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In its simplest form the analog optoisolator consists of an LED and a light-dependent resistor (LDR) in the same package: light output from the LED varies as the current passing through it; the light strikes the LDR, varying its resistance in turn; in effect this gives a '''current-controlled resistor'''.
An early manufacturer of these devices was Vactec Inc, who used the trademark ''Vactrol'', and this has since largely become genericized to cover all such available devices, whoever might make them.
This Perkin Elmer document contains a good introduction to such devices: [http://denethor.wlu.ca/pc300/optoisolators/analogoptoisolatorintroduction.pdf Analog Optical Isolators: Vactrols]
A more complete Perkin Elmer 'databook' contains not only the spec sheets of the various types of Vactrols, but also has many useful 'application note'-type circuits of how they might be used: [http://www.datasheetcatalog.org/datasheet/perkinelmer/VT500.pdf Photoconductive Cells and Analog Optoisolators (Vactrols)] (Note: current spec sheets are to be found at the Excelitas page linked below.)
There is also much useful info in the Wikipedia article: [http://en.wikipedia.org/wiki/Vactrol#Resistive_opto-isolators Resistive opto-isolators]
== Manufacturers ==
Current Vactrol product page at Excelitas: [http://www.excelitas.com/ProductPages/Analog_Optoisolators_and_Optocouplers.aspx Analog Optoisolators and Optocouplers]
Some Doepfer modules are known to use Macron devices: [http://macron.com.hk/specification.htm Macron specification page]
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