I want a touch sensor that works reliable if i touch the metal.
If i look around there is CapSense, Qtouch and special ICs and it's all
about capacitive that works behind wood, glass, plastic...
Well also complicated code.
Do you want a capacitive sensor (one electrode that also works through an insulator).
Or a resistive sensor with two electrodes, where your finger bridges the gap (dotted line on one diagram).
Resistive sensors migh also work more or less on mains hum pickup.
Leo..
No, they are not. I've experimented with several circuits like the two you posted and have been disappointed.
Those circuits operate by detecting weak electric potentials/currents induced in your body (or in the circuit itself) by fluorescent lighting, nearby AC wiring, etc. If you happen to be in a "quiet zone" or are well grounded, they often don't work.
I suspect that Force Sensitive Resistors (FSRs) should be pretty reliable as touch sensors, but I have not personally experimented with them. Perhaps others can comment.
I used the cap sense examples in the Arduino Playground to make different sensors.
I takes getting the right resistors to match your button area and capacitance. Also to have the button area surrounded by grounded area is supposed to help.
I have used aluminum foil layered with paper and rolled up as sensor. I have done the bare wire stuck in a pin hole and that with the top bent into shape to hold a foil-area button. And I have done the same with a 10 uF cap with one leg stuck in the pin hole and the other with a foil button.
The foil and paper was the hardest to tune but I could detect nearness and there was a big jump on touching the paper but really compressing the layers of paper and foil increased the capacitance. I wonder but haven't tried to see if compressing a spring will increase its capacitance. I think it should.
There's a capacitance jump on contact with all of those 'buttons'.
