Automatic Delay-Off Timer Circuit

The Automatic Delay-Off Timer Circuit operates by stepping down and converting 230V AC mains power into a stable low-voltage direct current to drive timing and switching stages. Mains voltage is stepped down through transformer X1 and converted to DC via bridge rectifier BR1, with filtering capacitors C1 and C2 smoothing the rectified output. Voltage regulator IC1 (LM7812) supplies a steady 12V rail to power the logic and switching components. The core timing function relies on IC2, a TLC555 timer configured in a monostable multivibrator mode. When a trigger event occurs via the push-to-on switches (S1, S2) or selectable resistor networks (R4 through R8) determining the RC time constant alongside capacitor C3, the TLC555 changes its output state. This output drives transistor T1 (a 2N2219 NPN transistor) which acts as a solid-state switch to energize the 12V SPDT relay (RL1). While active, the relay controls the connected external 230V AC appliance. Signal diode D2 and rectifier diode D1 provide necessary flyback and reverse-polarity protection, preventing inductive kickback damage from the relay coil. LED1, limited by resistor R9, serves as a visual indicator for circuit operation. Users can further customize or override settings using the bank of on/off switches (S3 through S7) connected to the discrete resistor-capacitor timing arrays, allowing variable delay intervals before the timer times out, cuts off transistor T1, de-energizes the relay, and automatically turns off the load.