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.