This is a simple circuit which can detect when you touch a sensor connected to one of the sensor inputs. It can be used to add a touch switch to your computer for example. It uses an AVR micro controller, the AT90S2313. This is overkill. I can add a lot more sensors than the two sensors I have now, but I had a 2313 to spare and I didn't need more than two sensor :). Anyway, the basic idea is really simple. Make one pin output and another input. Connect a resistor between these pins. The resistor together with the human capacitance (about 100 pF) forms an RC network. The AVR set the output to low and then make a transition from 0V to +5V. 5 µs after this switch, the logic level at the input pin is sampled. If someone is touching the probe connected to the input pin, the capacitor (=human) will not be fully charged, and the input will be a digital 0 and vice versa.
This is the schematic. The potentiometers are there to adjust the sensitivity. 0.5 MOhm is usually a good value.
This is the board layout (made in EAGLE)
|XC1||8 MHz Murata ceramic resonator|
|ISP_CON||AVR isp connector|
|SENSOR1||1x2 pin header, sensor 1 connector|
|SENSOR2||1x2 pin header, sensor 2 connector|
|SWITCH1||1x2 pin header, output for sensor 1|
|SWITCH2||1x2 pin header, output for sensor 2|
|SUPPLY||1x2 pin header, +5V and GND|
Almost any NPN transistor will do if you don't have BC547.
The source is written in C and compiled with AVR-GCC.
AVR source code
EAGLE schematic and board layout
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