Why is it that you always test 48 bulbs before you find the bad one in a 50-light string? The simple circuit in Figure 1
allows you to divide and conquer, greatly reducing the time it takes to
find the bad bulb. The circuit uses a pair of NE2 neon bulbs with
current-limiting resistors. You can use a pair of Radio Shack 272-1100
bulb-resistor sets. It's convenient to house the tester in a clear
piece of plastic tubing, with the probe tip emerging from one end and a
light-duty power cord emerging from the other end. You place the bulbs
in the tube such that one is close to the probe tip and the other is
near the power cord, so it's easy to remember which bulb lit last. The
probe tip connects to common point between the neon bulbs. It consists
of thin spring wire with all but the last ¼ in. insulated. You use the
bare tip to make contact with the crimp connectors in the base of the
bulbs. ... ... Read More on EDN.com: Simple tester checks Christmas-tree lights
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