This circuit can switch on / off a load when the set time arrives. It can be used to switch on AC loads such as TV, Radio, Music system etc. Its triggering pulse is obtained from a small table clock. The switch on / off the time can be set manually through the alarm setting of the clock.
Optocoupler IC MCT2E is used as the link between the clock and the circuit. The alarm buzzer gets around 3 volts when the alarm rings. This voltage is used to trigger the Optocoupler. The Optocoupler has an LED and phototransistor inside. When the LED inside the Optocoupler lights by receiving an external voltage, the phototransistor conducts.
When the Phototransistor conducts, the SCR BT169 fires and latches. This actuates the relay and the load will switch on / off. If the load is connected through the common and NO contacts, the load switches on. If the load is connected through the common and NC contacts, the load switches off when the time arrives.
When the Phototransistor conducts, the SCR BT169 fires and latches. This actuates the relay and the load will switch on / off. If the load is connected through the common and NO contacts, the load switches on. If the load is connected through the common and NC contacts, the load switches off when the time arrives.
SCR shows latching property when its gate gets a triggering pulse. The SCR continues the conduction even if the gate pulse is removed. It can be switched off only by removing the anode current. So a Push to off switch S1 is used to reset the SCR. Capacitor C1 has a buffering action at the gate of SCR for its smooth working. Diode IN4007 protects the SCR from back emf.
A small low cost table clock can be used for the purpose. Open its back cover and solder two thin wires at the buzzer terminals and connect to the pins 1 and 2 of the Optocoupler observing polarity. Enclose the circuit with the power supply in a case and fix the clock above it using glue. To connect the load, an AC socket can be fixed on the box.
A small low cost table clock can be used for the purpose. Open its back cover and solder two thin wires at the buzzer terminals and connect to the pins 1 and 2 of the Optocoupler observing polarity. Enclose the circuit with the power supply in a case and fix the clock above it using glue. To connect the load, an AC socket can be fixed on the box.
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