exceeding 10V. Perhaps the horn generates some back emf spikes that need to be clamped to avoid damage to the rest of the circuit. Also, many electronic designers aren't well schooled so perhaps there was a circuit used in another device that ran on mains. And when the engineer lifted the circuit for this exclusive battery operated version he didn't realize the zener was no longer necessary. I used to work with car radios and sometimes entire circuits were lifted for a new design. And a certain transistor was included in the lift but the truth was that transistor was never going to turn on. So millions of these radio units were shipped with a transistor that would never do anything but up the cost about 3 cents for each unit. Frequently when servicing these units I would take that transistor out and toss it in my spare parts pile. But one can only tolerate just so many 3906 transistors in ones spare parts box.
In America (not sure about England), we call these type detectors by their activation temperature, typically anywhere from 80-120 centigrade, and "rate-of-rise" if they have that feature.
If I am understanding the schematic and datasheet correctly, then there seems to be a mistake in the description of how the circuit works. The default voltage on the SENSITIVITY SET pin of the A5367 is 50% of the battery voltage (due to internal 1.1M resistor to ground and 500k + 600k to VDD). But this particular circuit also has an 18M resistor to ground on the SENSITIVITY SET pin. This lowers the sensitivity to ~48.52% of the battery voltage. Either way, the FET is actually turned on to begin with when powered by 9V. Which makes sense, since it can't very well do any smoke detecting if the detection voltage divider doesn't have a path to ground. The purpose of the FET seems to just be to disable the detection circuit unless the battery voltage is high enough. Not sure why that would be necessary though. When timer/hush mode is triggered, then the TIMER OUT pin is pulled to ground every time the A5367 samples the DETECT IN pin. That places 3 resistors (1.1M + 18M + 3.3M) in parallel, which drops the trigger level to ~41.76% of the supply voltage. So it makes it harder for the alarm to sound, but it will still trigger anyway if the temperature is hot enough. As for the purpose of the zener diode... It must be to protect the FET from static discharge since the thermistor leads are exposed. It would only come into play when the battery is low/removed (FET is turned off), but that's when people are most likely to be fiddling around with it. 10V is well below the maximum the FET can handle, but since the battery is only 9V anyway, that's just an ultra safe value to choose. However, there doesn't seem to be any way for the A5367 to know if the FET is bad. If the FET happened to be failed open, then the alarm would never sound in a fire since the DETECT IN pin voltage would always be higher than the SENSITIVITY SET pin.
The zener may be to protect the FET from static which many of the sensitive ones can be damaged by, maybe because the thermistor wires are exposed and it's a safety device? It seems that if the FET were to fail the device could give a false negative that the chip might not detect as a fault.
We have an infra red detector fitted in our kitchen as part of a central alarm system, the alarm company had to cut it out of the system with a bypass resistor as it would go off as soon as the grill was turned on or the toaster was on in the wron position so that the detector saw the element. The alarm is one that goes through to a central control room and we were getting several false alarms a day,they tried another detector but in the end came to the conclusion that it was in the wrong position and as we were not about to redo the kitchen and move the grill/oven or have the alarm re wired leaving a hole in the cieling it was cut out and we now rely on the smoke detectors fitted in the other rooms.
It seems that the 2N7000 transistor is susceptible to static electricity; just soldering the transistor with an ungrounded soldering iron might blow it out. Once the transistor is mounted in the circuit board it is usually not a problem. However, with the thermal-resister sticking out like that, where someone could touch it; the "mystery" zener diode provides some protection by limiting any external static charge that might accidentally be induced directly into the transistor and causing it to blow out.
Those long life Energizer lithium AAA cells are around 1.7V so a 10 year smoke alarm PP3 with 6 x AAAA lithium of similar chemistry would be 10.2V - Just a guess.
Clive, your reverse engineered schematic failed to indicate the connection to the beeper. The mystery diode may be related to any back EMF off of the piezoelectric beeper if they produce a Voltage when being pulsed. (or a voice coil hooter when they are used)
my thought too. i've had devices dying because they've been droppen on the floor. The reverse piezo effect should be considered when everything is in the range of the megaohms.
There are not many smoke alarm chips since getting them certified is costly. Image the legal quagmire from multiple deaths due to a smoke alarm not working. Just a thought.
@@konradw360 Fair bump. Still, would have thought that there would have to be some form of certification of a purpose made silicon chip since it's innards are buried within a layer of opaque black epoxy.
My work has an old, low-tech heat detector in the server room: wired up to the burglar alarm system as "fire trouble" (but not to the fire alarm system, strangely). Unfortunately it's normally closed (as would be expected, of course, for an alarm circuit), and the previous company which occupied the building wired the heat detector via a pair, sharing a UTP cable with numerous other pairs, through the 66-blocks, completely unmarked/unlabeled... so while I was ripping the 66-blocks out (we're 100% VoIP), I ended up setting off the alarm sounder. (Thankfully we didn't have any monitoring on the alarm system at the time...) Honeywell T631A1055 "Provide line voltage control of heating, cooling and ventilating systems in farm buildings or storage areas."
I have interconnected smoke and heat alarms in my house and they are crap. They're mains powered, but they need a battery. If you don't fit a battery, they peep. So you fit a battery and over the course of a year it does so many battery tests, the battery goes flat then it peeps again. So you replace the battery again. Eventually it even peeps with a fresh battery. So you bin it and buy a new one, Every 5 years. Until you get fed up or they become obsolete. So what you have is a safety technology so awful and so annoying that most normal people will disable it. That's terrible safety philosophy.
They're designed with a maximum lifespan (usually 10 years now) because they have a tendency to fail over time. They'll start beeping after they've been powered on for that predetermined time to force you to replace it. The thinking is it's better to annoy you a bit than have something nonfunctional and giving you a false sense of security.
Mike Page I have a similar linked system with good results alkaline back up batteries are lasting 5 years plus easily in fact so long a can’t really remember changing many or in what order certainly not a issue anyway I’m using the Ei range such as Ei 141 commonly used in uk. Are you using alkaline batteries? If so not sure what could be causing such poor performance maybe it’s the brand/make of alarms??
Most mains powered detectors are designed to need a battery so that they continue to provide protection in the even of a mains failure. Alkaline batteries should last longer than a year, but some brands (such as Duracell) don’t last even if not used in a device. I have linked mains powered detectors and I have no problems, but I use the Lithium 9V batteries.
@@Mark1024MAK - I use the heavy duty batteries specified by the manufacturers, and they don't corrode the smoke detectors _and_ they last at least a year. Alkalines last no longer, and they corrode the detectors.
@@iarejester In this case "annoying me a bit" means not sleeping because they always go at 2am, so I have to get a chair to stand on and butter knife (screwdriver doesn't work) and pop the lid, then try to get back to sleep.
Though it seems odd they would include the 10 V zener in a version clearly designed to run off a 9V battery. You would’ve thought they would only include it in a version designed to be wired into 12v.
@@michaelfisher9671 maybe its cheaper for them to use the same board for 2+ uses? i swear ive seen something like that but in a nightlight or something.
Hey big Clive, would you please take a part a protected vaping Mod, say one that supposed to be 200 W that runs off two 18650 batteries? Explain what the chips do inside & how is it possible to get 175 - 200 wants of supposed power from two 18650 batteries. I would find this extremely interesting! I bet a lot of other people would also. As far as I know, there are no other channels doing this. :-)
just using the voltage drop of the diode to bias the sensor by raising its (-) terminal voltage making it easier at lower ''smoke/heat levels'' to reach the input pins threshold/trip voltage, in hush mode the fet removesthat bias voltage so the sensor has to be ''more triggered'' to raise the voltage on the sens pin, I guess they didnt do this 2 voltage level switching internally (no doubt its an op-amp) to allow for different sensors with different resistance changes
The FORWARD voltage drop of a silicon junction changes according to the junction temperature at the rate of 10mV per °C e.g. an increase of 1°C will cause an increase of 10mV and a decrease of 2°C will cause a forward voltage drop of 20mV from the nominal 600mV. In your circuit, if the diode is to be connected with anode biased positive with respect to cathode, then this will offset the bias into the temperature resistor chain according to temperature. As the thermistor is mounted in a temperature sensitive location outside the enclosure and the diode in question is mounted inside the enclosure, this will provide a certain of hysteresis depending on the thermal lag between thermistor and diode. This thermal lag will provide for a rate of temperature rise rather than just a temperature rise. A combustion on a hot day is needed to be sensed pretty much at the same sensitivity as combustion on a cold day. However, the diode in the circuit in question, is reverse biased and I suggest that this is incorrectly installed. Keep up the good work Clive, best regards yorkybar.
bold claim, but I have a heat alarm that you can fit in the kitchen too, so that definitely isn't the "only" one. Also slight nitpick, they ARE interconnectable, they communicate by means of "High intensity infra red".
Americium emits beta particles which are fast electrons. The electrons act as current flow so the Americium is kinda like a battery. The fast electrons flow across an air gap. If smoke gets in that gap, because smoke consist of ionized particles, they disturb the flow of electrons and the chip detects a sudden change in the current.
9V is only the nominal voltage for said batteries. Actual new voltage is somewhat higher. Some of the rechargeables will exceed 11 volts freshly charged. So maybe the zener has a purpose?
I didn't even know these existed, I've been wanting to have an alarm in my heating element storage room (the "kitchen") but toast makes a conventional smoke detector... less than ideal. Cheers Clive.
now, with a bit of adjustment I guess this could be made more sensitive? it might be handy to have a warning when my aircon fails in the server room at work.
@@ripstick2217 UPS does have a monitor, limited options for alerting (email only). Other servers doo have sensoors but only for CPU and disk temps, no general air temp. Also my UPS is at the bottom of the rack as it's heavy (those lead acid batteries you know) so will be the coolest thing around.
Maybe the diode is there because the thermistor is located out side the enclosure so it might get a static shock. so the zenor might clamp any static voltage that might other wise disturb the chip.
I did before I read this then I noticed it while writing the comment. Mainly because I wasn't WARNED and everyone in the entire house heard it since it was off the tv and it was louder than normal cause I was laughing over the dead pan. Think that alarm should be glued to whoever got the money for that video..
May be the mysterious zener is compensating for ambient temperature? May be the change on the alarm temperature is caused by touching the diode with the hands which raised the temperature.
The only suggestion I have for the zenor diode is that it is somehow offering protection to the mosfet, and that the chip still requires a trickle current to do something so they cheaped out and killed two birds with the one stone.
analyzing brain is already asleep but Zv is dependent on temperature so that current will be based on temp so is that where the differential part of the detector is. And soldering on made it warmer so the higher trigger point
if you drop or knock on the detector the piezo could easily get >50V between the electrodes. in a high resistance circuit this might present a risk for the inner electronics of the IC
a factory where i used to work had to change their 'smoke' detectors to 'rate of heat rise' as the existing 'normal' ones kept getting tripped by the forklift exhausts..
I think this is some kinda of bias for gate of the fet. though the leakage current is very low Fets are voltage controlled so need very little current to affect them. Thats the only reason I could see how desoldering the diode would change sensitivity it must have changed the leakage of the diode they are very sensitive to heat.
Perhaps they use this board in a mains powered unit as well ? So could be a variation in supply which would affect the thresholds?. As you say a battery version won't be affected by this 🤔
Clive, any chance of a updated rechargeable comparison video for AA cells? On a few electronics pages of Facebook and apparently the Ikea Ladda batteries are blowing even the Eneloop batteries out of the water. I use quite a few rechargeable batteries and would like to see your take
That diode is protecting the chip from the mof, voltage hammer! I bet without it the circuit causes oscillations in the reading of the thermo since the thermo's circuit is just butt-strapped on to the circuit.
but there is no way that even a cheap infra-red camera will be as low in power consumption as a device like this - running off batteries for several years
@@uwezimmermann5427 maybe, but I see current primitive sensors as very rude way of measuring flame presence, like last attempt to recognize fire, but damage to property when they turn on will be quite serious already or non returnable, it's basically to save building carcass than what inside-is already a fuel
Could that 10v zener be used to allow a supply that is not from a 9v battery? Or maybe it is there to deal with the power coming from an interconnected alarm, would that put two 9v in series as one alarm feeds into the next one?
Zener might be in the circuit as an error and only useful when the board is totally populated and linked to other devices, if they are battery powered I ver several devices the excess could build up
Having no electronics knowledge, my only thought is that they might have left the diode in incase the alarm is run off a fixed supply, like those wires you sometimes see coming down from through the ceiling.
from what I understand the new style battery powered smoke detectors will "listen" for other smoke detectors (tone), thus not needing them to be directly wired together
I wonder if the "HORN" generates a bit of back emf and that diode just suppresses that. They might have chosen 10v knowing it will always behave as a diode with that VCC (maybe cheaper)
The transducer looks and sounds to be a piezo element. These behave like a capacitive load in a circuit, and not an inductive load. Hence, I don't suspect it's for back-EMF.
@@JoeyvanHummel yup. I realise that - but only if that circuit in the datasheet is the same as the implementation (if it was a speaker of some kind for example, back emf)
This zener. Might it be a protection over the FET? Its usual to have a reversed biased diode over a transistor. If you use a zener it will work as protection in both directions.
Perhaps they put the 10V zener in for over voltage protection. How high is the voltage of a brand new 9 volt battery. Battery chemistry and how many cells are in the 9V battery will determine the voltage. For example: Rechargeable range from high 9.6V (8 cell 1.2V NiMH) batteries, to 7.2V (2 cell 3.6V li-Ion) batteries.
Hmm. Working in a related field (industrial fire detection), I would be very suspicious of a "heat detector". Heat detection has a terrible reputation: first they are subject to inverse square law, so unless it's right inside the fire then the fire is probably already out of control before the detector goes off, and that's assuming that the heat is not carried away elsewhere by ventilation anyway. Also, they tend to be made of materials that degrade over time: who knows how his detector will perform after 10 years? And given the normal seasonal and cooking temperature changes in a kitchen, how sensitive could they make it? Not very, I'd expect. I would stick with smoke detectors: kitchens shouldn't normally be filled with smoke unless you regularly burn your toast!
So basically that IC could be employed anyway you devise to vary the trigger using a resistivity device. You could use something like a Load Cell as the sensor . Looking on E-Bay those chips are a bit pricey. en.wikipedia.org/wiki/Load_cell
there are many (i do mean loads) of pcb made with bent resistor and axial cap leads , they do seem to be from the better quality manufactures and they are quite hard to desolder but i usually have long enough finger nails so dont get burned to much :-)
They make certain models of these with output relays to signal to other equipment. Wonder If the diode has been carried over from a unit with a built in relay and that is the position of the suppression diode. They may have just installed the zener to fill the space on this model? Either that or they manufacture a 12v version of this detector?
If components remain available, and you have an automated production line (I know of component inserters that crimp the leads like that), it's *far* cheaper to keep cranking them out, vs. redesigning the circuit *and* replacing all the automation equipment. Upgrade for the sake of upgrading? Not if it costs you a quarter to half a million $ for *absolutely zero* return on investment.
My understanding of an Ion detector was it has 2 chambers, 1 sealed and 1 open. It ionises the chambers and compares the the difference between them to sense combustion products. This may be only in commercial detectors and different in these smaller domestic devices. interesting that the same chip is able to run a thermal detector as well.
Does this Zener diode have a temperature dependent leakage current/resistance that might affect the circuit? I ask because IN4148F have been used as breath temperature sensors in "Apalert" veterinary apnoea alarms.
Dont they need a trickle current flowing through the sensor at all times, in regards to the mystery diode, and that is the job of it? Check if you can get the heat sensor to trigger without the zener diode in the circuit.
50% of fires start in the kitchen, the other 50% start on BigClive's workbench.
Most underrated comment ever!
Bbbwaaahahahahaha .. true that
Yes the man who stuck a firework up a doll’s arse
Funny
Lol
I love how Clive explains things about rather common electronics and how/what they do. I already knew how the ionization detectors worked, but his explanation was a 1000% better than any I've heard about or read.
true :)
Yeah I had horrible electronics teachers back at school
I did a little research and it seems that zener diodes conduct a nearly constant current of less than 1 micro amp in the reverse direction until near the breakdown voltage. They may have used it as a constant current device. At higher currents, shorted FETs are used for constant current regulation.
10µA for this zener according to the datasheet
JAMES AZBELL The 10ua sounds like a more useful value anyway.
According to the data sheet, the sensitivity pin is at 1/2 VDD unloaded.
This would keep the FET turned on given there's 18M between the gate and ground which would keep the Zener shunted in normal operation.
When timer/hush is triggered, the timer out pin is pulled down to VSS, both lowering the sensitivity via the 3.3M resistor and sensitivity pin and causing the FET to turn off.
This means the Zener is now in series with the sensor divider and the reverse leakage limits the current within that string effectively lowering the sensors sensitivity.
It isn't unusual for the thermistor used in a heat sensor to be a high value - current savings of the high values gives you longer battery life. I work with thermistors in the 1-2M ohm region.
No wonder the battery is flat, that unit has been protecting people since 1967.
That's yow-man service for you!
The zener may be used as some sort of stabilizer. Back in the 1980s the signal diodes used in the production of smoke alarms were oven stabilized as to hold calibration longer than a few months.
This was back when manual calibration was done on each smoke alarm.(what a pain)
Your unsoldering of the diode might have changed that calibration. Check the polarity of the diode in the circuit to make sure it's not just being used as a signal diode.
Heat alarm with dead battery? It was in the dark green Connex box at the top of the stack on the container ship crossing the ocean in August. Further questions? ;-)
"50% of fires start in a kitchen."
Ok, I'll move my stove to the bedroom instead.
What if the other 50% start in the bedroom?
Some people don't understand statistics.
100% of fires in a home start as a result of combustion.
0% of statistics don’t tell you in which time period they came up with the figure.
You can then prepare breakfast in bed for your Missus, or mistress!
Wich is by you
How about comparing the heat detector to one of the same company’s smoke detector and see if the circuitry is the same and if it explains the diode.
according to the datasheet the 1N4740A has a leakage current of 10 µA at 7.6 V - which is a factor of 10 higher than the leakage current of the 2N7000. It might very well be used here as an extremely low constant current source for the resistive divider.
Is reverse leakage of a diode better thought of as a constant current? I would have thought it was more like a resistance
@@power-max yes, through most diodes it is a rather constant current, described by the term Is in the diode equation Id(V)=Is(exp(qV/kT)-1).
Very often there is a parallel shunt conductance which will lead to an increase in current for higher reverse voltages. Also when reaching breakdown the current will rapidly increase.
Fig 3 shows the reverse current over a logarithmic voltage scale, on a linear scale it will be much flatter.
www.vishay.com/docs/81857/1n4148.pdf
www.vishay.com/docs/84064/anphyexp.pdf
You'll find that it does have the functionality to connect with up to twelve other alarms. Up to twelve in this case means zero though.
Does "up to twelve" also include negative number of units as well as zero?
@@MiniLuv-1984 Yes.
The connection is audio, when one goes off it sets off others near by. That's a drunk guess, and how my alarms work.
@@Gunzee That is a clever way to do it. However, I've had another look at the datasheet of this chip just now to see if that was the case, and it doesn't appear to have that functionality. It does have an I/O pin to connect it to other alarms but it goes off to the part of the PCB that isn't populated.
Commercial units are usually 'rate of rise of heat' which uses 2 sensors, one is more insulated than the other,
Doing a quick search for 'zener diode as a temp sensor' i find this in the results
"The sensor is essentially a zener diode whose reverse breakdown voltage is proportional to absolute temperature." this is referring to the LM335.
I already knew that diodes can be temp sensors, but first time i knew about zeners, though i guess any junction could be.
Often 1N4148 diodes were used as temperature sensors... or cheap NPN transistors with one of the junctions reversed biased.
Lead "burnishing" (bent over on the solder side) is actually a very common auto placement feature as it stops components moving too much during wave soldering.
Finally. Friday evening, beer and a new upload from Clive. Perfection, I dare say.
@bigclivedotcom - Zeners don’t clamp as sharply as you may think, they act like a resistor and partially conduct before the clamp voltage is reached, then they clamp hard at their rated voltage, a curve tracer shows the characteristics.
Very well constructed and made. Looks like a decent alarm.
I suspect that the zener leakage current varies with temp in such a way as to counteract sensor current. If both diode and sensor heated together (e.g. on a hot day) alarm trigger point may be much hotter. A fire would heat the sensor first, the diode would take longer to heat up due to the boards thermal inertia, thus alarm triggered at lower sensor temp. I tried to get data on zener leakage vs temp, didn't find anything succinct so all this is just conjecture.
ua-cam.com/video/JffJpym7K8Y/v-deo.html ?
Depends if it's a Zener or avalanche diode. Zener is below 5.5V (ish). Zener leakage has a negative temp coefficient, avalanche has a positive temp coefficient. So (unless I've had a brain fart) a 10 V avalanche diode will compensate (a little) for ambient temp changes.
However, everything I've just dug out says that while maximum leakage at a specific temp (or temps) is specified and controlled, the temperature coefficient of leakage is not specified or controlled. What you get depends on the manufacturer and the batch. And the phase of the moon.
If it *is* to compensate for ambient temp (why not just use another thermistor?) that might explain Clive seeing it trigger at what appeared to be an absolute temp. Because, electrically, that's what it does if you heat only the thermistor. If you heat the thermistor and diode equally it won't see much change. So it's the time it takes for the heat to get to the diode that allows it to trigger on rapid changes.
That might also explain the difference in trigger temp after unsoldering/resoldering the diode. It could have altered the leakage slightly. But it's also possible Clive was holding the heat gun at a slightly different angle and the amount of heat reaching the diode was different.
My concern about the diode is that it's just an ESD trap because the thermistor is exposed and might work as an antenna for ESD.
Excellent observation. I was thinking the same as well. But if you look at the 2N7000 datasheet, it already has a built in ESD protection diode. Maybe they chose the 10v to protect the IC and not just the FET?
That was also my guess, because the 2N7000 is the only component I destroyed repeatedly by handling it without proper ESD precautions. But as The Dollar Guy states, and as I verified in the ON Semi and ST Micro data sheets, the data sheet in fact shows a Z Diode as body diode between source and drain. Most likely, I destroyed those chips with ESD into the gate.
On the other hand, they have an absolute maximum rating on 60V Vgd, and specify the minimum breakdown voltage of that Z Diode as "above 60V", so I am unsure whether relying on the internal diode for ESD protection is within spec.
ehsnils I thought of that too. Even if a mosfet has integrated ESD protection it‘s common practice to add an external (zener) diode for protection. And as mentioned before, there is the IC too.
Thanks for the headphone warning you're a true gentleman
@Ungregistered User nae luck ( that's unfortunate)
Wormfood Ditto, it allowed me to not falsely alert the neighbors to a "fire".
I don't understand headphone warnings. The "loud" sound is no louder than the rest of the video.
I think it’s just there for over-voltage protection of the FET used on some other circuit by the designer of this circuit, so he/she habitually always puts one in. Even when the supply voltage is known.
My thoughts on why the Zener changed its tripping threshold is due in part to the solder used. The solder used originally had a slightly different resistance value and that's why it was spec'd out during production and the solder you used is just different enough to produce the different triggering amounts.
in the theater where i worked a while ago they had Carbon monoxide detectors, i think. Apparently the heat detectors caused problems when the old school lights gave off so much heat that the air under the ceiling got hot enough to trigger them.
In our old historic theater(1926), the detectors are sensitive as hell. Any time we use haze or fog the city turns them off and puts someone on fire watch. Our new theater built in the 70's doesn't have any and when it gets remodeled this summer we expect to have to do the same.
Carbon dioxide detectors are not fire detectors so would have not been used to replace heat detectors
@@nickbird7742 I was there to fix the DMX routing and install some of my multiplexer boxes so I had nothing to do with them. They just told me that, when I asked about the detectors as the looked strange, so i dont know if it is true.
@@nickbird7742
carbon monoxide is a by product of (most) fires so it would work as a fire alarm :-)
@@YTANDY100 and by the time the carbon monoxide alarm triggers from a fire in your home you are all ready dead.
Bigclivedotcom The zener is acting as path to ground *and* as a constant current source (~10uA) in the reduced sensitivity mode when the FET is disabled. Without the zener, the sensor voltage divider would be floating, and disable the sensor entirely (100% reduced sensitivity). This simply raises the ground potential slightly, and shifts the sensitivity. Try reading voltage across the zener in normal vs. mute modes.
For some reason, I find the photo of the white circuit board especially gorgeous. Kudos to your photography, Clive, and to your printer. The zoomed, high-quality photos really help to see what's going on.
BTW, I only discovered your channel relatively recently, but I think I've watched at least 50 videos. Very addicting. Besides the electronics, as an American, it's nice to learn about the Isle of Man, the UK, Scotland, and even your quirky friends.
Keep up the good work, sir.
--pred
Huh. Heat detectors have come a long way from the old Simplex style with the finned metal button that snaps into a hardwired base. (As a child I "helped" my father install several Simplex commercial fire alarm systems in the 1980s, it amazed me how modular the systems were and how customizable they were for the time.)
It blew my mind when my old man explained that the entire system was essentially loops of wire with NO or NC contacts - whether it be heat detector, smoke detector, manual pull station - and being so simple made it a robust system, where system failures were typically from a detector itself (or some other work crew taking a saw through two 5/8" rigid steel conduits and disabling two zones' detectors, pull stations, and the horns/strobes as well). We re-pulled a lot of 18 AWG stranded wire to fix that! LOL
Clive, It's call a "rate of rise of heat detector", I used to install them back on the 80's, very effective
I did lights and sound for my high school theater several years ago when I was under 18. I've since become the local "expert" electrician and designed and run several shows. They call on me to design and plan every show. I design and run every show manually. I never leaned to program a cue or put a timestamp on a scene. I have an ETC Express 48/96 and I do everything manually every show. I know all the channels and all the groups by heart. Our catwalk is 30 ellipsoidals and the rest of the lights are above the stage on three pipes. Roughly 10 par cans per pipe. Anyway, I was trying to say that my theater didn't have any smoke detectors. I could do anything I want without a worry about smoke or heat. Smoke bombs and pyro were no problem.
I venture the guess that the NP junction of the Zener is being used to set the ambient room temperature to be compared to that of the exposed thermistor. If the room slowly changes temperature then they both change and there isn't the discrepancy between them to trigger the device. It explains why after you soldered the leads of the Zener, the thermistor needed to be heated to a higher level before the triggering happened.
When I worked on thermal meters we were required to wait over 24 hours after we soldered any component in the sense circuit before we could align and check calibration.
Shorting out that diode, if anything, will increase sensitivity, assuming an NTC thermistor. Assuming it is some sort of zener, when the FET is off it will raise the voltage at the bottom end of the voltage to the zener voltage. Since the chip triggers on a low voltage, this will decrease the sensitivity. When the FET is on, it drags the low end of the divider down to ground and increases the sensitivity.
Say it's a 3V zener. The input at 20°C will be 3V + 6V×[470K ÷ (102K + 470K)], or 7.93V. With the FET on, it would be 9V×[470K ÷ (102K + 470K)], or 7.40V.
It might be that the diode is some sort of polarity protection and than the sensitivity output is biasing it into the linear range.
Thanks for covering these, I've never heard of them before. An interesting experiment might be to substitute a 9v Zener diode for the 10v one and see what that changes the triggering temperature to. I'm, going to say I agree with the commenters that the diode used might have been intended for running the unit off a 12v power supply. I'm also happy you took the time to explain the uninstalled components, that's always been a thing I wonder about when 'chinese ebay specials' are torn down.
You might be interested to know there's a number of cheap smoke detectors available via eBay that only sound if you press the test button, they might be worth investigating in the future.
Regarding the Zener Diode - Because the sensitivity pin is actually a pulse train [smoke sample time], so the external mosfet actually turns on to pull the sensor down so that the actual resistance/temp can be measured by the IN pin. Hence the zener diode is for back emf protection in-case the sensor is inductive. This must be the operating method otherwise the IN pin will float at Battery Voltage defeating the purpose of the sensor. To proof, a oscilloscope can be hook up at the sensitivity pin to see the pulse train and the zener can be removed to proof that the circuit is still functional without it. I suggest a Part 2 is necessary.
Back to BigClive's bench...Awesome
Assume that the zener is used in it's most common fashion. It protects the voltage from exceeding 10V. Perhaps the horn generates some back emf spikes that need to be clamped to avoid damage to the rest of the circuit.
Also, many electronic designers aren't well schooled so perhaps there was a circuit used in another device that ran on mains. And when the engineer lifted the circuit for this exclusive battery operated version he didn't realize the zener was no longer necessary. I used to work with car radios and sometimes entire circuits were lifted for a new design. And a certain transistor was included in the lift but the truth was that transistor was never going to turn on. So millions of these radio units were shipped with a transistor that would never do anything but up the cost about 3 cents for each unit. Frequently when servicing these units I would take that transistor out and toss it in my spare parts pile. But one can only tolerate just so many 3906 transistors in ones spare parts box.
In America (not sure about England), we call these type detectors by their activation temperature, typically anywhere from 80-120 centigrade, and "rate-of-rise" if they have that feature.
If I am understanding the schematic and datasheet correctly, then there seems to be a mistake in the description of how the circuit works.
The default voltage on the SENSITIVITY SET pin of the A5367 is 50% of the battery voltage (due to internal 1.1M resistor to ground and 500k + 600k to VDD). But this particular circuit also has an 18M resistor to ground on the SENSITIVITY SET pin. This lowers the sensitivity to ~48.52% of the battery voltage.
Either way, the FET is actually turned on to begin with when powered by 9V. Which makes sense, since it can't very well do any smoke detecting if the detection voltage divider doesn't have a path to ground. The purpose of the FET seems to just be to disable the detection circuit unless the battery voltage is high enough. Not sure why that would be necessary though.
When timer/hush mode is triggered, then the TIMER OUT pin is pulled to ground every time the A5367 samples the DETECT IN pin. That places 3 resistors (1.1M + 18M + 3.3M) in parallel, which drops the trigger level to ~41.76% of the supply voltage. So it makes it harder for the alarm to sound, but it will still trigger anyway if the temperature is hot enough.
As for the purpose of the zener diode... It must be to protect the FET from static discharge since the thermistor leads are exposed. It would only come into play when the battery is low/removed (FET is turned off), but that's when people are most likely to be fiddling around with it. 10V is well below the maximum the FET can handle, but since the battery is only 9V anyway, that's just an ultra safe value to choose.
However, there doesn't seem to be any way for the A5367 to know if the FET is bad. If the FET happened to be failed open, then the alarm would never sound in a fire since the DETECT IN pin voltage would always be higher than the SENSITIVITY SET pin.
The zener may be to protect the FET from static which many of the sensitive ones can be damaged by, maybe because the thermistor wires are exposed and it's a safety device?
It seems that if the FET were to fail the device could give a false negative that the chip might not detect as a fault.
We have an infra red detector fitted in our kitchen as part of a central alarm system, the alarm company had to cut it out of the system with a bypass resistor as it would go off as soon as the grill was turned on or the toaster was on in the wron position so that the detector saw the element. The alarm is one that goes through to a central control room and we were getting several false alarms a day,they tried another detector but in the end came to the conclusion that it was in the wrong position and as we were not about to redo the kitchen and move the grill/oven or have the alarm re wired leaving a hole in the cieling it was cut out and we now rely on the smoke detectors fitted in the other rooms.
It seems that the 2N7000 transistor is susceptible to static electricity; just soldering the transistor with an ungrounded soldering iron might blow it out. Once the transistor is mounted in the circuit board it is usually not a problem. However, with the thermal-resister sticking out like that, where someone could touch it; the "mystery" zener diode provides some protection by limiting any external static charge that might accidentally be induced directly into the transistor and causing it to blow out.
Those long life Energizer lithium AAA cells are around 1.7V so a 10 year smoke alarm PP3 with 6 x AAAA lithium of similar chemistry would be 10.2V - Just a guess.
those AAAA cells would start our a 1.624-1.659, depending on how long they sat before getting bought. still doesn't explain the zener though.
Clive, your reverse engineered schematic failed to indicate the connection to the beeper.
The mystery diode may be related to any back EMF off of the piezoelectric beeper if they produce a Voltage when being pulsed. (or a voice coil hooter when they are used)
my thought too. i've had devices dying because they've been droppen on the floor. The reverse piezo effect should be considered when everything is in the range of the megaohms.
If it triggers on rapid changes in temperature, maybe it'll respond to ice cubes or freeze spray too
doubt it, it'll be rate of heat 'rise' not fall..
Regular UA-camr: (not clickbait)
Bigclive: (not a smoke detector)
Ohai.
"What's this little transistory thing?", asks Clive. Believe it or not I actually expected that! lol
Keep on keepin' on, Clive.
cheers!
There are not many smoke alarm chips since getting them certified is costly. Image the legal quagmire from multiple deaths due to a smoke alarm not working. Just a thought.
In most cases the final product is certified for a fire not a component.
@@konradw360 Fair bump. Still, would have thought that there would have to be some form of certification of a purpose made silicon chip since it's innards are buried within a layer of opaque black epoxy.
Did the circuit operate with the mystery diode de soldered?
16:38
nice clear description of this works thanks.
My work has an old, low-tech heat detector in the server room: wired up to the burglar alarm system as "fire trouble" (but not to the fire alarm system, strangely). Unfortunately it's normally closed (as would be expected, of course, for an alarm circuit), and the previous company which occupied the building wired the heat detector via a pair, sharing a UTP cable with numerous other pairs, through the 66-blocks, completely unmarked/unlabeled... so while I was ripping the 66-blocks out (we're 100% VoIP), I ended up setting off the alarm sounder. (Thankfully we didn't have any monitoring on the alarm system at the time...)
Honeywell T631A1055 "Provide line voltage control of heating, cooling and ventilating systems in farm buildings or storage areas."
I have interconnected smoke and heat alarms in my house and they are crap. They're mains powered, but they need a battery. If you don't fit a battery, they peep. So you fit a battery and over the course of a year it does so many battery tests, the battery goes flat then it peeps again. So you replace the battery again. Eventually it even peeps with a fresh battery. So you bin it and buy a new one, Every 5 years. Until you get fed up or they become obsolete.
So what you have is a safety technology so awful and so annoying that most normal people will disable it. That's terrible safety philosophy.
They're designed with a maximum lifespan (usually 10 years now) because they have a tendency to fail over time. They'll start beeping after they've been powered on for that predetermined time to force you to replace it. The thinking is it's better to annoy you a bit than have something nonfunctional and giving you a false sense of security.
Mike Page I have a similar linked system with good results alkaline back up batteries are lasting 5 years plus easily in fact so long a can’t really remember changing many or in what order certainly not a issue anyway I’m using the Ei range such as Ei 141 commonly used in uk. Are you using alkaline batteries? If so not sure what could be causing such poor performance maybe it’s the brand/make of alarms??
Most mains powered detectors are designed to need a battery so that they continue to provide protection in the even of a mains failure. Alkaline batteries should last longer than a year, but some brands (such as Duracell) don’t last even if not used in a device. I have linked mains powered detectors and I have no problems, but I use the Lithium 9V batteries.
@@Mark1024MAK - I use the heavy duty batteries specified by the manufacturers, and they don't corrode the smoke detectors _and_ they last at least a year. Alkalines last no longer, and they corrode the detectors.
@@iarejester In this case "annoying me a bit" means not sleeping because they always go at 2am, so I have to get a chair to stand on and butter knife (screwdriver doesn't work) and pop the lid, then try to get back to sleep.
Hrm. Might they have a version that uses a 12volt supply?
Yes My RV has one.
Though it seems odd they would include the 10 V zener in a version clearly designed to run off a 9V battery. You would’ve thought they would only include it in a version designed to be wired into 12v.
I'd guess it would run at a standard 12v as part of a bussed security system.
@@michaelfisher9671 maybe its cheaper for them to use the same board for 2+ uses? i swear ive seen something like that but in a nightlight or something.
I agree with the 12VDC supply theory as these are used a lot in RV type vehicles.
Hey big Clive, would you please take a part a protected vaping Mod, say one that supposed to be 200 W that runs off two 18650 batteries? Explain what the chips do inside & how is it possible to get 175 - 200 wants of supposed power from two 18650 batteries. I would find this extremely interesting! I bet a lot of other people would also. As far as I know, there are no other channels doing this. :-)
Very interesting video
just using the voltage drop of the diode to bias the sensor by raising its (-) terminal voltage making it easier at lower ''smoke/heat levels'' to reach the input pins threshold/trip voltage, in hush mode the fet removesthat bias voltage so the sensor has to be ''more triggered'' to raise the voltage on the sens pin, I guess they didnt do this 2 voltage level switching internally (no doubt its an op-amp) to allow for different sensors with different resistance changes
The FORWARD voltage drop of a silicon junction changes according to the junction temperature at the rate of 10mV per °C e.g. an increase of 1°C will cause an increase of 10mV and a decrease of 2°C will cause a forward voltage drop of 20mV from the nominal 600mV. In your circuit, if the diode is to be connected with anode biased positive with respect to cathode, then this will offset the bias into the temperature resistor chain according to temperature. As the thermistor is mounted in a temperature sensitive location outside the enclosure and the diode in question is mounted inside the enclosure, this will provide a certain of hysteresis depending on the thermal lag between thermistor and diode. This thermal lag will provide for a rate of temperature rise rather than just a temperature rise. A combustion on a hot day is needed to be sensed pretty much at the same sensitivity as combustion on a cold day. However, the diode in the circuit in question, is reverse biased and I suggest that this is incorrectly installed. Keep up the good work Clive, best regards yorkybar.
Could the zener be useful for the dual mains/battery powered version?
bold claim, but I have a heat alarm that you can fit in the kitchen too, so that definitely isn't the "only" one.
Also slight nitpick, they ARE interconnectable, they communicate by means of "High intensity infra red".
lmao good one :)
It looks to have another "missing" feature too. That bridge of metal you took off looks like it is connected over to the empty R5 component spot.
Americium emits beta particles which are fast electrons. The electrons act as current flow so the Americium is kinda like a battery. The fast electrons flow across an air gap. If smoke gets in that gap, because smoke consist of ionized particles, they disturb the flow of electrons and the chip detects a sudden change in the current.
9V is only the nominal voltage for said batteries. Actual new voltage is somewhat higher. Some of the rechargeables will exceed 11 volts freshly charged. So maybe the zener has a purpose?
Interesting thought, but both the IC and the 2N7000 have a lot more voltage leeway beyond 12V!
I didn't even know these existed, I've been wanting to have an alarm in my heating element storage room (the "kitchen") but toast makes a conventional smoke detector... less than ideal. Cheers Clive.
now, with a bit of adjustment I guess this could be made more sensitive? it might be handy to have a warning when my aircon fails in the server room at work.
Don’t your servers or UPS have built-in temperature sensors that you can then set to alert you over a certain temp?
@@ripstick2217 UPS does have a monitor, limited options for alerting (email only). Other servers doo have sensoors but only for CPU and disk temps, no general air temp. Also my UPS is at the bottom of the rack as it's heavy (those lead acid batteries you know) so will be the coolest thing around.
Maybe the diode is there because the thermistor is located out side the enclosure so it might get a static shock. so the zenor might clamp any static voltage that might other wise disturb the chip.
Anyone get flashbacks to the Fanny Flambeaux video when the alarm goes off?
Not until you said something.
I wonder if he's ever going to make a weird, fake product again? April Fools Day is coming. It would be nice if made something weird as a prank video.
I did before I read this then I noticed it while writing the comment. Mainly because I wasn't WARNED and everyone in the entire house heard it since it was off the tv and it was louder than normal cause I was laughing over the dead pan. Think that alarm should be glued to whoever got the money for that video..
May be the mysterious zener is compensating for ambient temperature?
May be the change on the alarm temperature is caused by touching the diode with the hands which raised the temperature.
I have found filling the beeper with silicon greatly reduces the sound level. Used that trick on a UPS that beeped when it was on battery.
Stabbing may also work?
The only suggestion I have for the zenor diode is that it is somehow offering protection to the mosfet, and that the chip still requires a trickle current to do something so they cheaped out and killed two birds with the one stone.
analyzing brain is already asleep but Zv is dependent on temperature so that current will be based on temp so is that where the differential part of the detector is. And soldering on made it warmer so the higher trigger point
if you drop or knock on the detector the piezo could easily get >50V between the electrodes. in a high resistance circuit this might present a risk for the inner electronics of the IC
The sensitivity output might be a chain of pulses and the zener diode is for smoothing the voltage. Hook a scope to it and see.
Pin 2 is the TX/RX pin for communications between devices.
a factory where i used to work had to change their 'smoke' detectors to 'rate of heat rise' as the existing 'normal' ones kept getting tripped by the forklift exhausts..
"It also says that 50% of all fires start in the kitchen...Excellent." -Big Clive, March 2019
We're cooking with gas, here, folks!
Hey Clive, great video. I'm wondering if it doesn't allow for silence while the temperature is beyond a certain threshold.
I had not thought of that... i suspect that he did not "hold" the button long enough, as the face says push and hold to test and pause.
I think this is some kinda of bias for gate of the fet. though the leakage current is very low Fets are voltage controlled so need very little current to affect them. Thats the only reason I could see how desoldering the diode would change sensitivity it must have changed the leakage of the diode they are very sensitive to heat.
Perhaps they use this board in a mains powered unit as well ? So could be a variation in supply which would affect the thresholds?. As you say a battery version won't be affected by this 🤔
I suspect the Zener is there because someone misinterpreted the diagram of the 2n7000.
Clive, any chance of a updated rechargeable comparison video for AA cells?
On a few electronics pages of Facebook and apparently the Ikea Ladda batteries are blowing even the Eneloop batteries out of the water.
I use quite a few rechargeable batteries and would like to see your take
Did you check the data sheet for the sounder to see if it spec'd in the diode?
Interesting little device, but, how are the certain types of cook supposed to know when they've "over-caramelised" their dinner? :P
The stinging of the eyes is a sure sign.
That diode is protecting the chip from the mof, voltage hammer! I bet without it the circuit causes oscillations in the reading of the thermo since the thermo's circuit is just butt-strapped on to the circuit.
I'm really waiting for a day when infra-red camera 🎥 will be so cheap that used everywhere as rapid precise and maybe smart fire detector.
but there is no way that even a cheap infra-red camera will be as low in power consumption as a device like this - running off batteries for several years
@@uwezimmermann5427 maybe, but I see current primitive sensors as very rude way of measuring flame presence, like last attempt to recognize fire, but damage to property when they turn on will be quite serious already or non returnable, it's basically to save building carcass than what inside-is already a fuel
Could that 10v zener be used to allow a supply that is not from a 9v battery? Or maybe it is there to deal with the power coming from an interconnected alarm, would that put two 9v in series as one alarm feeds into the next one?
Zener might be in the circuit as an error and only useful when the board is totally populated and linked to other devices, if they are battery powered I ver several devices the excess could build up
Having no electronics knowledge, my only thought is that they might have left the diode in incase the alarm is run off a fixed supply, like those wires you sometimes see coming down from through the ceiling.
from what I understand the new style battery powered smoke detectors will "listen" for other smoke detectors (tone), thus not needing them to be directly wired together
Certainly not this one. Haven't seen one like that either.
I wonder if the "HORN" generates a bit of back emf and that diode just suppresses that. They might have chosen 10v knowing it will always behave as a diode with that VCC (maybe cheaper)
The transducer looks and sounds to be a piezo element. These behave like a capacitive load in a circuit, and not an inductive load. Hence, I don't suspect it's for back-EMF.
@@JoeyvanHummel yup. I realise that - but only if that circuit in the datasheet is the same as the implementation (if it was a speaker of some kind for example, back emf)
This zener. Might it be a protection over the FET? Its usual to have a reversed biased diode over a transistor. If you use a zener it will work as protection in both directions.
Engineers brother-in-law had a couple thousand of those 10v @ 1W zeners lying about.... The 'BIL effect'. Michael in Colorado.
Perhaps they put the 10V zener in for over voltage protection. How high is the voltage of a brand new 9 volt battery.
Battery chemistry and how many cells are in the 9V battery will determine the voltage.
For example: Rechargeable range from high 9.6V (8 cell 1.2V NiMH) batteries, to 7.2V (2 cell 3.6V li-Ion) batteries.
Hmm. Working in a related field (industrial fire detection), I would be very suspicious of a "heat detector". Heat detection has a terrible reputation: first they are subject to inverse square law, so unless it's right inside the fire then the fire is probably already out of control before the detector goes off, and that's assuming that the heat is not carried away elsewhere by ventilation anyway. Also, they tend to be made of materials that degrade over time: who knows how his detector will perform after 10 years? And given the normal seasonal and cooking temperature changes in a kitchen, how sensitive could they make it? Not very, I'd expect. I would stick with smoke detectors: kitchens shouldn't normally be filled with smoke unless you regularly burn your toast!
I think it's because the zener leaks a small, almost constant current below its zener voltage.
So basically that IC could be employed anyway you devise to vary the trigger using a resistivity device. You could use something like a Load Cell as the sensor . Looking on E-Bay those chips are a bit pricey. en.wikipedia.org/wiki/Load_cell
My guess would be: It still beeps because the thermal resistor is still too hot. Not because the mesured temp drops.
there are many (i do mean loads) of pcb made with bent resistor and axial cap leads , they do seem to be from the better quality manufactures and they are quite hard to desolder but i usually have long enough finger nails so dont get burned to much :-)
They make certain models of these with output relays to signal to other equipment. Wonder If the diode has been carried over from a unit with a built in relay and that is the position of the suppression diode. They may have just installed the zener to fill the space on this model? Either that or they manufacture a 12v version of this detector?
2019 - a commercial mass product of lowpower type completly throughhole? that's really strange, it's like fallen through a 25+ years wormhole.
If components remain available, and you have an automated production line (I know of component inserters that crimp the leads like that), it's *far* cheaper to keep cranking them out, vs. redesigning the circuit *and* replacing all the automation equipment. Upgrade for the sake of upgrading? Not if it costs you a quarter to half a million $ for *absolutely zero* return on investment.
My understanding of an Ion detector was it has 2 chambers, 1 sealed and 1 open. It ionises the chambers and compares the the difference between them to sense combustion products. This may be only in commercial detectors and different in these smaller domestic devices. interesting that the same chip is able to run a thermal detector as well.
Does this Zener diode have a temperature dependent leakage current/resistance that might affect the circuit? I ask because IN4148F have been used as breath temperature sensors in "Apalert" veterinary apnoea alarms.
Dont they need a trickle current flowing through the sensor at all times, in regards to the mystery diode, and that is the job of it? Check if you can get the heat sensor to trigger without the zener diode in the circuit.
The diode comes from a model that is wired into the mains? And they just kept it?