Vactrol

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.

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. Like a lot of other products used in SDIY though, this is yet another device that is about to become end of life. As the photoresistors typically use CdS (Cadmium Sulphide) in their construction, Cadmium is classed as a hazardous substance under RoHS and production is due to end in 2015. Production would have ended sooner, but Vactrols where given a limited exemption whilst replacement materials were researched.

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.

Manufacturers

 * Excelitas, Analog Optoisolators and Optocouplers
 * Some Doepfer modules are known to use Macron devices, Macron specification page
 * The Silonex 'NSL' series, Silonex Audiohm Optocouplers
 * Fairchild H11FxM, Solid State Relay Optocoupler