This simple and inexpensive bulb tester is ideal for residential use including Christmas trees and small lighted displays. It's perfect for incandescent string lights that don't operate if a single bulb burns out. Not sure which bulb is faulty? Then this tester is a holiday must-have! To use, simply hold down the black button and move the tester down the length of the light set so that the sensor is near the top of each bulb. The red indicator will light when near a functioning bulb, but will not light when near a faulty bulb. Other features include mini light tester, bulb remover, fuse tester, and C bulb tester. Check out our 1000Bulbs.com video for visual instructions! Not for use with LED Christmas lights.