Automatic Power Cut-off Timer Circuit Using Supercapacitor

The Automatic Power Cut-off Timer Circuit uses a combination of analog components and a high-capacitance supercapacitor to manage power delivery to an appliance. The system interfaces with the AC mains through a centre-tapped transformer (X1), which steps down the voltage. The 1N4001 diodes (D1-D4) form a bridge rectifier to convert this alternating current into direct current, providing power to the circuit and charging the supercapacitor. The LM358 dual op-amp (IC1) acts as a comparator, monitoring the voltage level stored across the supercapacitor as it gradually discharges through the timing network of resistors. LEDs (LED1, LED2) provide visual status indicators for the operational state of the circuit. While the supercapacitor maintains a voltage above the reference threshold, the LM358 output drives the 2N3904 transistor (T1). This transistor acts as a solid-state switch, pulling current through the coil of the 5V SPDT relay to keep the AC mains contacts closed and power flowing to the connected appliance. Once the supercapacitor discharges below the predetermined threshold, the op-amp output switches state, cutting off the base current to the 2N3904 transistor. This de-energizes the relay, causing its contacts to open and physically disconnect the AC mains supply from the load. This discrete analog approach eliminates the need for programmable microcontrollers while providing reliable time-delayed power disconnection.