Opening car’s door is very simple activity, and off course it’s just a daily activity which require no special skill. Almost nothing can go wrong, but forgetting to close the door when we doing or thinking on many tasks is a common thing. This can lead to many consequences, from as simple as air conditioner inefficiency to as dangerous as ridiculous accident when driving car with improperly closed door. The circuit shown in the following schematic diagram can alarm you for such careless behaviour.
When the car’s door is opened, the first section of NE556 dual timer starts the timing cycle when normally closed arm/disarm switch is opened. Pin 5 becomes low to activate the alarm circuit after delay to notify the user if the door is still open. PNP transistor is kept off as long as all door switches are closed because pin 9 is high. Transistor turns on only after some predetermined delay for owner to enter car, the beeper won’t be sounded if the user closes the door within delay time.