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Discuss Relay controlled by power to an LED in the Auto Electrician Forum area at

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  1. Southdowns

    Southdowns New EF Member

    Hi, I have a 12V output from a control box that lights an LED. I want to use that output to switch power to a camera, but don’t want to lose the LED. The LED has a resistor in series.

    My first thought was to just put the camera in parallel with the LED, but then thought a relay with the coil in parallel WITH THE LED might be better.

    Is this feasible, and if so do I even need the relay? Do I need to do anything else?


    • Funny Funny x 1
  2. Spoon

    Spoon Forum Mentor

    Welcome to the forum mate.

    Is this the type of LED you are talking about? A small one...

    If so, then the resistor is to limit the current going through the LED.

    I'm not quite sure what you are trying to do?
    What is the control box?
    You want to use the signal from the LED to power a relay, and then you have a separate supply for the cameras?
    Will the circuit have enough power to drive the LED and the relay?
    What relay are you thinking of using?
    Do you have any pics?
    • Agree Agree x 2
  3. Southdowns

    Southdowns New EF Member

    Hi Spoon. Yes it’s a small LED in a switch.

    So the control box is a parking sensor control unit. It outputs 12V to the LED whenever the parking sensors are activated, below 10mph or when in reverse. I have two cameras, a reverse one that I’ll power from the reversing lights and connect to input one on the monitor. The second camera will be at the front and connected to input two on the monitor, and I want it to receive power whenever the LEd is lit. The monitor will take care of switching between the two cameras - camera one gets priority if in reverse, camera two shows if the LED is lit AND the car is NOT in reverse.

    I hope that makes sense. The relay is a simple automotive SPDT one.
  4. littlespark

    littlespark Electrician's Arms

    Scottish Borders
    You might want to check the voltage at your reversing light.
    It depends on the vehicle, but mine has a big flag sticker on the wiring loom that says not to connect anything (like relays) to the lamp input wires.

    I’ve had to wire my reverse camera from the 12v socket. It’s powered all the time when I’m driving. I may add a switch but it won’t be linked to the reversing light
  5. Strima

    Strima Electrician's Arms

    St Neots
  6. Shoei

    Shoei Regular EF Member

    You'll need a relay that will operate from a small signal to avoid overloading the control box, there are plenty to choose from, mainly for Arduino, Raspberry Pi and the like.

    The 1 channel 12V board here on eBay for example.

    The input signal drives the LED inside an optocoupler and only needs a few mA. Power for the relay comes from the 12V supply.
  7. Southdowns

    Southdowns New EF Member

    Thanks everyone. Strima, the power magic looks good, but doesn’t give me the control options I want (front camera only only when the parking sensors are activated), unfortunately.

    Shoei, that looks ideal, cheers :)
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