The purpose of a voltage regulator circuit is to take a variable input voltage and produce a steady output voltage. Two common regulator types are linear and switch-mode. Linear regulators are simple, but waste a lot of power in the process of regulating the voltage. Linear regulators can be thought of as self adjusting series resistors.
Connect the input of the circuit to a 12V power source, such as a solar-charged lead acid battery. Adjust the 10K trimmer potentiometer for the desired output voltage. Connect the load device to the output connections, and power up the load. For the best power efficiency, be sure to switch off the input power to the regulator circuit when the load circuit is not in use.
For use as a linear current booster, connect the circuit input to a 36 cell (18V, 12V nominal) solar panel. Put the solar panel in the sun and adjust the 10K trimmer potentiometer for the desired output voltage. Connect the lower voltage load device to the output connections and power up the load. In this mode of operation, there is no storage battery, so the load only works when the sun is shining on the solar panel. The regulator IC will accept input voltages up to 40V DC, this allows the circuit to be used with 36V (24V nominal) solar panels. If you use this circuit with input voltages above 22V, change the 100uF capacitor to a 50V part and replace the transzorb with a 47V part
There are a few unusal parts in this circuit. The 330uH inductor is available from digikey.com as part number TE2017-ND. Other inductors can be used, but they should be designed for use in switching power supplies. The output filter capacitor section uses a 330uF electrolytic capacitor in parallel with a 10uF tantalum capacitor. It is possible to use other capacitor combinations, such as a 330uF "Low ESR" type electrolytic capacitor and a 100nF ceramic disk capacitor or a standard 330uF electrolytic capacitor in parallel with a 1nF disk capacitor and a 100nF mylar capacitor. The small value capacitors improve the high frequency rejection of the electrolytic device