Auto bed leveling with piezo sensor

G29 command with Marlin firmware. It uses piezo sensor and the piezo disc is above the E3D head in yellow plastic (not visible).
This video is part of an article, sorry, it is in Hungarian:
www.ivanov.eu/?...

[@TheWhitde]

I like this method. Going to get a few Piezo's and experiment with them underneath 1st using a small Arduino and an OLED to get a feel of the input received
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[@mikesimpson4652]

Hi Peter,
Hungarian was no problem for Google translate. You have a nice looking setup but if I could ventrure a comment: Piezo discs act as very good detectors for temperature changes, so much so that with 10 Megohms of input resistance in the amplifier a rate of temperature change of only 0.1 degrees per second there will be an output of about 150mv from the disc and this will make the trigger point too sensitive or not sensitive enough depending on which way round it is connected. The simple answer is to have a resistance of about 1 Megonm across the input, essentially between pins 2 & 3 of the LM358N. A brief discussion about this on forums.reprap.org/read.php?1,635075,707280#msg-707280 and more info on piezos for bed leveling on www.mdpi.com/2411-5134/3/1/8/htm
Mike

[@ivanovp]

Hi Mike,
thanks for the advice and links! I'll add the resistor to the inputs.
Peter