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.
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Denne historien er fra September 2024-utgaven av Electronics For You.
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