Controlling car lights with minimum number of relays

Thread Starter


Joined Dec 27, 2021
I have an old German car that has “city lights” incorporated into the H4 headlights. These use a 5W bulb, so it doesn’t do more than light up the shape of the headlight enclosure when the “parking lights” are on. They remain on when the headlights are on.

I would like to replace the standard bulb with a brighter (10-20W equivalent) yellow colored LED to give them a more pronounced glow, like European yellow headlights. This is purely for fun and not meant to provide illumination. In fact, I want them to turn off whenever the headlights are on.

I assume relays are the obvious answer. Four wires enter the headlight bucket (12V for the city light, low beam, and high beam, and ground).
City Lights.jpg
I saw someone use two NC relays such that the city light ran through the primary circuit of both relays (terminals 30-87a). The positive side of one coil (86) tapped into the low beam 12V connector while the other attached to the high beam wire. As long as the headlights were off, both coils were deactivated, and the city light bulb turned on. Turn the headlights on, and one of the relay coils would activate, breaking the city light circuit.

I would like to eliminate one of the relays. Obviously, if I hooked both wires from the headlight to the relay (86), the two connected wires would create a circuit powering both the low beam and high beam at the same time. Not good. But it seems to me that I could add a diode to each wire before feeding them into the relay, thus preventing power from one beam feeding the other.

Am I on the right track here?


Joined Mar 14, 2008
Yes, one diode in series with the high-beam signal and one with the low-beam signal would allow either to activate the relay coil while isolating the two signals.