Negative and positive switch circuit possible?

Discussion in 'Automotive Electronics' started by Ramathorn15, Sep 10, 2017.

  1. Ramathorn15

    Thread Starter New Member

    Sep 10, 2017
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    Hey all!

    I'm new here but think I will likely be back, often.

    Here is my current situation I could really use a hand with.

    I am trying to make an aux led light that is switched on with the dome light of my Toyota but can also be switched on independent of the dome. The dome light is switched via the ground and the positive is always hot. I know I can add a relay using the ground as the relay switch for the light, then run a manual switch to complete the ground circuit independent of the dome. The issue with ground switching is only certain switches with led indicators will work when ground switching. I'd like to be able to use any switch and have it's led indicator work.

    I'd like to use a single 12v relay to make this work, but I don't believe it's possible. I have a very basic understanding of electronics (relay, diodes, resistors, etc.) But not enough to wrap my head around this one.

    Also, the dome lights fade on and off. I'm sure that it's a voltage increase, then drop when turning off? Would a relay that triggers at 6vDC expect to cause the same fading on and off when hooked to an aux light?

    Thanks for any help! Hopefully that makes sense.
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    That is almost certainly done by pulse width modulation (PWM) under the control of a microprocessor. Adding external controls to the light without risking interfering with the PWM or damaging the microcontroller would require knowledge of the existing circuit/control arrangement.
    Welcome to AAC!
     
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  3. Ramathorn15

    Thread Starter New Member

    Sep 10, 2017
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    Does anyone see an issue with this setup?

    1- 5 pin relay powered from battery and triggered by tapping into the ground switch wire from the dome.

    1- 5 pin relay powered from battery and triggered by a regular switch with a 12v supply.

    The end supply of both relays going to the same aux light.

    In that scenario, the aux light should turn on with the dome, but also be able to be turned on independent of the dome. I may also put a diode between the first relay and the negative trigger tap from the dome to hopefully protect the original circuitry.

    How's that sound?
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Risky, if you intend to apply a fixed voltage (12V) to the light when it is being driven by a PWM source of unknown configuration.
     
  5. Ramathorn15

    Thread Starter New Member

    Sep 10, 2017
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    Thank you.

    Wouldn't adding a diode between the relay and the PMW source protect it?
     
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Do you know if the PWM control operates as a high-side switch or a low-side switch? That would determine how you would need to connect a diode.
     
  7. Ramathorn15

    Thread Starter New Member

    Sep 10, 2017
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    Unfortunately I don't and I have no idea how to tell. Is it something relatively easy to determine? All I really have physical access to is the positive and negative wires off of the dome light.
     
  8. Ramathorn15

    Thread Starter New Member

    Sep 10, 2017
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    Thank you for your help by the way.

    The idea was that I would just use a wire tap off of the (-) for the factory dome light to diode then to the first relay. Wasn't sure if that was initially clear.
     
  9. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    That would be risky if the PWM is switching the low side.
     
  10. Tonyr1084

    Well-Known Member

    Sep 24, 2015
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    Wife's 89 Celica, may it rest in pieces, had a dimmer for the dash lights. It is PWM (I still have it). The way it's wired is the light is powered and when the lights are on the PWM dimmer would connect to ground. This is not a smart system. That is not to say it was dumb of someone to design it this way, I'm saying it isn't programmed to react in any certain manor except to take an input from a driver as to what brightness to set the lights at. Current path is: Battery to light switch. Light switch to dash lights. Dash lights to PWM dimmer. PWM dimmer to ground. The level of brightness is controlled by the user.

    Other systems that have been employed use ground as the hard connection point for the dash lights and the brightness controls how much current goes to the dash lights. So that's the difference. One system controls the incoming power TO the lights, the other controls the outgoing power FROM the lights. That's what AlecT was asking in post #6.

    Somewhere in your dome light circuit is a control. Likely a micro controller. Simply adding ground to the lights may not switch the lights on. I know you said there's power to the lights, so it sounds like you may have a low side switching setup. Grounding across the PWM might or might not cause issues. To be honest, you really need the complete circuit diagram before you try this. You COULD end up spending unnecessary money.
     
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