How Everything Works
How Everything Works How Everything Works
 

QUESTIONS AND ANSWERS
 
Question 783

How do flashing lights, chasing lights, and any type of Christmas lights work? - N
Years ago, many strings of Christmas lights consisted of about 20 or 30 light bulbs in series. In this series, electric current passed from one bulb to the next and deposited a small fraction of its energy in each bulb. The result was that each bulb glowed brightly so long as every bulb was working. If a single bulb burned out, the entire string went dark because no current could flow through the open circuit. If you replaced one of the bulbs in a working string with a special blinker bulb, the whole string would blink. The blinker bulb contained a tiny bimetallic switch thermostat that turned it off whenever the temperature rose above a certain point. At first, the bulb would glow and the whole string would glow with it. Then the thermostat would overheat and turn the bulb and string off. Then the thermostat would cool off enough to turn the bulb and string back on. This pattern would repeat endlessly.

But modern electronics has replaced the blinker bulbs with computers and transistor switches. Transistorized switches determine which bulbs or groups of bulbs receive current and glow at any given time and carefully timed switching can make patterns of light that appear to move or "chase." As for the problem with one failed bulb spoiling the string, a reader has informed me that the bulbs are now designed with a fail-safe feature. If a bulb's filament breaks, the sudden surge in voltage across that bulb activates this fail-safe mechanism. Wires inside the bulb connect to allow current to bypass that bulb completely. The remaining bulbs in the string glow a little more brightly than normal and their lives are shortened slightly as a result.

         

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