Here’s a clap switch free from false triggering. To turn on/off any appliance, you just have to clap twice. The circuit changes its output state only when you clap twice within a certain time period of 3 seconds.
The clap sound is sensed by the condenser microphone, and it is amplified by T1. The amplified signal provides a negative pulse which is applied to pin 2 of IC1 and IC2, and triggers both ICs. IC1 is wired as a monostable multivibrator, and when triggered causes pin 3 to go high. Pin 3 remains high for a certain time period depending on the values of R7 and C3. This specific “on” time (T) of IC1 can be calculated according the following formula:
T=1.1· R7· C3 seconds (R7,C7 in ohms and microfarads, respectively)
On first clap, output pin 3 of IC1 goes high and remains high for the preset time. Also, LED1 is energized for this period. The output of IC1 (pin 3), also energizes IC2 by providing supply voltage at its pins 8 and 4. IC2 is also wired as a monostable multivibrator, and after the first clap will be ready to receive the triggering signal from the second clap. R10 and C7 are connected to pin 4 of IC2, and they prevent false triggering during the first clap.
On second clap, a negative pulse triggers IC2 and its output pin 3 goes high for a time period depending on R9 and C5. IC2’s output provides a positive clock pulse at clock-input pin of IC3. IC3 is a decade counter which is wired as a bistable.
Each pulse applied at clock pin 14 changes the output state at pin2 (Q1) of IC3 because Q2 is connected to reset pin 15. A high logic state at pin 2 drives transistor T2, which in turn, energizes the relay K1. LED2 indicates the activation of the relay and the on/off status of the appliance. D1 is used as a free-wheeling diode to prevent damage of T2 when relay de-energizes.
The circuit can be built on the printed circuit board of figure 2. This is a single-sided board, especially designed to accommodate the Clap Switch circuit. Please note that the PCB has one jumper. The jumper is located above IC1 and is made from a copper wire.
Printed Circuit Board Artwork for the Clap Switch