Is there a cheap device, or circuit I could make up, which alerts people to car doors being left open for more than X minutes, to avoid batteries being completely drained?

I know of a device which goes between one of the battery terminals and the cable which usually connects to that terminal, and when the battery power gets below a certain level it breaks the circuit, but it's not cheap and in some ways it would be better to deal with it much earlier (e.g. after 5 mins), and get the door closed for security & weather reasons.

Maybe just a buzzer which fits under the hood/bonnet, which detects power to the door light circuit (in some models), and beeps for a second every minute or so.

I have a Toyota Corona 1993 and we left a door open which flattened the battery a few days ago. Fortunately this model beeps when headlights, etc. are left on, so that's not a problem for us.


  • If you intentionally want to leave a door open then in some cases you can use a tennis ball to trick the door sensor. Doesn't work on all cars though
    – GdD
    May 16 '18 at 8:39
  • Thanks GdD, but this is for UNintentional door leaving open issues. But why doesn't it work on all cars? Is it because they don't have a button in a place which a ball can press against? I guess you also need the door to be pushing against the ball hard enough for it to activate the button, so you might need to have something pushing against the door???
    – Terry
    May 16 '18 at 23:18
  • Good guess @Terry, it is the button sticking out, not all cars have that. Most car doors have a sort of notch that keeps them open partway, that usually works to keep the pressure on the ball.
    – GdD
    May 17 '18 at 7:20

You can fit a buzzer between a permanent live and the interior light door switch - simple and works.

Then you could refine it with a timer circuit : either brush off your electronic skills or you may consider re-purposing a wiper delay circuit - which may give you an adjustment delay facility...

My car does the opposite - if you leave a door open, it turns off the interior lights after 25 minutes approximately...

  • Thanks for that, Mike. I've got some 555/556 timer chips which I may be able to work out how to use. An alternative to the buzzer could be a very short horn blast, I guess. And another alternative could be to try to break the interior light circuit after X mins, as your car does, the down side being that the door stays open which isn't ideal for security & weather. But starting with the simple buzzer thing you first suggested, tell me about where I should get to the necessary connections you mentioned? Or is it too make/model dependent to say? I know very little about car electrics.
    – Terry
    May 16 '18 at 23:27

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.