Solar Phone Charging System Featuring Sun Tracking

Solar power has become quite a star in green power generation recently. Especially here in Sydney, with the help of government policies, more and more buildings have solar power system on their rooftop.

As makers ourselves, making a small solar phone charger is becoming a fashion. There are many great entries in instructables about solar charger with portability in mind. They are great if you spend a lot of time outdoors.
Well people, I’m proudly present this instructable to you so that you can make your very own solar powered phone charging system with sun tracking.
Why tracking?
The sun moves 15 degrees every hour, angle of the incident solar rays directly affect the power output of the solar cell. A solar tracking gives 60% more power from the same solar cell.

Your will need
A solar cell – at least 6V, 1 Watt
A motor with gearbox
A circuit board
2x LM393 comparator IC, can be any similar comparator
A L293D H bridge motor driver IC
A IRF9450 MOSFET, or any similar P channel MOSFET
5k, 100k, 10k resistors
A 50k potentiometer
An IRF540 or similar N-MOSFET
One phototransistors (Darlington IR)
2 photocells or 2 photodiodes
Battery holder
Some 1N4004 diodes
Wires
Some sort of housing – I used my iPod touch case
And some batteries, I use NiMH here, you can also use Li-ion but the circuit may need to change

Tools:
Soldering iron
Hot glue gun and hot glue
Drill could be useful too

Step 1: Let the Theory Begin (too Boring? Jump to Next Step Then)

Solar cell is a current source; its output current varies with solar intensity. Therefore we can just charge a NiMH pack without additional circuit to limit the current. To harvest maximum amount of ‘juice‘out of the solar cell, we should pick a battery pack that matches the voltage output of the cell at its maximum power point.
We will need a diode to prevent current flow into the solar cell, which may contribute a 0.7 to 1V drop. And normally solar cell’s voltage is marked using the open circuit voltage. you may need to lower a 10% or so to get to the VMP.
If you are getting a 6V solar cell, then it is best to charge 3 NiMH in series. I have used an 8V solar cell, and I have 4 batteries charging in series, so about 2V each cell.

Step 2: Tracking Circuit Design

For this circuit to work the pair of photocells or Light dependent resistor, LDR, has to act as eyes, giving lm393 comparators analogue signal about the sun’s position. And this pair is better built on a separate PCB from the main control board.
Voltage is divided through R1, 50k pot and R3, thus creating reference. The 50k variable resistor is use to set the NULL zone, or sensitivity. This way the system won’t be tracking every second of the sun’s movement. Saving energy spending to track the sun and also save the moment when the system rotates back and forth causing by its momentum.
The outputs of lm393 signal the H bridge chip to control the motor. Based on this design two comparators’ outputs will not be both HIGH at any given time; means there won’t be any error signal which cause the H-bridge to short out.
To conserve energy, L293d is usually disabled. The enabling signal came from LM393, when there is a HIGH from one of the comparators the voltage at EN1 will raise via the 1N4004 diode. Note here R6 has to be much bigger than R4 and R5.
Ground all the other inputs on L293d. this simple act can save about 20mA of current. I don’t know why L293d drain so much current on idle.
The small circuit at the left of the schematic, involving a phototransistor, a 10k resistor and a MOSFET, is designed to switch on battery power backup on the tracking system when the sun rise from the east while the panel is still left pointing to the west.

The phototransistor has to be a Darlington pair. You can test it by using a multimeter on continuity mode, placing the + test lead to the shorter pin on the phototransistor, and the – test lead on the longer one. It should be an open circuit when not infrared exposure. But once you turn the diode toward the sun your multimeter should peep.
This phototransistor is best place right behind the solar panel, and pointing to the east as shown below. Here the phototransistor is normally an open circuit, shutting off the MOSFET and the current can only flow from the solar cell to the batteries via diode D3. While the sun comes from the east will shine on the phototransistor making it a short circuit, raise the voltage across R7, and thus turn on the MOSFET. Now the diode D3 is bypassed and the current can flow from the batteries to the tracking circuit.

As for the MOSFET, any N channel MOSFET will work. But low gate to source threshold voltage is preferred.

It is time for the all important photocell sensor. You should construct the sensor on a different board, so it can be relocated easily. Connect two of these in series, and bend them outward slightly shown below. This configuration allows a bigger difference in solar energy on each cell, when the device is not facing the sun directly. This difference is small, but through the amplification of the comparators, controlling the motor is possible.

Step 3: Harvest the Solar Energy

There are many possible ways to use the energy stored. I use it to charge my phone. Boost convertor is required to regulate the voltage; the construction of such circuit will not be covered in this instructable. Designing a switch mode regulator is a tedious work, to achieve maximum efficiency also requiring a lot of tweets. It is best to buy one. Like minty boost from adafruit, or other boost module out there. I purchased a ptn04050 module from TI, and built a small supporting circuit around it.

To protect your NiMH battery pack, it is best to have a low voltage protection circuit. You can either buy a Lipo protection module, or build one according to this schematic.

The way the circuit function the IRF9450 acts as a switch, it only turns on when the gate-source voltage is high. As the circuit is just connected to the battery, gate-source voltage is zero. The MOSFET does not conduct. The push button PB1 is able to connect the gate or the MOSFET to the ground temporally. Switch on the MOSFET, and the rest of the circuit. The Vref is produce by the small circuit consist of a resistor and a BC549 NPN transistor. By tying the collector and base together on the transistor, the voltage across the collector and the emittor is constant at 0.6V.

This circuit will sustain itself until VOUT is less then 3.4V, determined by R2 and R3. It does not use any power on idle. Great for a care-free system.

Step 4: Putting All Together

Now that the tracking circuit is done, secure everything in the housing you chose. Place the photocells anywhere you want as long as direct sunlight is achieved. Connect your solar panel to the port named solar in the schematic. Then your product may look like this.

Enjoy making your own solar system, the world could be greener everyday.