Help wiring some electronics.

Lankysean

So, occasionally I do a little work wiring LEDs and such into peoples cars because it's easy and it's a good way to make some fast cash. Anyway, I have this one customer who loves unique and new ideas and we came up with an idea for the back of his car but I'm having a little trouble getting the wiring together. I'm pretty sure all I need is a relay, but I'm a little new at the relay game so I was wondering if someone here can help me out.

Basically I have a 2 color LED, it has two positives (one for each color) and a common ground. I need to take 2 different positives from the car (brake light and running lights) and connect them to the LED but never have them both hot at the same time. I did something similar but it was ok to have them both on at the same time because it was just for brightness not for color.

If anyone has any ideas I'll be happy to hear them. Thanks!

proXimity

So basically you want the light to be on with the running light and then change color when the brakes are applied?

Lankysean

proXimity wrote:

So basically you want the light to be on with the running light and then change color when the brakes are applied?

exactly that.

Daenris

A pnp transistor should work for this. Have the brake light wire go to the base of the transistor as well as to the led. Then have the running light wire run through the transistor and to the led. When the brake lights are off, the transistor should allow current to flow, causing the running light LED to be illuminated. Then when the brake lights come on, the transistor should no longer allow current to flow, so the running light LED will turn off, but the brake LED will be on.

Lankysean

http://www.amazon.com/Fairchild-Semiconductor-General-Thru-Hole-Transistor/dp/B002N6R6LU/ref=sr_1_1?ie=UTF8&qid=1319983377&sr=8-1

You mean one of these? How would you go about wiring this?

Daenris

Yeah, but if these are high power LEDs or something, make sure you get a transistor that can handle whatever current you're putting through the LEDs.

I'm pretty amateur in terms of circuits, but I believe something like this should do it:


So, you send the Brake signal to the base of the transistor as well as to your Brake LED. Then you run the Running signal to the transistor input, and run the output to the LED. The transistor should only allow current to flow if the Brake signal is NOT active.

Lankysean

I see, but that would still allow for both currents to reach the LED if the break is activated right? See, I need either or, but never both at the same time.

proyebat

What is pictured is a PNP transistor (versus an NPN transistor), which means that it defaults to a closed circuit between the Base and Emitter (Running light on) when the brake circuit has no electricity flowing to the brake light. When the brake light is on, the electricity running through that part of the circuit turns off the transistor, turning off the running light, which is what you want.

An NPN transistor defaults to off, which changes the circuit so that both lights are on at once, or none at all. Not what you would want.

Lankysean

Awesome, thanks!! I'll order a PNP transistor today and see what happens when it comes in, thanks again!!!