Using a Transistor to Switch LEDs

Printed From: the12volt.com
Forum Name: Lights, Neon, LEDs, HIDs
Forum Discription: Under Car Lighting, Strobe Lights, Fog Lights, Headlights, HIDs, DRL, Tail Lights, Brake Lights, Dashboard Lights, WigWag, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=146452
Printed Date: May 01, 2025 at 11:04 AM

Topic: Using a Transistor to Switch LEDs

Posted By: zain5505
Subject: Using a Transistor to Switch LEDs
Date Posted: September 21, 2020 at 5:06 PM

Hey, new here but have exhausted all searches on this forum regarding this topic. I have a low voltage signal from a dimming dome light that I wanted to use to trigger a 12v relay and power some leds to be in sync with the dome/courtesy lights. The problem is that it does not have enough juice to trigger the relay, it just chatters. I've tried a couple different relays.

Instead, I picked up a TIP120 NPN transistor to use as a switch with the low voltage signal. Plan is to connect the collector to ground, base to resistor to signal wire, and emitter to ground of leds. Does this sound right? I also have no idea what resistor to use and need some advice on that.

Thanks in advance, this is totally new to me so I am learning as I go posted_image

Replies:

Posted By: i am an idiot
Date Posted: September 21, 2020 at 7:17 PM

Which voltage switches on the domelight? If it is the negative that switches, negative wire of domelight directly to the base of your transistor. No resistor needed.

Posted By: zain5505
Date Posted: September 21, 2020 at 7:26 PM

+ , It reads a little under 5 volts. Thanks!

Posted By: i am an idiot
Date Posted: September 21, 2020 at 9:43 PM

What is the year make and model of this vehicle?

Posted By: zain5505
Date Posted: September 22, 2020 at 6:16 AM

2016 dodge challenger

Posted By: zain5505
Date Posted: September 22, 2020 at 4:40 PM

i am an idiot wrote:

What is the year make and model of this vehicle?

Sorry, forgot to add your quote. 2016 Dodge Challenger. To be a little more specific, I tapped the positive lead for the courtesy LED in the footwell because it was easiest to get to and it functions with the dome light.

Posted By: i am an idiot
Date Posted: September 22, 2020 at 7:31 PM

You will need a pnp transistor. positive trigger wire from domelight to base. positive 12v to Collector. Emitter to positive of LED strip via a resistor. Start with 1000 ohm resistor.

What is the operating voltage and current requirement of the LED strip?