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.