Wiring 2 electrical inputs to one source

[Stel 10+ year member]

Depending on the vehicle, the door lights are activated by the computer not a direct switch. Apply 12v where it wasn't meant to be and the computer gets a bit upset. A diode could keep it from being a problem...

That is something to watch out for, most cars go with the cheapest method which is a simple switch on each door. But if IC's are involved a diode should be also be placed in series between the new 12v + feed and the path back to the integrated circuit to prevent current flowing in that direction. Or use a relay, I like avoiding relays unless needed since they are mechanical devices and more likely to fail at some point, and it's a bit more wiring. Though with the currents involved, it may be easier in this case just to go with some relays.

Or depending on the car you could probably tap directly into the domelight switch which would allow them to turn on with the door open or whenever you turn the domelight on with the switch. But I would definitely use a relay as opposed to running 8 leds directly off a door trigger.

I think he already has a switch installed, a fog light switch and I assume he wants independent control from the dome light.

 

[nismos14 5,000+ posts]

Any diagram on how to wire the relays in parallel? Is it just jumping the power wire off one input and to the other input as well? And a standard bosch relay should do the trick right?

 

[Stel 10+ year member]

I would use a "single pole double throw" type. Here's a diagram (the one labeled SPDT):

http://en.wikipedia.org/wiki/Relay#Pole_.26_Throw

NOTE: for the A terminal I mean the one that is normally closed (completing the circuit) when no power is supplied to the coil. You can use a multimeter to be sure which is which.

Connect the existing +12v feed in the door to the A terminal and connect the light +12v input to the C terminal (this is the "common" terminal). Connect the fog switch (which is also +12v) to one of the coil terminals and to the B terminal. Connect the other coil terminal to ground. This way with the fog switch off and no power to the relay coil, the circuit behaves exactly as before, with the fog switch on the coil turns on and opens the old circuit and closes the new circuit which turns on the light.

 

[nismos14 5,000+ posts]

So the existing door switch would still turn on the lights using ^ approach?

 

[Stel 10+ year member]

So the existing door switch would still turn on the lights using ^ approach?

Yes, when the fog switch is off they should behave exactly how they did before.