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#### 1spideyfan

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I would like to make a very short LED light string for a Christmas ornament. I would like to make the string with a total of 9 lights; 3 red, 3 yellow, 3 green. I would like to power this string on a 3v coin lithium battery. What I currently have...3mm LED's, some perf-board, momentary tactile switch, copper wire and the battery. Also, I just so happen to have some 470 ohm and 100 ohm resistors intended for white LED's, but I not sure if these can be used. What other components might I need? Please help.

#### AllanJ

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You need to know the working current draw and the voltage drop (the latter is one of the parameters) of the LED. If only a maximum current draw is specified, I suggest starting with 2/3 of that value.

(Unscientific method follows)

In every electrical or elextronic circuit, voltage measured across any two points equals the current flowing between those points times the resistance between those two points.

Subtract the LED voltage drop from the supply voltage. Divide the result by the current draw. Now you have the value of resistor needed. Use the next larger size (ohms, not inches) of resistor if you don't have the exact value.

If it does not work and you are using a battery or other dirct current source, reverse the power leads.

You can have several LEDs with the same current draw each, all in series with one resistor. Subtract all of their voltage drops from the supply voltage. LEDs have positive and negative terminals. In a series, positive of one connects to negative of the next.

It is possible that slight variations in the LED's in a series string with one resistor could cause one to burn out and a domino effect then burns all of the rest out.

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