LMC/TLC555 based contactless capacitive sensor, built of components, that everybody already has

Discussion in 'The Projects Forum' started by Skfir, Feb 16, 2018.

  1. Skfir

    Thread Starter Senior Member

    Aug 19, 2010
    Hi everyone. Sorry for apparently a sloppy post - I am a pianist, not an engineer and this is my very first submission. So, being a pianist = being poor. Recently I needed a contactless capacitive touch sensor. Of course there are many specific ICs for this purpose, but I decided to make a sensor out of parts, that every hobbyist already had in his toolbox. After some Googling I found out a number of schematics, built on a 555 timer that claimed to be capacitive sensors, but in reality were still old-good resistive ones. So, I decided to design the sensor myself and apparently it works fine.

    This sensor can be used not only in order to switch something on and off, but also as a moist sensor, for, say, building an automated plants watering system.

    The sensor consists of two 555s, one of them is a generator, the other one is the switch. When you touch the plate, you change the capacitance on the generating 555, so the frequency lowers. Consequently, the voltage drop on C1 increases. This in turn switches on the transistor, which triggers the second 555. What is funny in my schematic is that the control pin of the generating 555 is connected to the C1, which establishes a weird positive feedback. When you don't touch the plate, the voltage on C1 and the control pin is low, which decreases the duty cycle of the generator, which in turn makes the voltage on C1 even lower, which in turn again decreases the duty cycle and so on. When you touch the plate right the opposite thing happens. So... Yeah... Sorry, I am a pianist and it is my very first submission. I didn't really know ho to make a part list, basically any resistors/diodes/capacitors will do. What about the 555s, these should be either LMC555, or TLC555. The transistor should have Vt, higher than 1V. If a transistor with Vt>3V is used, then R17-R18 divider can be removed. I tested the circuit at supply voltages from 6 to 13V. 12V should be the best I think.
    Last edited: Feb 18, 2018