Does this kind of relay exist?

It is a simple job just to use another relay, the contacts in series with the hot wire to your fog/driving lights. This relay should be normally closed. Tie the solenoid of the relay to your headlight hot wire. When the headlights are on, the relay will open, shutting off your second set of lights.

You could do the job with other types of relays, semiconductor logic circuits, but this method is probably the easiest.

Yes there is.

Jeeps for one have a relay for the fog lights that has an extra pin to turn them off when the high beams are activated.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

eKennedy wrote:

Yes, if both sides of the coil have 12V the relay will shut off but in order to turn on, one of the sides needs ground.

This will never happen because while one of the inputs will be 0V, it will not be GROUND. What you're saying is true for digital logic circuits with a "pull-up" resistor, but not a relay coil.

While there may be special purpose relays that can actually do this, they are most likely specialized and not generally available at an auto parts store. Two generic automotive relays sounds like the way to go, probably cheaper too.

I don't see how the ground in the headlights would effect the ground for the relay switch.

Isn't it completely separate besides the fact that both hots (relay hot and switched hot) generally share the same voltage source? So the ground for the headlight bulb should not find it's way to being helpful in the relay's circuitry (the circuit that actually operates the switch in the relay).

From my experience with regular ol' relays, the polarity doesn't matter, but one side needs a hot and the other needs a ground for the switch to close. Just switching a hot lead to off doesn't make that lead suddenly a ground point.

If you hook one relay so that it closes the switch when the lights turn on (ie hook it to the wire that is hot only when the headlights are on). From there you can run a new (switched by the relay) wire from a fused hot from anywhere to power your DRL's. The relay just needs the cue from the headlight hot to know when to turn on the DRL's; it will hardly draw any current from the headlights. 2 would work as well, but is not necessary and would just cost about 2 times as much to configure.

Driving lamps are *not* suitable for use as DRLs, for several reasons:

1) They are *way* too bright -- typically 8 to 10 times brighter than allowed by even the very permissive US standard. This will create massive glare to other road users and will improve nobody's safety. 2) Their beam pattern is *way* too narrow -- that high brightness nosedives more than about 8 degrees to the left or right of the lamp axis. DRLs, in order to be effective, need to have a beam at least 30 degrees wide.

You'll be much better to pick a different set of lights.

Very simple to do with a normally-closed relay. In practice, these are sold as "changeover" relays -- you simply use the NC contacts for the DRLs. If you want, you can use the NO contacts for your headlamps, or alternatively you can take the power feed (terminal 30 of the relay) to something that has power only when the engine is running. The fuel pump circuit, for instance.

DS

Are you referring to fog lamps or aux low beams, such as Hella XL? 'Cos *driving lights* are high beam only lamps, and only for use as such: not for use with traffic in the vicinity ahead, and never to be operated with low beams or separately.

As if the existing low voltage high bum drls on our roads aren't trouble enough! :}

Personally I would use a second relay. Take the switched 12V that goes to a headlight and run that into the relay with the other side of the coil tied to ground. Then have the switched side of the new relay connected in series with the 12V feed to the DRL relay. That would be normally closed.

The lights I installed are Optilux (by Hella) 2500 H3 55w Halogen Driving Lights.

After reading all the opinions here, and really taking a look at how high they're aimed, and really how bright they are, I've given up on the idea of using them to serve as DRL's.

Thanks again, eKennedy

With the headlights OFF and the "DRL" relay energized, current flows from the relay's "ground" pin into the headlight feed circuit, THROUGH the filaments of the headlamps, and thence to ground. Since the relay coil impedance is much greater than the impedance of the headlamps, the filaments "look" like a direct path to ground to the relay. The current flowing through the headlamps won't be enough to heat them up at all. Thus the relay energizes. When you turn the headlamps ON, however, that line is fed by a low-impedance drive from the headlamp switch and thus floats up to the same supply voltage as the feed side of the "DRL" relay, current through the relay coil ceases to flow, and the "DRLs" go off.