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Doornob touch sensor Answered

I took apart a stud finder to see how it worked. Did a quick google search and figured out they work similar to tough sensitive lamps (capacitance). So after soldering an on off switch to replace the push button on it, I taped it to my door, and fine tuned the sensitivity wheel. Now when someone touches the doorknob (on either side of the door), It lights ups.


you can make a simpler one using jfets

I reverse engineered a few portable "door knob alarms" similar to the one I have pictured here, and they work by way of detecting 60 Hz stray inductance that is everywhere. Tuned so that it does not pick up like an antenna, but rather like a touch switch, one touches the metal and that person becomes the antenna themselves, this triggers the alarm. It is a great principle to understand as you can make all kinds of "touch" switches and they are quite portable if run by battery ( warning: if they will be switching AC "mains" current, make sure the "touch pad" whatever it is, is completely "isolated" electrically from the AC current).