This circuit will keep the car interior lights switched ON for some time after the ignition is turned OFF so that the driver is able to exit the car comfortably without missing anything important inside the car. Nowadays, a switch-off delay for the car interior lighting is a naturally a standard feature. However, with certain models having only spartan fittings, or older-model vehicles, you’re left sitting in the dark as soon as you climb in and close the door. That calls for an aftermarket accessory!
The only holes drilled in the circuit board were the four fixing holes, and the entire assembly was firmly attached to the surface of the heatsink for power transistor T3 (the author used a finned heat sink rated at 7.2 °C/W). The heatsink is at ground potential. A value of 1 Ω was used for R3 with satisfactory operation of the darlington. The light goes on when the door is opened. After the door is closed, it continues to illuminate the interior of the car at full brightness for around 30 seconds, after which it slowly dims. Approximately 1 minute after the door is closed, the quiescent current drops to zero.