Simple ON/OFF TIMER

Imagine leaving home in the evening and returning to find freshly cooked rice ready in the rice cooker.

- PANKAJ DATTA CHOUDHURY

Or consider needing a power outlet socket for mobile phone charger that automatically turns off after a set time to prevent overcharging and extend lifespan of the mobile phone and charger. These scenarios can be made possible with a simple on/off timer that can be easily assembled at home.

This simple on-off timer can control any household electrical device that needs to be switched on or off after a specific duration. It employs inexpensive components wired in an analogue circuit, providing extended timer duration without the complexity of digital ICs or microprocessor programming. Higher loads can be controlled by selecting higher current relays, switches, and wires, or external contactors. EFY lab tested the prototype on a breadboard, as shown in Fig. 1.

Circuit and working Fig. 2 shows the circuit diagram of the simple on/off timer. It is built around transformer X1, bridge rectifier BR1, 12V voltage regulator LM7812 (IC1), dual op-amp LM358 (IC2), transistor BC547 (T1), 12V SPDT relay, one 1N4007 diode (D1), signal diode 1N4148 (D2), two LEDs (LED1, LED2), and a few other components.