Any auto electricians on here?

[Garth]

I need to wire something up with a relay and wanted some advice before I start...

I currently have something fed with a live from the light switch so that it varies the voltage with the interior light dimmer. Now I want to continue using this but also introduce a relay so that when the lights are off, I get a 12v supply, but when the lights are on, I want the variable voltage I currently use.

This is a pretty simple circuit and I can find the necessary feeds, I just need to know what kind of realy will be suitable for this and any advice on wiring it up if necessary.

Thanks 

[cameluk]

A simple SPDT relay should do

connect the NC pin to 12V
connect the NO pin to the dimmed voltage
connect the common pin to the device you want controlled
connect one side of the coil to ground, and the other side to 12V controlled from the light switch

Maplin do a suitable relay N00AW

[Garth]

Nice one, thanks mate you're a star 

[Garth]

Camel,

I'm afraid I'm being thick. I've got the relay now, but your instructions don't help much when I don't know which pin is which  

Can you tell me which pin connects to what as per your description before please? I've attached a photo of the relay showing the circuit diagram. The pins are just numbered.

Thanks a bunch  

[Garth]

I reckon I've worked it out:

85 : Ground
86 : Ignition switched live
87 : Dimmed 12V
87a: 12V
30 : Load

Correct?

[cameluk]

Spot on   

[Garth]

Yessss! I'm not an idiot after all