How do proximity-type Christmas-light testers work?
If you have read the article How Christmas Lights Work, then you understand why the strands are so sensitive to bulb failure. If any of the bulbs in the strand is loose, the entire strand will not light — read the article to learn why. These testers detect the alternating electromagnetic waves given off by any wire carrying power. If you have read the article How Electromagnets Work, then you have seen that current running through a wire generates a field that can affect a compass. In the case of household power, the current is alternating at 50 (European standard) or 60 (U.S. standard) oscillations per second. These oscillations set up a field near the wire that is fairly strong. The bulb tester picks up this field, and a simple amplifier amplifies it. The amplified signal is either strong enough to light the LED in the tester or it is not. The level of amplification is set so that only a str
