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I want to place a small LED on top of a police car. What size LED should i use and what OHM resistor? I have a 12 volt transformer. And can i wire it to my AC terminals?

-- Ray S. Lambert (lamb2260@bellsouth.net), October 09, 2004


1. Size of LED- that depends on the scale. Choose one from your local electronics supplier that fits the scale. They come in all kinds of sizes from about the size of a pencil eraser down to ones little bigger than a dot from a pencil.

When you buy the LED, ask how much current it can handle. If you are getting them from Radio Shack, usually the info is on the back of the packages.

Then you want to use the following simple law V=R*I


Typically, you would have a 20 mA LED, and 12V supply.

So 12=.02*x, solve for x,

x=600 ohms

So, now you need to get a 600 ohm resistor. It is not going to be too critical if it is 550-650 ohms, but don't try something like a 330 ohm one.

You have to use DC to power LED's. In order to use the AC terminals from your power pack, get a "bridge rectifier" from the same place you are getting the rest of the electronic stuff. Get one rated at 2 amps, and you can run a boatload of LED's on it. (using 20 mA leds, 100 of them or so)

-- James Powell (wx732@pacificcoast.net), October 11, 2004.

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