touch-pad potentiometer - maintains the set value

maintains the set value la Touch-pad keys normally use a simple digital memory, but they can be operated to give an analogue out- put voltage as is shown here in an inexpensive circuit that is easy to build. The circuit is based on IC1, an oper- ational amplifier with very high input impedance, which is connected as an integrator. When touch-pad Se1 is touched with a finger, capacitor C2, 7-34 an MKT type, charges via the skin resistance, which causes the output voltage of IC1 to drop linearly to zero. When the other touch-pad, Se2, is touched, the opposite hap- pens: the output potential of IC1 will then rise linearly until it reaches the level of the supply voltage. The beauty of the circuit is that when you take your finger from the pad, 9 - 30 V july/august 1984 lb 51 per hour towards zero or towards the supply voltage, depending on which of the key pads was touched last. To keep these leakage currents small, it is necessary to keep the circuit well away from moisture or hum...