LONG DURATION TIMER Using Supercapacitor

When charging devices like mobiles and laptops, it is easy to forget to switch off the power once they are fully charged, often resulting in devices remaining connected to the AC mains supply for hours.

- PRADEEP VASUDEVA

A simple solution to this oversight is presented here. The circuit shown in Fig. 1 can automatically switch off any device connected to the AC mains supply after a period ranging from one to over four hours. Extending the delay period is possible by adjusting the values of the resistors used in the circuit.

Circuit and working

Fig. 2 shows the circuit diagram of a long-duration timer using a supercapacitor. The circuit comprises a centre-tapped transformer (X1), four rectifier diodes (1N4001, D1-D4), a dual op-amp (LM358, IC1), a transistor (2N3904, T1), a 5V SPDT relay, two LEDs (LED1, LED2), and additional components.

The 230V AC is stepped down to 6V-0-6V AC by transformer X1, then rectified by diodes D1 and D2 and stabilised by filter capacitor C1, resulting in around 6V DC to power the circuit. The circuit utilises one op-amp (IC1A) of the LM358, while the second op-amp remains unused. A supercapacitor (C2) serves as the heart of the circuit.