We have a friendly donation jar in the Hackerspace, and one of our members (cheers, Xintian!) wanted to make it interactive. He modified a touch-switch 555 circuit to use with an existing blinking LED strip, so when someone throws a coin in to the jar, the LEDs light up.
Bill of materials:
* 555 timer IC
* 10nF capacitor
* 100nF capacitor or larger, or a bunch of them to experiment
* 47kOhm resistor
* battery (9V, or 4x1.5V is good too)
* 200-330Ohm resistors + LEDs or LED strip
* metal foil
The wiring is shown on the attached picture. The principle of operations is something like: the Trigger pin of the 555 has high impedance and the voltage of the human body can trigger it through the touch plate. When the circuit is triggered, the output stays high for an amount of time determined by R1 and C2.
Some notes and modifications:
Touch plate: it is aluminium foil wrapped around the mouth of the jar, to provide contact with people's hands when dropping in the coins.
Output: in our case, the output just provides the voltage for the existing blinking LED circuit, that was previously battery powered. The output pin and the negative terminal of the 9V battery are attached to the + and - terminals of that circuit, replacing D1 + R2 in this circuit.
Capacitor / resistor values: the circuit diagram has the capacitor and resistor values (C2 and R1) used in our circuit, but it takes some trial and error to find some good values (other examples of this circuit online has different values listed). The effect of different values is different on time, and maybe different sensitivity. With some values the circuit ended up "getting stuck" in on mode. So far smaller C2 seemed to work better for stability, but larger looks nicer with blinking. If not using blinking circuit yourself, smaller value should be okay.
Breadboard: we assembled the circuit on a breadboard, though in this case, that might not be an ideal solutions with the stray capacitances around that can mess with the circuit since the trigger pin is so sensitive. Soldering onto a prototyping board might be a better idea, and also trimming the legs of the components used. Will experiment with that later.
And finally, here's the circuit in operation: