Converting variable voltage to constant voltage for relay

Discussion in 'Automotive Electronics' started by David Spivey, Sep 11, 2015.

  1. David Spivey

    Thread Starter New Member

    Sep 11, 2015
    I am trying to install LED lights under my car, and have them automatically controlled. The LED controller will use about 8 amps. I want to signal a 10 amp relay so that when the dome lights or the headlights come on, the under car lights will come on too. The dome lights come on automatically when the car is unlocked, and go off 30 seconds afterwards. The headlights do not. However, the dome lights fade on and fade off via variable voltage. The voltage changes that happen during this cycle cause my relay to go crazy. I'm afraid I would damage the relay or the LED controller if I keep it like that. What should I buy in order to make the signal from the dome lights only be sent to the relay when the lights are full brightness?

    In essence, I want to ignore the voltage until it is at its peak, then allow it to flow to the signal of the relay. What does this? Is there a premade component that will help?

    Be careful with me; I am a novice at electronics.
  2. crutschow


    Mar 14, 2008
    That could be done with a comparator, such as an LM339, but a transistor circuit may be easier to build so a two transistor circuit is shown below (LTspice simulation).
    It switches the relay on at an input of about 7.5V and switches off at about 5.3V giving a switching hysteresis of 2.2V (as determined by the relative value of R2 and R3) to minimize any chance of the relay chatter you observed.
    In is the voltage from the dome light and V2 simulates that voltage.
    V1 is the car battery.

    The transistors can be just about any general purpose NPN and PNP transistors that have at least a 30V and 200mA rating.
    D1 is a general purpose diode.
    D2 is a 5.6V zener diode.
    Note that the relay contacts are not shown.

    Relay Driver.GIF
    Last edited: Sep 11, 2015
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
    would it would be easier to disable the dimming feature on the dome light?
  4. David Spivey

    Thread Starter New Member

    Sep 11, 2015
    Thanks, guys, for your recommendations. I appreciate your help.

    crutschow, Thanks for the schematic. Unfortunately, I didn't know what I was asking for. I have decided this problem can best be solved with a photocell circuit instead. I really don't want to tap into the existing power for the lights after all. Sorry to have caused you any trouble.

    GopherT, there is no feature to disable the fade in/out.