5v Regulated Solar Power Supply Circuit

This project uses the 1.2v rechargeable battery and solar panel from a Solar Garden Light. These lights can be bought for less than $5.00 in most $2.00 shops or similar shops that sell general household items. We are also using the housing for this project as we could not buy the case, battery and panel for $5.00 in an electronics shop. It is incredible that a solar panel, rechargeable battery and plastic housing can be bought for less than $5.00! We have already described the operation of the Solar Circuit, but unfortunately it cannot be used to generate a voltage higher than about 4v, so a new design had to be created.
5v Regulated Solar Power Supply Circuit - schematic

The circuit we have designed is shown above and provides a regulated 5v output @ 10mA. If a higher current is drawn, the output voltage will drop. At 15mA, the output voltage drops to 4v. This supply has been specially designed for a microcontroller project, but it will also work for circuits such as amplifiers, FM transmitters etc. The circuit consists of an oscillator transistor and a regulator transistor. The solar panel charges the battery when sunlight is bright enough to produce a voltage above 1.9v. A diode is required between the panel and the battery as it leaks about 1mA from the battery when it is not illuminated. The regulator transistor is designed to limit the output voltage to 5v. This voltage will be maintained over the capability of the circuit, which is about 10mA. The oscillator transistor must be a high-current type as is is turned on for a very short period of time to saturate the core of the transformer. This energy is then released as a high-voltage pulse. These pulses are then passed to the electrolytic and appear as a 5v supply with a capability of about 10mA. If the current is increased to 15mA, the voltage drops to about 4v. The transformer is wired so that it gives POSITIVE feedback. The transistor turns on via the 1k resistor and this produces expanding flux in the core. The flux cuts the turns of the secondary winding and produces a voltage that ADDS to the turn on voltage and...

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