Hey everyone, I'm looking to put in a blinking LED light to use as a fake alarm. I know what I need to make it and everything but I just have a question about where to wire it. Ive read on here taht the ignition wire becomes a ground when the truck is off but Id rather not touch that unless I have to...Can the switched power (red one i think) that goes to the radio be used to do the same? Does it become a ground when the truck is off? if not does anyone know of another option or am i going to have to use the ignition wire. Thanks in advanced

 

Every load is an effective ground for a low-current device (like a single LED) when the power is off. Even a brake light bulb's "hot" side can be used as a ground. This applies until the parasitic load (the LED in this case) begins to approach the donor circuit's normal draw - like maybe around 10%. Then the circuit becomes a "voltage divider", and that's NOT an effective ground.

 

alright so you're basically saying any "hot" wire while the truck is on will be a good enough ground for the LED to turn on only when the vehicle is off...If I understood that right then perfect, thanks for the help.

 

if you use a hot wire that is only hot when the truck in running then the light will only be on when the truck is running...I dont thing that is what you are looking for. Tap off your power from your cigarette lighter (if you are running a full 12v LED, otherwise you will have to pick an input of proper voltage rating.

Also, all LEDs are polarity sensitive, make sure you pay attention to the wiring that you have going to anode and cathode.

-dan

 

Easy way around this, use a automotive 5 pin relay.

Pin # 30 : Power in ( from the cigar lighter is fine for an LED ).
Pin #87A : Power to the LED ( the side of the LED that is round )
Pin #85 : Any hot in run position ( you can use the radio ckt for this )
Pin # 86 : Ground.

Ground the lead from the LED that has the flat side on it ( this is the same as the side of the diode that has the line on it )

Pin 87A is the Normally Closed contact output on the relay, so when the coil has power applied to it ( pin # 30 ) the contacts are opened, and held open as long as power is applied to the coil.

This way you are not trying to derive ground via another circuit.

This is a bit involved vs. the hook up 2 wires method above ( which might work fine ), but will work 100%, without any concerns.

Watch your LED choice, that it is 12 V or you will need to add a resistor inline with it, to make up for a lower operating voltage.

 

Dan I think I may have confused you...I meant that I would be pulling the power from somewhere that is constant, like the cigarrette lighter, then using a ground that is hot while the truck is on and not while it is off, which I believe would show ground and in turn complete the circuit for the LED. I may be way off on this though, thats just what I thought I understood. Anyways Ill prob just go with the relay, thanks guys

 

Yes, as long as the wire is ALWAYS hot with the truck ON. The brake light hot wire (for example) doesn't qualify - it's only hot when the brake pedal is pressed.