Touch Sensitive Light Switch

This low-cost circuit enables turning room lights on and off simply by touching a round metal sensor. The light is turned on by briefly touching the sensor, and off again by touching it slightly longer. With reference to the circuit diagram, when the sensor is briefly touched, hum and noise induced on the body are amplified by cascaded gates NI, N2 and N3. A pulse train with a swing of nearly the supply voltage (4.7 V) and a frequency equal to that of the mains voltage (50 or 60 Hz) is ap- plied to a bistable set up around N4 and Ns. C2 is charged via D2, and the bistable latches in a high output state. Triac Trii is triggered via driver Ti, so that the lamp lights. When the sensor is touched for about 2 seconds or longer, the pulse train charges Ci via Rs and Di. Inverter Ns pulls the input of N4 low when the voltage on Ci is sufficiently high. Bistable N4-N5 toggles and Ti breaks the gate current for the triac, so that the lamp is turned off. The circuit also works in a relative...