Clock Controlled Relay Driver. Design Trick 14

Don’t miss your favorite TV programme or don’t forget to irrigate garden. This clock controlled circuit will actuate a relay to switch on the AC load to perform the function at the right time. Operation of the circuit is simple. It receives the triggering pulse from the buzzer terminals of the clock and an SCR triggers to activate the relay. The only drawback of the circuit is that, it is necessary to switch off the load manually by turning off the power. You can do experimentation to make an automatic operation also.

The mini digital or needle type table clock uses 1.5 or 3 volt DC which appears in the buzzer terminals when the alarm sounds at the set time. Open the clock and remove the buzzer (not needed, it can be there) and solder thin wires to the soldering points connecting the buzzer. Observe polarity. Then connect the wires to the points shown in the diagram. So when the DC appears in the buzzer terminals, the Optocoupler activate to trigger the SCR. Optocoupler has an LED and a Phototransistor inside. When the LED lights by obtaining the power from clock buzzer terminals, the phototransistor conducts. The voltage appearing at the emitter of phototransistor (pin4 of Optocoupler) then triggers SCR. Relay clicks and the load gets AC through the Common and NO (Normally Open) contacts of the relay.Once SCR triggers, it will be in the latched state and conducting , even if the the gate voltage is removed. So it is necessary to remove the Anode current of SCR to switch off.

Set the alarm time in the clock and turn on S1. When the needle of clock crawls to the set time, the load will be switched on. After completing the required time, switch off the load by opening S1.

Caution : Take care while connecting AC load to prevent shock. Do not touch the relay contacts when it is connected to mains.