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10-20-2007

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.

Touch sensor

Schematic and board layout

This is the schematic. The potentiometers are there to adjust the sensitivity. 0.5 MOhm is usually a good value.

Touch sensor shematic

This is the board layout (made in EAGLE)

This is the board layout (made in EAGLE)

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  

Files:

AVR source code
EAGLE schematic and board layout

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