light to frequency

This web page shows the active element for making an oscillator that gives frequency related to light. The element is a Cadmium-Sulfide photoresistor (photocell). The two photos show how the resistance changes more than a factor of two when you shade it from room light.

A circuit that can use this photoresistor is the 555 integrated circuit, $.60, in the astable configuration. This gives a buzz right into a speaker. But it uses a 9V battery pretty quickly. The battery-conserving circuit is a three-transistor astable multivibrator.

A Temperature-to-Frequency Circuit

The same circuit can be used with a $.20 thermistor to get a buzz that is related to temperature.