In essence, this circuit is a 15 to 20-second courtesy light extender for cars. It is activated in the usual way by opening a door but it also samples the negative lock/unlock signals from a car alarm or central locking and does two more things. First, when an unlock signal is received, it turns on the courtesy light for 15-20 seconds before you open the door. Second, when a lock signal is received, it turns off the courtesy light immediately, with no fade-out. This is done to eliminate false triggering of the burglar alarm through current drain sensing. When a car door is open or the unlock relay is activated, the 33µF capacitor discharges through diode D1 and this keeps transistor Q1 turned off. Circuit diagram: Courtesy Light Extender Circuit Diagram This allows Q2 and Q3 to turn on and the courtesy lamp is activated. When the door is closed, the courtesy lamps stay illuminated and the 33µF electrolytic capacitor starts charging through the associated 1MO resistor. As the voltages ...