Touch sensor

AT90S2313

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