What size resistor do I need to change the alarm LED to blue? I have the LEDs they are 5mm.

 

1/8 inch should do it.

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Assuming that your LED is 3.0V 20ma (typical blue LED), you need a 470 Ohm resistor. If you can find out exactly what your LED needs (should be on the package if you got it at radio shack/fry's) enter the numbers here to get a better figure. Err on higher resistance if you're unsure.