Introduction: Solar Powered AA Battery Charger

This Solar Powered AA Battery Charger was made from vintage parts I had in stock. The solar cell is an early 1970's hobbyist solar cell put out by International Rectifier. The transistors are RCA 2N404 Germanium transistors which were manufactured in 09/65. These transistors were probably the most popular transistors manufactured in the USA, in the late 50's to the late 60's due to their extensive use in the computers of that era. I decided to use germanium transistors because the solar cell only puts out 1 volt (in bright light) and I wanted to get the best efficiency. Germanium transistors are turned on at .25 volts compared to silicon transistors which are turned on at .7 volts. The charger will start working at voltages substantially below 1 volt (lower light conditions) albeit with lower output, but if I used silicon transistors, it would barely work at 1 volt. This circuit can be duplicated using silicon transistors, such as PNP 2N3906 or NPN 2N3903, if two solar cells are used. This circuit can then be fed with higher voltages with the resultant higher output current to charge bigger batteries than the AA type. If the circuit is to be used at higher voltages with silicon transistors, I would change the delicate 1N34 signal diode to a Shottky diode such as an 11DQ09.

Step 1: How It Works

I intentionally kept this circuit as simple as possible while still demonstrating the basic building blocks of an Astable Mulivibrator and Boost Converter circuit. The transistors Q1 and Q3 are configured to form a circuit called an Astable Multivibrator. This circuit is a fundamental building block of electronics, still being used extensively in the electronics industry. Today most versions of this circuit would be found on an IC chip. The original circuit was invented by two French scientists in 1919. https://en.wikipedia.org/wiki/Multivibrator

This circuit is a type of oscillator but instead of producing sine waves, it produces square waves or some variation of a square wave. These circuits produce signals of extremely high harmonic content and if this is not desired, other waveform creating circuits would be used. In this case, harmonics are not so much of a problem so this basic circuit is adequate. The signal is taken off the collector of Q1, but it can also be taken off the collector of Q3. The only difference will be that the two signals are 90 degrees out of phase with each other. The frequency and duty cycle of the waveform is governed by the relationship between the 10k resistors and the 1nF capacitors. The frequency is voltage dependent but even with a regulated voltage, the Astable Multivibrator is notorious for it's frequency instability. The frequency and duty cycle are not very important in this circuit as the input voltage rises according to the input voltage anyway. The purpose of this circuit is to provide a "chopping" action for the DC input so that the voltage can be passed on to the Boost Converter. At 1 volt the frequency was measured to be approximately 130 kHz.

The signal is taken off the collector of Q1 and coupled through the 15nF capacitor to the base of transistor Q3 which is configured as a switch. Resistor R5 is used to provide a load for the signal and gives a proper impedance match to the previous stage. Transistor Q3 is either on or off and when on, passes current through the 1mH inductor L1 to ground. (Note that when the transistor is on and a signal appears on the collector, the diode has a blocking action to the signal, keeping it from flowing out to capacitor C4) During the period when the current is not flowing through the transistor, the magnetic field which had been established by the current flowing through the inductor collapses, inducing a counter electromotive force which flows through the diode to charge up the capacitor. The capacitor stores the charge and during each subsequent discharge from L1, more voltage is added which builds up the charge to a value that can be many times the input voltage. In this circuit, the voltage is built up from one volt to a no load value of 4.76 volts. This circuit is only capable of trickle charging two AA rechargeable batteries as the output current is limited to a couple of milliamperes.

Step 2: Cut a Piece of Perfboard to a Size Just Smaller Than the Inside of an Altoids Can.

Cut a piece of perfboard to a size just a little smaller than the inside of an Altoids can. At the same time cut a piece of thin cardboard to the same size as the perfboard. This is to act as electrical insulation between the bottom the circuit and the Altoids can. Once the circuit has been completed, the cardboard is glued to the bottom of the Altoids can with a bit of hot melt glue and the perfboard is glued to the cardboard also with a bit of hot melt glue.

Assemble the components in an L-shape as shown in the above photo. This is to accommodate the battery holder.

Step 3: Connect the Components With Wire and Solder As Shown

Connect the components with wire and solder on the bottom of the perfboard as shown in the above photo. Note: I personally find that it is better to copy the layout of the schematic and work from left to right as you mount the components. I then wire them together and solder each individually. Take special care to use a heatsink to keep heat from damaging the transistors and especially the diode. Germanium devices have are less heat tolerant than silicon devices.

Step 4: After the Circuit Is Wired and Soldered, Wire Up the Solar Cell and Battery Holder.

After the circuit is wired and soldered, wire up the solar cell and battery holder to the circuit. The completed assembly should look like the above photo.

Step 5: This Is How the Mounted Perfboard Will Look Inside the Altoids Can.

Once the perfboard is glued and mounted inside the can, the battery holder is allowed to freely sit in the space made by mounting the components in an L-shape. Any untidy wires can be secured to the board by a drop of hot melt glue. Be careful not to allow any glue to get on the transistors or diode, as this will destroy them through heat.

Step 6: The Solar Cell Is Mounted on the Upper Lid

The solar cell is mounted inside the upper lid being secured with hot melt glue or double sided tape. The wires should be secured with hot melt glue in places where it is shown in the picture. The reason that I mounted the solar cell on the inside, was because it could be concealed and protected when the device wasn't in use. If so desired, you can cut a hole in the can and mount it on the outside of the lid.

Step 7: Solar Charger Completely Disguised When Lid Is Closed.

The solar charger is completely disguised when the Altoids lid is closed. Anyone want a mint?

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