Resistive opto-isolator

Resistive opto-isolator (RO), also called photoresistive opto-isolator, vactrol (after a genericized trademark introduced by Vactec, Inc. in the 1960s), analog opto-isolator or lamp-coupled photocell, is an optoelectronic device consisting of a source and detector of light, which are optically coupled and electrically isolated from each other. The light source is usually a light-emitting diode (LED), a miniature incandescent lamp, or sometimes a neon lamp, whereas the detector is a semiconductor-based photoresistor made of cadmium selenide (CdSe) or cadmium sulfide (CdS). The source and detector are coupled through a transparent glue or through the air. Electrically, RO is a resistance controlled by the current flowing through the light source. In the dark state, the resistance typically exceeds a few MOhm; when illuminated, it decreases as the inverse of the light intensity. In contrast to the photodiode and phototransistor, the photoresistor can operate in both AC and DC circuits and have a voltage of several hundred volts across it. The harmonic distortions of the output current by the RO are typically within 0.1% at voltages below 0.5 V. RO is the first and the slowest opto-isolator: its switching time exceeds 1 ms, and for the lamp-based models can reach hundreds of milliseconds. Parasitic capacitance limits the frequency range of the photoresistor to ultrasonic frequencies. Cadmium-based photoresistors exhibit a "memory effect": their resistance depends on the illumination history; it also drifts during the illumination and stabilizes within hours, or even weeks for high-sensitivity models. Heating induces irreversible degradation of ROs, whereas cooling to below −25 °C dramatically increases the response time. Therefore, ROs were mostly replaced in the 1970s by the faster and more stable photodiodes and phototransistors. ROs are still used in some sound equipment, guitar amplifiers and analog synthesizers owing to their good electrical isolation, low signal distortion and ease of circuit design.

History In 1873, Willoughby Smith discovered the photoconductivity of selenium. In the early 1900s, the studies of the external photoeffect in vacuum tubes resulted in the commercial production of photoresistors. In 1918, American and German engineers independently suggested the use of vacuum photocells for reading optical phonograms in the film projectors in cinemas, and Lee de Forest, Western Electric and General Electric produced three competing systems using such photocells. In 1927, the first commercial sound film, The Jazz Singer, was produced in the United States, and by 1930 sound films had replaced silent films. The success of sound films stimulated the search for new applications of photocells. Various types of photocells were considered: vacuum, gas-discharge, photovoltaic and photoresistive, but the industry favored slow yet cheap selenium devices. By the mid-1930s, selenium photocells controlled assembly lines, elevators and looms. Fire alarms with selenium sensors came into mass production in the UK and then in the US. Norbert Wiener proposed, and Truman Gray built an optical scanner for inputting and processing data in analog computers. Kurt Kramer introduced a selenium photocell to medical research. In 1940, Glenn Millikan built the first practical selenium-based oximeter to monitor the physical condition of the Royal Air Force pilots. It was a RO where the light source and detector were separated by the ear lobe of the pilot.

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