Tuesday, May 10, 2016

Reverse engineered: Flickering tea lights

Perhaps you have seen the flameless battery-operated candles, such as these tea lights.

You can buy this online for 99 cents. Oh, plus 9 cents tax and only $10.65 shipping. You can buy these in stores for the same 99 cent price, or down to almost 50 cents each in quantities of 20 or more.

Now that ours have finally died (120 hours total run time!) I was given permission to take one apart to see how it works. It was a lot simpler than I imagined. This device is simply a switch, battery holder, battery, and flickering LED soldered together (no wires; no other parts).

Flickering tea light unveiled

Most LED's (Light Emitting Diodes) require about 1.5 volts. They require a current-limiting resistor if the voltage is much more than the diode's forward voltage drop. But this device uses a 3 volt Lithium button battery with no external current-limiting resistor. That would burn out a typical LED. How does it survive? Where does the "flicker" come from? It turns out that this LED has tiny circuitry inside that limits current to the LED and creates the random flickering. It runs until the battery is completely depleted. I would guess it has a "joule thief" circuitry inside, too.

For more about this LED see the EvilMadScientist web site. This has quickly become a favorite site--they should make tee shirts!

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