How to find a faulty bulb in a Christmas lights string
I just saw a great and simple project that does just this:

http://www.youtube.com/watch?list=PLFA57ACAC0F0DE0D1&amp;feature=player_detailpage&amp;v=cwiLQWJq2LQ



The project is by Alan Yates:
http://www.vk2zay.net/

As I understand it, it uses a high impedance gate of a JFET to detect fluctuations in the E-field in the wires due to noise on the mains. The signal is amplified using a BJT to make sound on a piezoelectric speaker. If a light is burned out it the E-field will exist on the wire going into the light, but, not on its exit wire. Using this principal it is easy to locate the burned light. He applies this to incandescent light string, but, the same principal would apply to an LED string.