Editing Opto-isolator (section)

===Resistive opto-isolators===
{{main|Resistive opto-isolator}}
<!-- [[File:OEP series optocouples.jpg|thumb|right|Obsolete photoresistor opto-couples (Russian OEP series pictured) retain a niche in modern [[guitar amplifier]] market.]] -->
The earliest opto-isolators, originally marketed as ''light cells'', emerged in the 1960s. They employed miniature [[incandescent light bulb]]s as sources of light, and [[cadmium sulfide]] (CdS) or [[cadmium selenide]] (CdSe) photoresistors (also called light-dependent resistors, LDRs) as receivers. In applications where control linearity was not important, or where available current was too low for driving an incandescent bulb (as was the case in vacuum tube amplifiers), it was replaced with a [[neon lamp]]. These devices (or just their LDR component) were commonly named ''Vactrols'', after a trademark of Vactec, Inc. The trademark has since been [[genericized trademark|genericized]],<ref group=note>According to the [[United States Patent and Trademark Office]], trademark registered in 1969 for "photocell combined with a light source" is now dead ([http://tess2.uspto.gov/bin/showfield?f=doc&state=4005:pnh880.2.4 USPTO database record serial number 72318344]. Retrieved November 5, 2010). The same trademark, registered in 1993 for "medico-surgical tubing connector sold as a component of suction catheters" is now live and owned by Mallinckrodt Inc. ([http://tess2.uspto.gov/bin/showfield?f=doc&state=4005:pnh880.2.2 USPTO database record serial number 74381130]. Retrieved November 5, 2010).</ref> but the original Vactrols are still being manufactured by [[PerkinElmer]].<ref>Weber, p. 190; PerkinElmer, p. 28; Collins, p. 181.</ref><ref group=note>Vactec was purchased by [[EG&G]] (Edgerton, Germeshausen, and Grier, Inc.), a defense contractor, in 1983. In 1999 EG&G purchased formerly independent PerkinElmer, and changed own name PerkinElmer (see [[reverse takeover]]). An unrelated company, Silonex (a division of [[Carlyle Group]]) brands its photoresistive opto-isolators ''Audiohm Optocouplers''.</ref>

The turn-on and turn-off lag of an incandescent bulb lies in hundreds of [[milliseconds]] range, which makes the bulb an effective [[low-pass filter]] and [[rectifier]] but limits the practical modulation frequency range to a few [[Hertz]]. With the introduction of [[light-emitting diode]]s (LEDs) in 1968–1970,<ref>Schubert, pp. 8–9.</ref> the manufacturers replaced incandescent and neon lamps with LEDs and achieved response times of 5&nbsp;milliseconds and modulation frequencies up to 250&nbsp;Hz.<ref>PerkinElmer, pp. 6–7: "at 1 [[Foot-candle|fc]] of illumination the response times are typically in the range of 5&nbsp;ms to 100&nbsp;ms."</ref> The name ''Vactrol'' was carried over on LED-based devices which are, as of 2010, still produced in small quantities.<ref>Weber, p. 190; PerkinElmer, pp. 2,7,28; Collins, p. 181.</ref>

Photoresistors used in opto-isolators rely on bulk effects in a uniform film of [[semiconductor]]; there are no [[p-n junction]]s.<ref name=P3/> Uniquely among photosensors, photoresistors are non-polar devices suited for either AC or DC circuits.<ref name=P3/> Their resistance drops in reverse proportion to the intensity of incoming light, from virtually infinity to a residual floor that may be as low as less than a hundred [[Ohm]]s.<ref name=P3/> These properties made the original Vactrol a convenient and cheap [[automatic gain control]] and [[Dynamic range compression|compressor]] for telephone networks. The photoresistors easily withstood voltages up to 400&nbsp;volts,<ref name=P3>PerkinElmer, p. 3</ref> which made them ideal for driving [[vacuum fluorescent display]]s. Other industrial applications included [[photocopier]]s, industrial [[automation]], professional light measurement instruments and [[Exposure (photography)#Automatic exposure|auto-exposure meters]].<ref name=P3/> Most of these applications are now obsolete, but resistive opto-isolators retained a niche in audio, in particular [[guitar amplifier]], markets.

American guitar and organ manufacturers of the 1960s embraced the resistive opto-isolator as a convenient and cheap [[tremolo]] modulator. [[Fender Musical Instruments Corporation|Fender]]'s early tremolo effects used two [[vacuum tubes]]; after 1964 one of these tubes was replaced by an optocoupler made of a LDR and a neon lamp.<ref>Fliegler and Eiche, p. 28; Teagle and Sprung, p. 225.</ref> To date, Vactrols activated by pressing the [[Effects unit#Stompboxes|stompbox pedal]] are ubiquitous in the music industry.<ref>Weber, p. 190.</ref> Shortages of genuine PerkinElmer Vactrols forced the [[Do it yourself|DIY]] guitar community to "roll their own" resistive opto-isolators.<ref name=C181>Collins, p. 181.</ref> Guitarists to date prefer opto-isolated effects because their superior [[Ground (electricity)#Separating low signal ground from a noisy ground|separation of audio and control grounds]] results in "inherently high quality of the sound".<ref name=C181/> However, the [[distortion]] introduced by a photoresistor at [[line level]] signal is higher than that of a professional electrically-coupled [[Variable-gain amplifier|voltage-controlled amplifier]].<ref>PerkinElmer, pp. 35–36; Silonex, p. 1 (see also distortion charts on subsequent pages).</ref> Performance is further compromised by slow fluctuations of resistance owing to [[light history]], a [[memory effect]] inherent in [[cadmium]] compounds. Such fluctuations take hours to settle and can be only partially offset with [[Feedback#Electronic engineering|feedback]] in the control circuit.<ref>PerkinElmer, pp. 7, 29, 38; Silonex, p. 8.</ref>