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.

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