Intro: Joule Thief Battery Charger
Many of you are probably aware of the "joule thief" circuit. If you aren't, it's a very simple voltage booster that is normally used to power an LED off of a mostly dead AA battery, but it can be used for other applications where a voltage boost is needed. One such application is charging a battery off of a battery with a smaller volatge rating. There are designs out there for battery chargers using a 9 volt battery and then step down the voltage to the level of the battery to be charged. But this circuit uses any single battery with a rating of 1.5 volts, such as a AA, C, or D battery, and bumps the voltage up to a level that can charge a battery with a voltage rating of 5 volts or lower. This curcuit is extremely simple, dirt cheap, and can be built in an afternoon using parts taken from other old devices.
Step 1: Parts and Pieces
Parts for the charger include:
2N3904 NPN transistor
1K ohm resistor
Toroid transformer core
SPST Switch (not pictured)
A word about some of the parts:
The toroid core can be any size, but nothing fancy or big is needed. A small toroid is probably better just because it is more compact. Only one toroid is used in the circuit, but two are pictured for size reference.
The diode can be just about any diode, but the lower the forward voltage drop, the better. Germanium diodes work the best. To find the forward voltage drop, simply hook a diode up to a battery, and measure the difference in voltage with and without the diode in the circuit. If the voltage drop is about equal to the voltage being supplied, make sure the diode isn't hooked up backwards.
Step 2: Make the Transformer
The toroid transformer needs to be wound first. Cut two pieces of magnet wire to roughly equal length. About three feet each is plenty for a small toroid. Next, wind them parellel to eachother around the toroid transformer core, making sure at no point the wires cross. glue can be used to secure the wires to the core so the transformer does not unwind. The number of coils is not a big deal, as long as each wire has an equal number of coils, and each wire is wound around 10 or more times. Keep winding all the way around the core, and when you are done you should have four loose wire ends coming off the core.
Step 3: Schematic
Now that the transformer has been prepared, its time to look at the schematic and begin assembling the circuit. The schematic is very simple. Its obvious which components are which, but just incase:
B1 is the AA, C, or D battery cell powering the circuit
S1 is a SPST switch
T1 is the toroid transformer
R1 is the 1K ohm resistor
Q1 is the 2N3904 transistor
D1 is the diode
the voltage out connection goes to the battery to be charged.
Step 4: Assmble the Circuit
I chose to make my charger as small as possible, and designed to use any 1.5 volt battery as a power supply, and then be able to charge any other battery. This means that i did not use any battery holders, but if there is a specific type of battery you want to chrge or use as a power supply, you are welcome to use a battery holder. I also removed the switch from the circuit. If you want to make it small like I did, just refer to the pictures for how to connect the parts.
When connecting the transformer, you must pay attention to the phase of the windings. The dots on the schematic represent the phasing. I'm not going to go over the general idea of phasing, just give the specifics to this transformer. On the reansformer, you have coil A and coil B, and side 1 on the left and side 2 on the right. Each end of the coil should be on the opposite side of the transformer from the other. i.e., one end of A is on side 1 (A1) and one end of A should be on side 2 (A2) and the same is with coil B. Choose two ends, one from each coil and from opposite sides on the transfromer, and connect them. Putting it simply, connect A1 and B2 OR B1 and A2.
This forms the positive connection to the circuit. But which of the two ends of the transformer is connected to what part of the circuit does not matter for the rest of the construction.
Step 5: Finishing Touches
You can mount your circuit in an enclosure, or however suits your needs. I wanted my circuit to be flexible in terms of what battery is used as a power supply, so i mounted mine on a popsicle stick (very high tech) with a magnet to hold the popsicle stick to the power supply battery. A springy wire extends down to make contact with the positive battery terminal, and a wire with an attatched magent makes contact with the negative terminal. This allows the circuit to be mounted on a battery of any size. The voltage output terminals are just two wire with alligator clips on the end, so the circuit can be hooked up to a battery holder for the battery that is charging or hooked onto the battery by some other means.
Step 6: Use It!
The circuit can now be used to charge batteries from a battery. This might seem silly, until you realize any batter up to 5 volts can be charged off a AA, C, or D battery. I have charged my ipod (almost fully) off a C battery, and my phone (up tp 1/4 full) off of a D, but this can also be used to charge batteries for other devices. the circuit might not be the most efficent boost-type charger circuit, but it uses very common parts and is great when you lose power and dot have any other options. Not only does it act as a charger, but a LED can be powered off of it like a traditional joule thief. (as a side note; I have found this circuit to not be compatible with certain cell phones, even when the battery is removed and charged seperately. Some cell phone batteries have protection circuits built in, and will only be charged while in the phone. Using this on a battery with a protection circuit built in could lead to the damage of that circuit. If the battery pack has more than 2 wires or contacts, then it probably has a protection circuit. aways test on an extra battery)
Congratulations on the completion of your new joule thief battery charger!
With regards to the epilog laser challenge:
I do fabrication of miniature, portable, battery powered guitar amps for many of my friends who are in bands. When I was building one, someone questioned if they could have custom engraving on the front aluminum panel. Unfortunately, I do not have any means to engrave the panel, but if I win the laser engraver it could solve that! I also do custom woodwork to gun stocks, and being able to laser engrave stocks would really expand my capabilities for customization.