Do you have some deceased fluorescent lamps? If Yes, then you might still recycle some of its guts to fashion some simple yet usable circuits.
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Step 1: Car to Portable MP3 Player Power Supply
The first gadget is a 12 V to 1.8 V power supply, also known as step down or buck dc-dc converter. These type of SMPS (Switched Mode Power Supply) are very efficient in terms of energy loss by heat dissipation compared to the linear type (circuits like 7805, 7509, LM317, etc). This works under the principle of energy storage in a magnetic field and then just regulating the output voltage by means of duty cycle control on a PWM (Pulse Width Modulation).
Open up your lamp and look for a small ferrite core transformer (some brands might have two, so try different brands or sizes)
Step 2: MP3 PSU:Stuff You'll Need
Here is what you'll need:
-NE 555 (Timmer- oscillator IC)
-two 1N4001 or 1N4148 diodes or any similar (for low frequency)
-10k trimpot or potencimeter
-100n ceramic or polyester capacitor
-10n ceramic or polyester capacitor
-1000u X 25 V polarized capacitor
-Any N-Channel enhanced mode Mosfet for 5 A or greater current
-470 Ohm resistor
-A Schotsky diode (This is the most important item, since is should be a diode that can switch at high frequencies, thus the "fast recovery" tag) I got this from a deceased laptop power adaptor. They are very common.
-FROM YOUR LAMP, remove the xtransformed and if it is is still in goog shape, use it. If not, you can still use the core and rewind some 200 turns of magnet wire (30 or 32 AWG). Inductance of this coil should be in the mH range.
Step 3: Schematic
This is the schematic. 555 drives the Mosfet and it pulses current thru the inductor L and the load. Once Mosfet is off, the magnetic fiel collapses and a voltage with oposite polarity appears on L terminals, thus the Schotsky diode conducts this energy in the form of current to the load, but at the load the polarity remains unchanged. The resulting formula is this: Voutput=Vinput/Duty Cycle. Because power losses are like 15-20% you just need a small heat sink for the mosfet. Believe me, I had one linear converter using a LM317, and it got very hot, even though it had a large heat sink.
Step 4: End Result.
This is my version of the circuit.
Step 5: Adjust Your Output Voltage
Now, connect the NiCd to your dc-dc converter and adjust it to output 1.5 to 1.8 volts. Then power up your MP3 and fine tune it to get the full battery scale. This circuit will be able to charge your MP3 battery and at the same time power up your MP3.
Step 6: Joule Thief
This is a circuit that does the oppositte to the Buck converter, it "boosts up" the voltage, hence it is call Boost converter.
This circuit is known for its alias "Joule Thief". This is simply a self oscillating Boost DC-DC converter.
Using the toroidal ferrite core found on some flourescent lamps, you can wind the transformer needed for this gadget.
This is what you'll need:
-Any small signal NPN or PNP (bipolar transistor) like BC548.
-A 1.5 V battery
-The toroid from your lamp.
-1 meter long piece of magnet wire.
Step 7: Winding
Now bend your magnet wire in half and wind it around your toroid.
Cut the ends and using a Ohmmeter, identify the ends of L1 and L2.
Solder the end of L1 to the begginig of L2. So polarity on L1 and L2 are oposite. (for more detail go to these link: http://www.evilmadscientist.com/article.php/joulethief)
This is how it works:
When the transistor conducts, current flows thru L1 and the the LED is off (fig. a). This creates a magnetic field on the coil and makes L2 shut off the transistor. When this happens (fig. b), the energy of the magnetic field induces a voltage on the coil called "back emf" and this ADDS up with the battery so you have 3 V or more (it like adding batteries in series).
Because this cycle occurs at high frequencies (kHz, depending on your inductor and transistor, etc), the LED seems to be all the time ON.
Step 8: Final Result
Here is how i just soldered it to a Alkaline battery.