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[[File:Optoisolator.jpg|thumb|right|100px|Schematic symbol for a phototransistor optocoupler.]]The '''vactrol''', also known as '''resistive opto-isolator''' (RO), or '''photoresistive opto-isolator''' in its simplest form is an analogue optoisolator or '''optocoupler''' consisting of a [[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.<ref name="ts">[[Tim Stinchcombe]], Muff Wiggler wiki, June 2012</ref><ref name="wp">Wikipedia:[http://en.wikipedia.org/wiki/Vactrol#Resistive_opto-isolators Resistive opto-isolators]</ref>
The name is a trademark introduced by Vactec, Inc. in the 1960s, has since since largely become genericized to cover all such available devices, whoever might make them. Through acquisitions etc., Vactec became part of Perkin Elmer, and recently it appears the Vactrol trademark has passed to a company called Excelitas.<ref name="ts"/><ref name="wp"/>
There are reports of success using the Fairchild H11FxM family.<!--for what?--> These devices use a silicon photo-detector (rather than an LDR), which behaves like a [[FET]], and so they don't suffer from the lag in response that LDR-based devices have, i.e. they can be modulated at audio rates.<ref name="ts"/>
==Manufacturers==
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{{reflist}}
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*There is also much useful info in the Wikipedia article: [http://en.wikipedia.org/wiki/Vactrol#Resistive_opto-isolators Resistive opto-isolators]
*This Perkin Elmer document contains a good introduction to such devices: [http://denethor.wlu.ca/pc300/optoisolators/analogoptoisolatorintroduction.pdf Analog Optical Isolators: Vactrols]
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