12V LED indicators

Discussion in 'The Projects Forum' started by eduncan911, Feb 13, 2012.

  1. eduncan911

    Thread Starter Member

    Nov 14, 2011

    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!
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
    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.

    Last edited: Feb 13, 2012
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  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    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.
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