Simple 12V timer for a newby

Discussion in 'The Projects Forum' started by BigBlueCar, Sep 23, 2013.

  1. BigBlueCar

    Thread Starter New Member

    Sep 23, 2013
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    0
    Please help a simple newby with a simple circuit. Be gentle with me.

    I used to build lots of stuff many, many years ago so I pulled out the books, the patchboard and a load of components to prototype it. Unfortunately, I'm not as smart as I used to be so I'm floundering.

    I need to build a 12V timer circuit for my car to hold a relay on for about 5 mins when the ignition is switched off. Adjustable using a pot would be a bonus. I also need something low power so that I don't get battery drain from the quiescent current.

    Here's how I see it........... When the ignition is switched on it will trigger a relay to power my bluetooth kit and a dash cam. The relay stays held on as long as the ignition supply is on. So far, so good. When the ignition is switched off, I want the timer to hold the relay on for a further timed period and then drop it.

    If the 12V ignition supply is resumed part way through timing, it will hold the relay and the timing will be cancelled, stating again the next time the 12V supply is dropped.

    I'd like to do this without any relay bounce as the 12V+ is dropped and the timer takes over.

    I've found loads of 555 circuits but they all need a -ve pulse to trigger rather than a permament loss of the 12V+ ignition supply.

    How can I do it?

    Cheers.
     
  2. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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  3. BigBlueCar

    Thread Starter New Member

    Sep 23, 2013
    2
    0
    Yeah, thanks for that.

    It's not an automotive modification, it's just a timer for a dash cam which would have been plugged into a 12V outlet and 500mA fused. I'm hardly looking to completely take over the entire CANbus system or operate the car from my mobile phone.

    But rules are rules. It makes me smile when people are asking some really wild and out-there questions at mains voltages and big old currents.


    Guess I'll just keep bodging until I find something that works. Good job 555s are cheap - I'll get through a few before I'm finished.
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,151
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    I could be wrong but I thought anything plugged into the accessory outlet was fair game here, since it is not modifying the vehicle. Well maybe not anything.

    My van has two outlets, one switched by the ignition and one not. It's really handy and would make your project simple. Is your outlet switched or unswitched? If it's on the ignition switch, then you'll need a separate source of battery power to give you your delay, and getting at that might put this project outside the terms of service.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,564
    2,379
    What you could use and I could not see it violating forum rules? Is on many auto's now for a while, the Cigar lighter and auxiliary socket are on when the ignition is on.
    If you have this feature, and plug into this, then it is an auxiliary and not modifying any factory wiring?
    Max.
     
  6. wayneh

    Expert

    Sep 9, 2010
    12,151
    3,058
    Max, did you miss this?

    In my van, even the switched outlets (and the radio, window motors, etc.) remain hot for a minute or so even after the key is removed. They all go off immediately when a door is opened. I think the OP is wanting to do something similar.
     
  7. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    If the cigar lighter outlet goes off when the ignition goes off, a circuit could be built that was within the rules.
     
  8. wayneh

    Expert

    Sep 9, 2010
    12,151
    3,058
    How do we get +12V once the outlet goes dead, while staying within the rules? Getting the 12V is easy - it's the rules that are the challenge.
     
  9. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,871
    1,394
    It may be more trouble than the OP wants to go to, but a battery pack is a "legal" option. I can't imagine that a dash cam would draw much power in only 5 minutes. I would suggest a MOSFET instead of a relay.

    But, I see your point, and the OP may be long gone anyway. :)
     
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