# 12V LED indicators

#### eduncan911

Joined Nov 14, 2011
29
Hello:

I feel outclassed by posting such a simple project here.

I am currently debugging some MOI boards in Dell systems, and need to monitor when a few circuits are activated and deactivated. They are in a hard-to-reach place, so I would like to wire in parallel a few low-amp LEDs to indicate when the circuits have been activated.

The circuits are 12VDC, and draw about 2.5A each. Therefore, a couple of low-powered LEDs would not affect the circuits' power draw at all.

This would be a throw-away, one time use. So I'm looking to build it on the cheap. I'm assuming the following parts:

* Low volt LED (could you recommend some specific ones, Radio Shack perhaps?)
* A resistor to place inline to lower 12V down to ~3V, or whatever the LED needs.

Thanks again!

#### tracecom

Joined Apr 16, 2010
3,944
* Low volt LED (could you recommend some specific ones, Radio Shack perhaps?)
* A resistor to place inline to lower 12V down to ~3V, or whatever the LED needs.

Thanks again!
With a 12V supply, a 680Ω 1/4w resistor would be suitable for any LED with voltage drop from 2V to 4V (which includes pretty much all common LED's.) Here are some assemblies I built using 1k resistors; they work fine at 12V, just not as bright.

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#### mcgyvr

Joined Oct 15, 2009
5,394
An LED is a current driven device.. Most require 20mA. The resistor in series is what limits the current. To find the value of the resistor you take the input voltage-forward voltage of LED divided by the current (in ohms)
So assume a 20mA LED with a forward voltage of 3V. Math is as follows.
(12-3)/.02 = 450 ohms (pick a resistor very close to that..larger is better)

For the resistor wattage its input voltage times current
So 12 * .02 = .24 watts..You should always double the wattage and pick that rating so you want a 1/2Watt resistor around 450 ohms.