I already have a 12v flashing led that I intend to put into my car as dummy immobilisor warning LED. What I would also like to do is wire it up so that the LED only flashes when the cars ignition is off. ie. it stops flashing once the car has started.
My initial thought is to use a basic 12v relay with the switched igniton live supply energising the coil of the relay. With the flashing LED being fed via a "normally closed" contact of that relay, in my simply theory once the relay is energised by switching on the cars ignition the contact should open and hence stop the led flashing. My question to you educated folk is, will my idea work correctly, and if so what relay would be the best option, (I wouldnt know where to start). And is there a simpler way of acheiving the same result as in not requiring a relay etc etc.

 

A relay would work fine the way you have described.

 

I already have a 12v flashing led that I intend to put into my car as dummy immobilisor warning LED. What I would also like to do is wire it up so that the LED only flashes when the cars ignition is off. ie. it stops flashing once the car has started.
My initial thought is to use a basic 12v relay with the switched igniton live supply energising the coil of the relay. With the flashing LED being fed via a "normally closed" contact of that relay, in my simply theory once the relay is energised by switching on the cars ignition the contact should open and hence stop the led flashing. My question to you educated folk is, will my idea work correctly, and if so what relay would be the best option, (I wouldnt know where to start). And is there a simpler way of acheiving the same result as in not requiring a relay etc etc.

To find a relay, Google "12v Automotive relay". You'll get plenty of hits.

 

And is there a simpler way of acheiving the same result as in not requiring a relay etc etc.

What are you using to flash the LED?

 

A flashing LED that looks home made will only invite people to break in. Career criminals KNOW which systems to avoid. Home made junk is often the easiest way to defeat a theft deterrent device. It's like standing in a cage of hungry lions with a sign that says "I'm scared". You're going to get eaten up.

 

This replicates an existing genuine immobilisor in our other car, which has a similar flashing led, but connected to a thatcham cat 2 unit that immobilises three components. The dummy one is also being incorpprated with a hidden kill switch ;-)

 

My favorite anti-theft devices are an actual alarm, but instead of playing a siren it plays a blood curdling scream; and a removable gas pedal. How far are you going to get without a gas pedal? Talk about a low speed pursuit with some woman screaming bloody murder as you idle down the street!

 

There is an easier way to wire the LED if you have access to a Schematic of your cars wiring,
or, you are very familiar with Automotive Electrical Wiring.
Simply wire the LED to the Ignition Switch, one side where the Battery Power comes to the Switch,
and the other side to any Load that is disconnected by the Switch.
The LED will be grounded by the Load that the Ignition-Switch disconnects.
You can do this at your Fuse Block as well.
Connect the LED between any circuit (fuse) that is Hot all the time,
and any circuit (fuse) that is switched by the Ignition Switch.
Be sure to include 2 ~1-Amp Fuses to protect your very small LED wiring,
ONE FUSE FOR EACH WIRE, BOTH WIRES ARE HOT,
so that you don't burn your car to the ground when that small wire gets damaged.

This can be done on any Car, they all work on the same principles, and does not require a Relay.
.
.

 

Post #8 is what I recommend, because it is by far the simplest and lowest power way to get exactly what you are asking for. And adding the fuses is a very wise idea because indeed both wires will have the full battery voltage present when the car is running. And if it is an older car with breaker poits there is a simple anti-drive away trick, which is to diisconnect the 12 volt feed to the cigarette lighter and instead connect the lighter across the points. Push the lighter in when you leave and the engine will not start, even when hot-wired. And nothing shows. And most punks will have no clue.

 

connect the lighter across the points.

What a great idea. Only, I haven't seen breaker points or inductive discharge ignition in years. Aside from my yard equipment. I still like the idea of a removable gas pedal and an alarm with the soundtrack from one of those old 50's or 60's horror movies where the woman is screaming bloody murder.