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The '''vactrol''', also known as '''resistive opto-isolator''' (RO), or '''photoresistive opto-isolator''' in its simplest form, theis an analogue optoisolator consistsconsisting of a [[LED]] and a [[light-dependent resistor]] (LDR) in the same package:. lightLight 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>
AnThe earlyname manufactureris ofa thesetrademark devicesintroduced wasby Vactec Inc, whoInc. usedin the trademark ''Vactrol''1960s, and this 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. 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"/>
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