I need to replace my 12VDC (bulb based) brake lights but I am unable to find a three-function light (Running/Brake/Off) with the same specs. I did however find a two-function light LED fixture (ON/OFF) with the exact same dimension.

Similar to these:

Brake Light Example

I want to install the two-function LED based light brake in my car but using it as a three-function light.

I thought of building a small circuit that gives the light fixture max 50% on each wire using a resistor and adding a potentiometer for configurable dimming.

So when the car is running the LED light gets max 6V via wire1, and when I brake, it get max 12V via both wire1 + wire2.

Does this approach make sense? Can you suggest anything else?

If it does, I have no idea how to build that circuit starting from my three wires, as simple as it sounds.


1 Answer 1


I am going to assume you currently have three wires going to your old brake light, ground, Running, and Brake:

Running    Brake    Lamp

   0         0       off
  12v        0       dim
   0        12v     bright
  12v       12v     bright

The following circuit should accomplish what you are looking for using one LED:

enter image description here

When neither RUNNING or BRAKE are on, the LED is off. When RUNNING is on, but BRAKE is off, 12v minus a diode drop for D1 goes through the pot R1 to reduce the voltage to the LED fixture. You'll probably end up with a value of only 10 to 20 Ω, but I suggest starting with a larger value pot just in case. Because of diode D2, there is no feedback into the BRAKE circuit.

When BRAKE is on, regardless of the condition of RUNNING, 12v minus a diode drop for D2 goes directly to the LED fixture. Because of diode D1, there is no feedback into the RUNNING circuit.

  • \$\begingroup\$ Sorry for not being specific. It is a LED sealed fixture (somthing like this i879.photobucket.com/albums/ab358/autonetsperformance/…) I bought off the shelf, so no specs on it whatsoever. \$\endgroup\$ Jun 21, 2014 at 18:20
  • \$\begingroup\$ Sorry for being impolite btw; thanks for your help! I understand much better what I should do. I just need to find the proper resistance values. I will buy a 500ohn variable resistor this way I have more flexibility. \$\endgroup\$ Jun 21, 2014 at 18:28
  • \$\begingroup\$ I did my calculations think you were talking about one LED, looking back I see you did say LED fixture, which I assume runs directly off of 12v. I'll revise my circuit. \$\endgroup\$
    – tcrosley
    Jun 21, 2014 at 18:36
  • \$\begingroup\$ @tcrosley Most of these things seem to be <1A so 1N4004 or 1N5404 diodes should do the trick. \$\endgroup\$ Jun 21, 2014 at 18:39
  • \$\begingroup\$ Thanks for your reply tcrosley. Two questions: 1. Is D2 really needed? I just thought of connecting the brake line straight to the fixture and it seems to work. 2. What is the section (the loop) on the right of R1? \$\endgroup\$ Jun 24, 2014 at 20:51

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