The low voltage output on the PI GPIO pins is between 0.4 and 0.6V High is 2.9 to 2.4. So there is a danger of switching the transistor on with the GPIO set to low. A simple solution is to put a potential divider in circuit with equal values the centre will be 1.2V when high that should also cure the random switching. It's worth having a bag of 2N7000 mosfets in stock. These make easy work of such issues. The source - gate turn on voltage is a minimum of 1V so the 0.6 volt output when low wont switch them on. They also have a very low on resistance so can drive quite heavy loads. I usually put a resistor between Gate and ground just to make sure the device switches off. Andy
Thanks Andy! I did hit on the voltage divider idea as you'll see in one of the follow on videos. It hadn't occured to me to try a 2N7000 but that is a great suggestion. I wonder is the MOSFET will be less prone to partial conduction in the presence of RF. Given the low voltage output on the GPIO lines it makes wonder why virtually every online tutorial uses a 2N3904 and does not use a voltage divider at the base of the transistor. Does no-one test their circuits before they publish?
@@kk4das122 in their defense the GPIO pins are connected to the IO of the broadcom chip and the actual spec is not well published. I found a few bits on the web that gave the info. I suspect that many people fall into the trap of thinking that GPIO low = 0V. It's very unfortunate that the low output is right on the usual transistor threshold voltage and I suspect that is many cases it seems to work OK. Regardless you should have some sort of buffer / circuit that takes cane of such things. There are quite a few chips that will do level and logic conversion, but the cost is more components (The chips are cheap) The Pi 5 has a different GPIO interface, I think I read that they have taken the I/O control of the Pi Pico and used that to buffer the broadcom I/O. However I've not got a Pi 5 as yet so just going by what I've read. The 7000, they seem pretty solid, a resistor from gate to ground to ensure that it turns off and decouple that with a 0.01 ceramic and it should do the job nicely. The big plus is that it does not draw any current as such from the GPIO so that helps protect things. You have more danger from pick-up on the wires than anything else, so it pays to take a bit of care with that. Just about to have a look at the other videos... Andy G0POY
The low voltage output on the PI GPIO pins is between 0.4 and 0.6V High is 2.9 to 2.4. So there is a danger of switching the transistor on with the GPIO set to low. A simple solution is to put a potential divider in circuit with equal values the centre will be 1.2V when high that should also cure the random switching.
It's worth having a bag of 2N7000 mosfets in stock. These make easy work of such issues. The source - gate turn on voltage is a minimum of 1V so the 0.6 volt output when low wont switch them on. They also have a very low on resistance so can drive quite heavy loads.
I usually put a resistor between Gate and ground just to make sure the device switches off.
Andy
Thanks Andy! I did hit on the voltage divider idea as you'll see in one of the follow on videos. It hadn't occured to me to try a 2N7000 but that is a great suggestion. I wonder is the MOSFET will be less prone to partial conduction in the presence of RF. Given the low voltage output on the GPIO lines it makes wonder why virtually every online tutorial uses a 2N3904 and does not use a voltage divider at the base of the transistor. Does no-one test their circuits before they publish?
@@kk4das122 in their defense the GPIO pins are connected to the IO of the broadcom chip and the actual spec is not well published. I found a few bits on the web that gave the info.
I suspect that many people fall into the trap of thinking that GPIO low = 0V. It's very unfortunate that the low output is right on the usual transistor threshold voltage and I suspect that is many cases it seems to work OK. Regardless you should have some sort of buffer / circuit that takes cane of such things. There are quite a few chips that will do level and logic conversion, but the cost is more components (The chips are cheap)
The Pi 5 has a different GPIO interface, I think I read that they have taken the I/O control of the Pi Pico and used that to buffer the broadcom I/O. However I've not got a Pi 5 as yet so just going by what I've read.
The 7000, they seem pretty solid, a resistor from gate to ground to ensure that it turns off and decouple that with a 0.01 ceramic and it should do the job nicely. The big plus is that it does not draw any current as such from the GPIO so that helps protect things. You have more danger from pick-up on the wires than anything else, so it pays to take a bit of care with that.
Just about to have a look at the other videos...
Andy G0POY