Light meter solar cell and CdS cell
This project is a light meter with Solar Cell power and cadmium sulfide (CdS) Cell control, combined to give a Meter indication of relative lightlevels.
The use of both light controlled devices like this results in a greater change in Meter readings over certain levels of light than that obtained with either device alone. With very low levels of light the Solar Cell produces very low voltage, the CdS Cell has high resistance, -and the Meter deflection is little or nothing at all.
As light intensity increases, the Solar Cell output increases, the CdS Cell resistance decreases, and Meter deflection increases. The 50K Control is wired as a rheostat to provide some control over how much Meter deflection is obtained with a given light level. This allows the Meter to be compared with a calibrated light meter over a large range.
A chart or graph may then be made so that the Meter reading can be used to determine photography exposure requirements. To demonstrate the effect of the Solar Cell and CdS Cell, begin with a bright light source and a Meter reading near maximum. Use a solid object large enough to block the light to the Solar Cell but not to the CdS Cell.
The Meter reading should decrease to zero. Now cover the CdS Cell but not the Solar Cell. The Meter reading should again decrease. To obtain a zero Meter reading there can be no light allowed to leak through to the CdS Cell, as it is very sensitive.
The Solar Cell used is a selenium type which responds best to the blue to red region of light. The photons of light energy are converted directly to electric energy by this Solar Cell. The cadmium sulfide (usually referred to by its chemical symbol CdS) Cell is a resitance device that changes resistance due to the presence of light energy.
The change in resistance is inversely proportional to the light level. That is, high light causes low resistance and low light levels cause high resistance in the Cell. Typical resistances might be 100 ohms in bright sunlight and 5 megohms in total darkness. In the space below you may want to make a chart or graph of this Meter's sensitivities as compared with a photography light meter.