Switch of car accessories based on V

Discussion in 'The Projects Forum' started by Scorp1us, Dec 21, 2013.

  1. Scorp1us

    Thread Starter New Member

    Dec 21, 2013
    2
    0
    I have an always-on accessory in my lighter socket. But it keeps killing my battery if I don't run it every day. I was hoping I could use a simple circuit that would only work when the lighter is over 12V (meaning it 14V because the alternator is on)

    I think I would just need a transistor and a resistor?

    Many thanks!
    PS. I tried to search for this, but I didn't even have a good idea what to google.
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,094
    3,033
    Depending on your car, you may be able to program the outlet to be keyed instead of battery-direct. My minivan offers that option.

    Just about anything else you want to do to the vehicle will qualify as an automotive modification - a topic forbidden by the Terms of Service here.

    I'm not sure how the mods will feel about it, but I think altering your ACCESSORY to auto-off on low voltage should be an OK topic. We had a low-battery auto-off thread here recently. Search a bit and you should find it very useful.
     
  3. #12

    Expert

    Nov 30, 2010
    16,261
    6,768
    I just tried a TIP 120 circuit. Too much Vbe drop. This will have to be done with a relay (if you can't just "tell the car" to make it a "run only" outlet).

    Zener, resistor, transistor as a switch, relay coil.
     
  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
    5,997
    3,754
    What load are you drawing (amps) from the cig lighter?
     
  5. Scorp1us

    Thread Starter New Member

    Dec 21, 2013
    2
    0
    It varies, it is a dash cam, but supports a charging port for phones, which draws 1600ma when there is a phone plugged in, not including the dash cam.

    Why I want this is so I don't have to modify my car at all. I want to put the circuit inline - actually in the line to the dash cam - and let the acc port work as it currently does, when I need to charge my phone in the car. So TOS should not be a problem, as this would go on the dash cam wire. :)
     
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,777
    1,103
    Can you confirm what the lighter socket voltage actually is when the engine is running?
     
  7. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    My prediction:

    Engine running, > 14V
    Engine stopped, < 13V
     
  8. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    My prediction:

    Engine running, > 14V
    Engine stopped, < 13V

    I have a circuit on my other computer using a TL431 driving a PFET as a high-side switch which would do this. I can post it later today...
     
    #12 likes this.
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