Thread: Help needed to trace cable

What you can do is install a normally closed relay. Feed it from a 12v supply and controll it from a powered when on source.

theres no such beast in a standard disco1.

there is a pair of fuse terminals in the internal fuse panel that would be ideal to tap into to setup micks idea,

Im curious as to the intent of it.

Originally Posted by Mick_Marsh

What you can do is install a normally closed relay. Feed it from a 12v supply and controll it from a powered when on source.

Hi Mick,
Thanks for reply but you lost me. I can find any number of always on power sources but have no idea what a closed relay is, but I am sure I can source one from Jaycar or similar. Are you able to supply a simple wiring diagram.
Thanks.
Pete

What Mick is suggesting is a relay that when power is supplied opens a set of contacts IE power on switch opens no flow of power, when power is turned off relay goes to the unenergized state IE switch close and power the flows through the contacts. you will find relays marked NO or NC being "normally open" or "normally closed" in the energized state.
Hope this helps.
Sparky

Commonly known as changeover relays. When you apply power the moving contact disconnects the normally closed (NC) contact and engages the normally open (NO) contact instead. Use the relay to disconnect power from a battery feed wire when you turn on the ignition.

Example:

Britax Mini Relay RC 1230R 12volt 30 40A Changeover | eBay

In this listing they show a diagram, you'd connect 30 to an always-live supply (fused) and your chosen device to the 87a terminal. 85 from a switched ignition source and 86 to earth.

Here is the correct relay for the job from jaycar

here is how it works internally


Here is approximately how you want to utilise it (your load goes where the illuminated light is)

In your circumstance you would omit the switch in the line to earth unless you wanted an easy way to have the item power back up when the ignition is on.

I recommend using the appropriate size spade crimps and decent crimpers to ensure reliability,

Many thanks folks, I believe I will be able to manage that, I will give it a go. All I want to do is have a flashing red led operate when I switch off. The original one on the alarm system does not work and no one seems to be able to fix it. The alarm system works (been checked) but indicator light does not, I will therefore attach this instead. (Makes sense to me)

Originally Posted by Petetheprinta

Many thanks folks, I believe I will be able to manage that, I will give it a go. All I want to do is have a flashing red led operate when I switch off. The original one on the alarm system does not work and no one seems to be able to fix it. The alarm system works (been checked) but indicator light does not, I will therefore attach this instead. (Makes sense to me)

Doh, you should said that first! All you do is wire the flashing LED with the positive leg to a permanent battery supply and the negative leg to the ignition live wire. When the ignition is on there is no voltage difference and the light goes out. Easy peasy. Dunnit before.

I thought you wanted to conscientiously wire a DVD player that went out with driving, or something virtuous like that.