Cars have a range of dashboard warning lights, such as the oil pressure warning light or engine management light, which are designed to alert you if one of a variety of systems isn’t quite right. Many cars show these warning lights briefly when you turn on the ignition, but they should soon go out again. If one or more of these lights stays on once the engine is running, it could be a sign there’s something wrong with your car.
Virtually modern cars have a built-in alert alarm to warn that you have left the lights on when you are getting out, There are, however, millions of older cars that have no such useful device and their owners might find the present circuit just what they’ve been waiting for.
The anode of diode D1 is connected to the lights switch and the lights. The cathode is connected to the 12 V buzzer. The other terminal of the buzzer is connected to one of the door-switch contacts or to terminal 31B of the interior lights cable. When one of the front doors (fitted with a switch, which, in older cars is not always the case with rear doors) is opened while the external car lights are still on, the buzzer begins to hum. Note that the circuit allows for only two-door switches: S2 is the driver’s door contact and S3 the front passenger door’s contact. If you don’t want the buzzer to be operated by the front passenger door, fit diode D2 as shown and disconnect the wire between the contacts on the two doors.
Note that the circuit is intended only for cars whose door contacts switch the interior lights to earth (which is the case in most cars built in the past 10 years). Diode D1 may be a light-duty type such as the 1N4001. but D2 must be a somewhat heavier-duty type, such as a 1N5401 since it carries the current through the interior lights.