We can make a simple
touch switch using only two inverter gates, two resistors, and two
capacitors. The schematic diagram of the circuit is shown in the figure
below. At power up, the output (of U1A) will be high, and the inverting
output will be low because U1A gate will be triggered to ground level by
C2. After triggered, the low level of U1A input is maintained by U1B
output via R2.
If we touch the pad at this condition, where the output is high,
then the U1A input will go high because we “short” the voltage of C1 to
the input pin, and the low level previously caused by low level of U1B
output voltage connected via R2 can’t be maintained because our skin
resistance is much lower than 10M.
After U1A input goes high then U1A output will go low, and now U1B
will go high to maintain high voltage level of U1A via R2, so we can
release our finger without loosing the last state. Touching the pad
again after we release our previous touching will toggle the output as
the condition is reversed.
After we touch the pad, we have to release before 1 second (R2C2
time constant) elapsed. If we touch the pad longer than R2C2 time
constant then the output will oscillate (about 1 Hz).