This voltage regulator for devices that use solar cells is very simple, require few electronic components. If using a rechargeable energy storage device as a voltage regulator circuit provides charging when the output voltage of the cell is high, and is a minimal charge when the battery output voltage of the cell is low.
Since the supply voltage is supplied by a solar panel, regulator always work, even when the battery is fully discharged or disconnected. This is the best surge protection possible against all circuits powered by solar battery.
This voltage regulator is built using precision voltage stabilizer type TL431LP National Semiconductor product. When the output voltage rises above the solar battery set by P1, is a current through R3-R2-D1. When this current reaches about 5 mA, the transistor T1 begins to conduct. Collector's BD642 is conveniently connected to the ground, allowing installing it directly on the radiator.