switching 12v circuit with only 7v trigger

Discussion in 'The Projects Forum' started by pontiff, Nov 11, 2014.

  1. pontiff

    Thread Starter New Member

    Nov 11, 2014
    2
    0
    Hi all,

    Forgive me if I am not explaining things very well but i have a problem i was hoping someone could point me in the right direction for...

    I have a car stereo which for the auto dimming to work properly, needs 12v to trigger it to change from day to night mode. For some reason (I am trying not to get bogged down in the 'why' of) the illumination circuit of the car is only producing a steady 7v. I am not sure if this is normal or not, but when I get time I will check out another car and see if it is common or not.

    Sooooo What I am trying to achieve is using this 7v 'trigger' voltage to switch a 12v circuit to the stereo so it will go from day to night mode.

    I had tried to wire in a generic automotive relay, but the 7v was not enough to close the coil it just made a 'clicking / humming' noise so i suspect its trigger/coil voltage was too low.

    What options are there available use this 7v to switch a 12v circuit to the illumination of head unit?

    I should mention that the 12v in question that I was going to use was the ignition wire that runs to the head unit which from what I read from VW does not carry much current at all, it in itself is merely a trigger to tell the headunit to actually turn on.

    Any guidance would be greatly appreciated.

    Cheers :)
     
  2. ScottWang

    Moderator

    Aug 23, 2012
    4,853
    767
    If you using 7V can't active to change the mode then you need a dc to dc converter, as mc34063 converting from 7V to 12V.
     
  3. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    1,438
    368
    I think that the 7V wire is probably controlled from the dimmer and carries a PWM signal, hence, the buzzing relay.

    Find a device on the car that has an non-dimmable 12V lighting feed and use that instead.
     
  4. ScottWang

    Moderator

    Aug 23, 2012
    4,853
    767
    If the 7V is a pwm signal, maybe you can using a 2 diodes and 2 caps double voltage circuit to get the voltage around 12V.
     
  5. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
    1,190
    156
    Please watch the thin line you walk cause this forum doesn't allow automotive mods here as per the forum rules ... Most people leave the day and night off or tie it to the headlight switch or use the factory wiring .. What color wire are you talking about ?? Cause there should be a yellow wire for constant power and a red wire for ignition on power and the blue wire should be for illumination ???
     
  6. pontiff

    Thread Starter New Member

    Nov 11, 2014
    2
    0
    apologies I did not realise, i wont ask anymore, i will try the suggestions above thank-you.
     
  7. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
    1,190
    156
    No your fine just giving a heads up boss... If you have any more questions please ask...
     
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