This project is a light powered circuit which is very easy to hook up but very profound in its implications in this day when everyone is so concerned with the use and generation of energy. This circuit is useful as a light meter for photography. Most common light meters are very much like this simple circuit.

The Solar Cell changes light energy directly into electrical energy. The Meter provides a visual readout for relative light intensity and the 50K Control (connected as a rheostat) allows you to adjust the sensitivity of the circuit over a wide range of light levels. The Meter and rheostat (Control) form a simple voltmeter across the Solar Cell.

When the full 50K ohms of resistance is in the circuit, the full scale meter voltage is about 12 volts. When it is adjusted for maximum sensitivity (zero resistance) the full scale voltage is about 0.16 volt. The output voltage of the Solar Cell is sufficiently high so you should have no trouble pinning the Meter in the most sensitive rheostat setting (0), but you should not be able to pin the Meter with the minimum sensitivity setting (10).

Your VOM may be used to calibrate the Meter by setting the Control for some convenient full scale voltage. Place the VOM across the Solar Cell in parallel with the Meter and Control circuit when checking Meter calibration. It may become possible some day to make sufficient amounts of solar-energy-generated electrical power to do many of the common household tasks.

The present problem is the cost of the solar cells and equipment required to obtain a sufficiently useful installation. It will only be a matter of time though before we will see many light powered things in common use. The photovolatic cell used in this kit is a selenium type, which responds best to the blue to red.region of light.

Other solar cells are available that are made of silicon. These are far more efficient than selenium solar cells but are also much more expensive. If you are interested in this aspect of science we suggest that you take a look at the SOLAR POWER ELECTRONIC LAB KIT, which is available at your local Radio Shack store. With this simple light meter you may want to check the relative light levels around the house and compare these with some obtained outdoors in sunlight and in shade.