Most cars are provided nowadays with automatic interior lighting that comes on when one of the doors ( or the tailgate of a hatchback) is opened. This can have a drawback if one if the doors of the car are not closed properly and the car is left for a few days in the garage. This may effectively flatten the battery. There is, however, a remedy for this.
For the proposed circuit, the usual lights switch takes over the control of the interior lights from the doors switches. The circuit is based on a 555(7555, 555C) timer and acts as a monostable that switches on the lights for four minutes as soon as it is switched on, THis time may be altered by changing the values of R2 and C2. Network R1-C1 ensures that IC is enabled as soon as the voltage is applied.
The output of IC1 controls relay Re1 via T1. The only requirement of the relay is that it should be actuated by a voltage of 12 V and a current of less than 200 mA. As soon as the timer switches the lights off, the current drain drops to 6 mA if a 555 is used and to 0.5 mA if a 555C is used.
Before the circuit is built into the car, check how the door switches are connected. If they are in the +ve battery line, use variant an if in the -ve (earth) line, use variant b.
Finally, connect the circuit to the junction of the wires from the door switches, otherwise, it will react to only one door.
* These days, even in the garage, it is best for security’s sake to properly shut and lock all doors and, if fitted, set the alarm. Editor).
* In quite a few modern cars, the circuit described (or one like it) is a standard fitting.Editor.