Bi-color LED lightbar control from hi/low beam

Thread Starter

TimbrSS

Joined Feb 4, 2010
2
I'm looking to hook up a bi-color LED lightbar to my headlights. Desired function is High beam turns on yellow LED, Low beam is white LED. The light bar has 3 wires. Red for power, then white wire turns on yellow LED when grounded. Black wire turns on White LED when grounded. I want a plug-n-play solution, so don't want to add another power wire to the system... so I want the relay board powered by whichever light is on (hi or lo). The low beam and high beam are never on at the same time.

I'm trying to figure out the best way to do this. For packaging in a small box, I'm Currently thinking using a "DC 12V 2 Channel Relay Module with Isolated Optocoupler" off ebay.
The relay will complete the ground circuit when activated.

Just trying to figure out the best way to do the power for the relay module, which will also power the light bar. Do I just run a diode from IN1 and IN2 to the DC+ on the module to supply it with power? Do I need a capacitor from DC+ to DC- to power the board as I switch from high to low beam?

Is there a better way to do this that I am not thinking of.
 

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ElectricSpidey

Joined Dec 2, 2017
1,882
Are you sure you can take the power needed to run the add on LEDs from the existing high/low beam system?

If so it looks like what you have drawn here could work. (assuming high side switching for the beams)

The cap is probably optional, and the diodes must be rated to handle the current.

But, I would recommend using a separate power line with a fuse to power the LEDs and the relay board.
 

Tonyr1084

Joined Sep 24, 2015
6,365
You don't need the diodes from the headlights, you just need one from the high and one from the low. For that fact, you may need just one from one of the high beams. If you turn the lights on (low beam) the LED bar comes on with white connected to ground through the relay's NC/C (Normal Closed / Common) contacts. When you switch to high beams power from the beams switches over the relay from NC/C to NO/C (Normal Open / Common). This will disable the white section of the LED's and activate the yellow LED's.

Give me 22 minutes and I'll bang out a picture for you.
 
Last edited:

Tonyr1084

Joined Sep 24, 2015
6,365
OK, using just one of the headlights (the high beam) connected to the relay coil you can switch from yellow to white by turning on the high beams. The yellow LED's will be disconnected from ground and the white LED's will activate. When you switch back to low beams the relay will drop out and switch the light bar back to yellow.

I've not drawn a switch for the light bar connected to the power because I would recommend you have an independent switch for the light bar. That way you can turn it off if you're in an area where the law does not permit such lighting accessories (a.k.a "Unauthorized Use of Lighting" - which can get you a ticket). Nevertheless, you can wire it however you choose. IF you want it to light up when the headlights come on then you'll need a second relay. I can modify the drawing easily enough to accommodate that if needed.
1629809980028.png
 

Tonyr1084

Joined Sep 24, 2015
6,365
OK, modified drawing: (Note: Using blue wire to represent your White wire)
1629810780029.png1629810805227.png
This configuration will turn the light bar on whenever you turn on the headlights. Low beams turns on the white LED's and high beams turns on the yellow LED's. When the lights are off then so is the light bar.

You don't need to run wires from both high beams and from both low beams. No need for diodes at all. Just two SPDT relays. In fact, you can probably go to the junk yard and salvage a couple relays. Fan control relays are robust enough for the task. They will likely not have the NC contacts to connect to. But you don't need the NC contact. If you want to spend a few bucks, any automotive store will have the necessary relays.
 

Thread Starter

TimbrSS

Joined Feb 4, 2010
2
Thanks for the responses.

I guess I wasn't clear enough on the use. The light bars are actually replacing the headlights (we'll save the legal discussion for another time). I was looking to make a plug and play solution, so I could just power everything through whichever light (hi/lo) is on. I did not want to run another power wire from battery to each light bar. They are pop up headlights (Corvette) and I think for some reason the control module needs to see a load on the active headlight for it all to work correctly.

I already have the relay modules that are pictured in the my first post. Just waiting on amazon for some connectors and diodes to give it a go.

In my mind, the diodes are necessary in my drawing. When the low beam is on, it is powering the relay module, providing power to the light bar, and activating relay 1 so that the black wire is connected to ground, turning on white LED. The diode is needed to keep the trigger from relay 2 from seeing the low beam voltage.

Then when the high beam is activated, Relay 1 turns off, Relay 2 turns on, grounding the white wire, causing the LED bar to be Yellow. The high beam is providing power to the board, the LED, and the Diode keeps relay 1 trigger from seeing the voltage from high beam.
 

Tonyr1084

Joined Sep 24, 2015
6,365
Whole new ballgame.

First I'm going to make an assumption - that the low beams are still on even when you switch on the high beams. Disregarding the light bars - is this a problem? I know when I turn on MY high beams having the low beams remain on aids in throwing light out in front of the car.

You want yellow light (low beams) and white light (high beams). What's the difference? Oncoming traffic will be blinded either way.

Since you want the low beam (yellow) to be deactivated, you only need one relay (or relay module). When you turn the lights on - the low beam will be commonly the first to light up. When you switch on the high beams that can activate the relay (single relay) and shut off the low and switch on the high (white). But changing the color of the light - unless you're doing this for purely show purposes and NOT for use on the roadway - will not make any difference in lighting up the road ahead. Low beams have a shade internal to the light or lamp assembly. Switching the high beams on means you are now adding additional light without the benefit of the shade. The shade is intended to NOT blind oncoming traffic.

I know you want to ignore the legal part of it - but it's unavoidable. The police WILL make you restore normal headlight operation. I've seen guys put HID lights on in the wrong type of lamp assembly. When they drive down the road, even with "Low Beams", they're blinding oncoming traffic. Without the benefit of the shade inside the lamp or assembly you're casting light ahead - up and down. The shade prevents the "up".

Whether single lamp style or dual, there is a system in place to control the direction of the beams being cast forward. Two points here - first, headlights cast a beam directed down toward the ground and ahead of the vehicle while high beams are intended for casting as much light forward as possible. Second, having a light bar will simply cast light forward but not have the benefit of being a controllable beam. It's effectiveness will likely be worse than standard incandescent headlights. Now, if your light bar casts that much light so as to be MORE effective - you're going to be getting tickets left and right.

I can answer your question as to HOW to accomplish what you want to do - but I'm not going to participate in this discussion any further because of the legality of what you want to do. Even if you claim you want this for show only.
 
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