Here's a very handy part: the Clairex CLM8500 combines an LED and a very good, fast CDS cell in one package! By varying the current to the LED (use a current-limiting resistor to avoid exceeding the max current and burning out the LED), you can make the CDS cell go from about 500K ohms down to less than 10K ohms. Great for a DC-controlled potentiometer to vary audio level, lighting levels, etc. Easy to use, but RARE! Pick up several today before they run out! Price is per piece!

These are frequently found in the protection circuits of power amplifiers and the tremolo circuits of guitar amps. The CLM-8500 is one of the later ones that Clairex produced, using an LED as the light source instead of a lamp. For those who want a lamp-based system, I also have CLM-4012 opto-couplers which offer a slower response from the 12Volt lamp inside.