Hi All I am looking for some help with a project I've been working on. I have a adjustable shift light in my truck I would like to remove the 3v led and replace it with a 12v relay to control another circuit. I assume I need a transistor circuit but I don't have a clue what I need for parts.Any help or if someone could point me in a direction would be appreciated. Thanks Rod

 

Welcome to the Forum, Rod.
Is there a schematic of how that LED is wired?
Is the plan to use that single relay module seen at the bottom of the case?

 

Can you measure the voltage from each side of the LED to ground when it is both OFF and ON?

 

I dont have a schematic,the best I can do would be a picture of the circuit.originally I was going to use the single 3.5v relay but if I need to build a circuit to drive it. Could I just use one of the other 12v relays on the other board? Thanks for the reply!

 

I will measure the voltage to be sure but I thought is had 3.2v on either side

 

I will measure the voltage to be sure but I thought is had 3.2v on either side

Measured from ground?

 

Yes but I will need to confirm.I remember thinking that was odd to have voltage on both legs

 

If it's a conventional LED the voltage on one leg should be appx 3 volts higher or lower then the other when the LED is on.

 

Yes but I will need to confirm.I remember thinking that was odd to have voltage on both legs

It will be near the same when the LED is OFF, but not when it's ON.

 

Ok the voltage is the same on both legs when it's off 3.31v. When it's on the + side is 3.3v but the negative side is 0v

 

Here's a pic of the circuit I want to add the relay to

 

Could I just use one of the other 12v relays on the other board? Thanks for the reply!

I believe the negative side of the LED is a switched ground with can be used as the trigger source for an input on that 4 channel relay board.
Connect the negative side of the LED leg to an input on the relay module and set that input to L with the jumper.
Can probably leave the LED installed.

 

Here's a pic of the circuit I want to add the relay to

What is that module?
Can you post a link?

 

The module is from a 12v automotive shift light. Its triggered by engine rpm and I need it to trigger on a relay instead of turning the led on. I have removed the led already if that makes a difference.

 

Removing the LED shouldn't matter.
Is there anything else you need help with?

 

I would use an optocoupler and a relay.

 

I would use an optocoupler and a relay.

Below is an example circuit for that:

 

Since the LED is already current limited by the existing circuit you can reduce or eliminate R1.

 

Since the LED is already current limited by the existing circuit you can reduce or eliminate R1.

Likely, but I didn't want to take a chance on blowing the opto in case the removed LED had a built in resistor.

 

I'll try using the neg.side of the led for a trigger to see if that will work,my concern with that is the relay will be in the normally open position most of the time. The optocoupler circuit would be a possibility