The touch switch is based on a Schmitt trigger withhold contact. which is built from IC1a, IC1b, and R3. The output of IC1b is fed back to IC1a via R3. resulting in both gates maintaining their stable status. As long as the circuit is quiescent, that is, the anode of D1 is low and the cathode of D2 is high, the gates maintain their status.
Schmitt trigger is a comparator circuit with hysteresis that is implemented by applying positive feedback to a comparator or differential amplifier’s noninverting input. It is an active circuit that converts a digital input signal to an analog input signal. The circuit is called a trigger because the output holds its value until the input changes enough to cause a change. When the input exceeds a predetermined threshold in the non-inverting configuration, the output is high. When the input falls below a different (lower) threshold, the output is low; when the input falls between the two levels, the output retains its value.
A touch switch is a type of switch that operates solely by being touched by an object. It is found in many metal-clad lamps and wall switches, as well as public computer terminals. A touchscreen is a display with an array of touch switches. A touch switch is the most basic type of tactile sensor.
Consider that output A is low. When the touch contact is shorted with a finger, the potential across R8 rises. This rise is translated into a short pulse by R1-C1. This pulse renders the input of IC1a high via D1. The output of IC1b and Output A will then also go high and, as explained before, this status is maintained.
At the same time, the potential across C2 will rise gradually. When this capacitor is fully (or nearly so) charged, the circuit is ready to change state at the next pulse across R1. This pulse will make the output of IC1a low, whereupon the output of IC1b becomes low, and the potential across C2 drops. Output A is then low again and the circuit is back to where it started.
