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. 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.
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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.
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