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A Joule thief allows you to boost the voltage of a dying battery.
Without the joule thief, you cannot light and led with a 1.5 volt battery
- Get the following parts along with some copper wire:
- Clip off 4 feet of wire and then fold in half. Twist the top of the folded end
- Make a knot on the top of one of the wires
- Make a knot at the bottom end of the wire with the knot on the top
- Using the loop end wrap the wire around the toroid
- Take one knotted end with a straight end and twist
- Take sandpaper and sand off ends
- Use a multimeter to test for continuity
- With soldering iron, tin the ends
- Bend the legs of the transistor
- Fold the center leg (base) under and solder to resistor
- Take the LED and solder longer leg to right side of transistor (emitter)
- Solder the shorter side of the led to the left side of the transistor (collector)
- Solder the resistor to one single lead of the toroid
- Solder the other single leg from the toroid to the longer lead of the LED
and the right side of the transistor
- Cut three inches of wire and strip off both ends
- Solder one end of the wire to the left side of the transistor/short end of the LED
- Connect circuit to battery (wire to gnd and double wire from toroid to positive terminal) and let the light shine
So how does it work?
Imagine the condition when the battery is first connected. The transistor is off, but voltage can flow down the left side of the transformer (wound toroid) in the diagram, current limited by the resistor. This turns on the transistor, and a larger amount of current can flow through the other side of the transformer and through the resistor. Because the voltage applied across the LED is well below its forward voltage, no current flows through the LED, and the LED does not yet turn on.
The larger current flowing through the right side of the transformer induces a current in the left side of the transformer that opposes the existing current on the left side-- and the current on the left side quickly drops down to zero. When this happens, the transistor shuts off. With no current flowing in the left side of the transformer, the toroid acts like a simple inductor, which is like a flywheel for current. There is already current flowing though it, which cannot shut off immediately, and the resulting current (flow of electric charge) towards the collector of the shut-off transistor leads to a temporary increase in the voltage at that point, which continues until the voltage exceeds 3 V, at which point the current finds a way out-- through the LED, which lights up as current flows through it.
As the last of the current fades from the toroid, the LED shuts off again, and we again return to the initial state: no current flowing through either side of the toroid transformer. And the process begins again. This whole cycle happens at a typical rate of about 40 kHz, far too fast for you to see the blinking, and the light from the Joule Thief appears to be continuous. This is a simple example of a switching power supply driven by a transformer-coupled oscillator, made of only four components.
As a side note, this is not the most efficient circuit around; it's beauty is that it works with such a low voltage. So, this is a great circuit to use with a dead or dying battery, and less so for use with a brand new battery.