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# Need power, resistors for L.E.D "Zone Alarm"

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#### machavelli

##### New Member
I am making a visable alarm system for my house. This will be used to see if any window or doors opens, if so it will light up an led in the location it opened at. So far i got a project box, etched my house layout on it, drilled holes where my windows and doors are (for the led's). What i need to know is what kind of power and resistors would i use. The led's range from 2.2v to 3.5v and are 20ma. Any help with this would be great.

It depends what power pack ( or battery) voltage you have for you project box.
I take it yo have mounted a reed switch to every door & window and run wires from each to your box. A lot of wires, did you have fun laying them?

You can calculate your own resistor value:
battery (or power pack) voltage minus 2.2 or 3.5 gives you the voltage drop at the resistor (Vr).
Resistance value is Vr divided by .02 (20 mA).

So, for a 9 V battery, its 9V-2.2V = 6.8V / .02A = 340 ohm. The nearest standard resistor value is 330 ohm, use that.
Since that resistor is dissipating power you need to know that too. Power (watts) is voltage x current, for my example its 6.8 x .02 = 0.14 watt. Get the nearest bigger standard wattage, 0.25W.

Klaus

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