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 +5 V. 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.
Schematic and board layout
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)
Component list:
| Name | Value | Package |
| R1 | 4.7 kOhm | 1206 |
| R2 | 1 MOhm | |
| R3 | 1 MOhm | |
| R4 | 3.3 kOhm | 1206 |
| R5 | 3.3 kOhm | 1206 |
| Q1 | BC547 | TO-92 |
| Q2 | BC547 | TO-92 |
| IC1 | AT90S2313 | DIL20 |
| XC1 | 8 MHz Murata ceramic resonator | |
| ISP_CON | AVR isp connector | |
| SENSOR1 | 1 x 2 pin header, sensor 1 connector | |
| SENSOR2 | 1 x 2 pin header, sensor 2 connector | |
| SWITCH1 | 1 x 2 pin header, output for sensor 1 | |
| SWITCH2 | 1 x 2 pin header, output for sensor 2 | |
| SUPPLY | 1 x 2 pin header, +5 V and GND |
