Vehicle Interior Lighting With Switch-Off Delay

Nowadays, a switch-off delay for the vehicle 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 author built this circuit using ‘normal’ components (with leads), but in the SMD manner, which means fitting the components on the copper side. 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.