Describes a method to detect the presence of a human body for use as a touch sensor.
The technique is very interesting. Two pins of a microprocessor are connected to each other using a large resistor, around 500k. Attached to the ring is a touch pad of conductive material, covered with an insulator, for example plastic.
If the sensor is touched (on the insulating plastic) it is in fact a capacitor. This capacitor is charged via the resistor. The charging time is dependent on the value of the capacitor. If there is no finger the value is very low, thus charging is completed very rapidly. But if a finger is touching the sensor the capacity is much higher and charging takes much longer (well, much, 3 microseconds to be exact).
So the program does the following:
discharge the capacitor by making pin one low
wait a bit to make sure it is discharged
charge the capacitor by making pin one high
wait 4 to 5 microseconds
see whether the pin two is now high
if it is high, the capacity was low, thus no finger, but if it is still low the capacity is high, thus a finger
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