Radio Shack Bargain Parts

Mike I. mirwin1 at
Mon Mar 20 03:02:10 CET 2000

Hi Scott,
Photocell and LDR are pretty much synonymous terms. They are not light 
to current/voltage generators, rather they behave as nice resistors 
(with a somewhat sluggish response to sudden changes in illumination - 
10's of ms typically. OK for phasers since you modulate with a slow sine 
or triangle). Selenium and silicon photovoltaic cells (solar cells) are 
the light to electricity converters. One of the first VCF's I ever built 
used an LED shining on two identical CdS photocells, replacing the 
tuning R's in a state variable filter. They should be fine for VC 
resonance in a VCF as long as you can arrange an LED driver to produce a 
suitable change in resistance per volt change in CV. Cesium "photocells" 
were the original ones from vacuum tube days - these were "generators" 
relying on photoelectric emission. Cadmium selenide CdSe photocells 
are also made. By the way, the CdS photocells have terrible 
manufacturing tolerances - you have to go through a lot of them to find 
4 or 6 that are reasonably matched. The DIY archives have some info on 
these. Also take a look at Mike I.

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