I need to design a circuit that will operate in a car, and the circuit needs to know when the vehicle is turned off. The circuit will run from its own battery, and it will basically be an alarm to alert a driver he/she needs to remember to do something before leaving the vehicle. I originally thought to just plug into the cigarette lighter, as most modern cars shut this down when the engine is stopped. But some do not, and in that case my intent was to just sense whether any AC (from the alternator) was riding on the DC. So now, if the DC voltage goes away, OR its AC component goes away, I can assume the engine has been shut down. Now there will be some cases where my circuit won't be usable, and I'm just trying to identify and minimize these cases. An electric car, for example, will shut down at a stop light, so if that car normally supplied voltage to the lighter socket when the whole vehicle was shut down, my circuit would not be usable in that case. I'm also not sure at all how cars with 24 or 48VDC batteries handle their lighter. They may use 12V regulators or switching supplies, which means I'm never going to detect an alternator through that.

So the questions are, what bases have I not covered, and are there any solutions to the NFG cases I'm missing.

  • \$\begingroup\$ Can you wire it in? Usually there is a switched accessory wire used for things like the radio that are turned off when the key is in the off position \$\endgroup\$ Commented Dec 5, 2014 at 15:39
  • \$\begingroup\$ Usually in a 12v car the battery voltage will be ~13.5v when running (alternator charging battery) and more like 12v when off. Vehicles with odd power systems, like 24/48v providing a regulated 12v on the lighter socket will be an exception, but then many 24v vehicles put 24v out on their lighter sockets anyway, so you need to beware! \$\endgroup\$
    – John U
    Commented Dec 5, 2014 at 16:38
  • \$\begingroup\$ No, can't wire it in. Its more desirable to make something that anyone could use, without needing to get too technical or have it "installed". Regarding the voltage level, I've seen some cars dip down to a near trickle charge maybe 12.5 volts. In the electrically noisy environment of a car, I wasn't sure I could rely on that small voltage differential. As long as my device runs off its own internal battery, and is only using the cigarette lighter for detection, it won't be a big deal whether its 12V or 24V system, but thanks for the warn. :-) \$\endgroup\$
    – Randy
    Commented Dec 6, 2014 at 22:20

1 Answer 1


Nearly every car has a 12V accessory supply which is switched with the key. Typically this powers the radio, and maybe other things like indicator lights, power windows, etc. Of course the specifics of which things are on this supply vary from car to car, but every car has something like this. It is used for any accessory which doesn't need to be on when the car is off, and which might otherwise drain the battery between starts.

Once you've found it, then you could power your device directly from it, or indirectly via a 12V relay.

  • 1
    \$\begingroup\$ Unfortunately, not all cars have an accessory supply. My car doesn't, so I had to whip up a similar detector circuit when I installed a radio in my car. The radios that the manufacturer delivers listen on some kind of serial bus, and the cars main controller sends a shutdown command when the ignition switch is turned off. An adapter that can interpret the signal and provide a simple on/off costs more than a car radio, so I faked it. Mine just checks the voltage on the 12V supply. If it drops below a certain level, the radio goes off. Sucks when idling at a red light with the AC on. \$\endgroup\$
    – JRE
    Commented Dec 5, 2014 at 15:56
  • \$\begingroup\$ Ditto. My reasonably new SCION has no separate ACC supply. I'm pretty much stuck with a standard cigarette lighter, and that would be fine if not for some older vehicles that leave them powered at all times. But I appreciate you're input Phil. \$\endgroup\$
    – Randy
    Commented Dec 6, 2014 at 22:26
  • \$\begingroup\$ @Randy huh...I didn't know that. Crazy Japanese manufacturers. I guess you could take the steering column apart, and connect directly to the ignition switch :/ \$\endgroup\$
    – Phil Frost
    Commented Dec 7, 2014 at 14:10
  • \$\begingroup\$ @PhilFrost - not just the Japanese. My car is a Skoda, with all of its running gear from VW. So, at least Skoda and VW deliver cars without ACC. \$\endgroup\$
    – JRE
    Commented Dec 10, 2014 at 9:22
  • \$\begingroup\$ @Randy it's not just older cars... my 2007 Ford Ranger has both a cigarette lighter and an "accessory" outlet, both of which are unswitched (always on). I actually find this to be a convenience, because sometimes my phone's battery is dying, so I can leave it charging when I'm running errands. \$\endgroup\$
    – Doktor J
    Commented Dec 24, 2014 at 17:53

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