You’re walking the dog at night, and your little flashlight, which has been getting dimmer, conks out. Instead of stumbling home in the dark, wouldn’t it be great if you could somehow eke a little more energy out of the batteries inside?
That’s not as crazy as it sounds. When a battery-powered device stops working, we say the battery is “dead”—but it’s not really. It still contains chemical energy, and the voltage isn’t zero; it’s just not high enough to run a current through the light bulb or LED (or whatever load you have).
But with a little physics chicanery, you can indeed get that light to run longer. Much longer! I’m going to show you how to build a simple electrical circuit, pairing a transformer and a transistor, to tap that residual energy. It’s waggishly called a “joule thief” circuit. Get it?
It’s not only fun to build and kind of mind-blowing, it’s also a great illustration of Faraday’s law of induction, the same principle that’s used in electric generators—and induction cooking stoves, for that matter. Let’s do this!
Basic Battery and a Light Bulb
I’ll start with a very simple circuit. This is a 1.5-volt AA battery connected to a small incandescent light bulb using a single copper wire.
Photograph: Rhett Allain
It’s a complete circuit. Electric current comes out one end of the battery, runs through a bulb with a tungsten filament inside, and then returns to the other end of the battery. Because that filament is super thin, the current heats it up—to like 4,500 degrees Fahrenheit—so that it glows white-hot. (Luckily, tungsten has the highest melting point of any pure metal.)
As long as the circuit is complete—which in a flashlight means the switch is left on—current will continue to flow, gradually using up the battery’s chemical potential energy. As the voltage drops, it produces less current. At some point, there won’t be enough to produce any light.
In a way, this circuit is also a joule thief. If you leave the switch on, current will continue to flow even after the light goes out, and it’ll use up the battery’s remaining energy. But who cares about a joule thief that drains a battery and doesn’t produce light?
LEDs vs. Incandescents
Today, most devices use LEDs instead of incandescent bulbs. An LED doesn’t produce light by making things hot; instead, it’s a solid-state device with an energy gap. When electrons in the current fall to a lower energy level, they release the extra energy as light. It’s way more efficient, because you don’t have all that wasted thermal energy. The only downside is that a white LED requires 3 volts, so now we need two of those AA batteries:
Photograph: Rhett Allain