Resistive opto-isolator

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[2] and have a voltage of several hundred volts across it.

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.

[13] Various types of photocells were considered: vacuum, gas-discharge, photovoltaic and photoresistive,[14] but the industry favored slow[15] yet cheap selenium devices.

[19] Norbert Wiener proposed, and Truman Gray built an optical scanner for inputting and processing data in analog computers.

In 1940, Glenn Millikan built the first practical selenium-based oximeter to monitor the physical condition of the Royal Air Force pilots.

[21][22] In the early 1950s, Teletronix used the "T4" optical attenuator in the LA-2 compressor, furthermore, for their unique sound, they are still used today by Universal Audio in their reproductions of the LA-2.

By 1960, ROs based on incandescent lamps and CdS/CdSe photoresistors were used in feedback circuits in the industry, for example, for controlling rotation speed and voltage.

In the early 1960s, the introduction of sensitive and compact CdS/CdSe photoresistors resulted in the mass production of cameras with automatic exposure.

[23][24] However, these photoresistors were not adopted in medicine because of their memory effect and rapid aging[24] – they required regular recalibration that was not acceptable for medical practice.

[24][25] ROs retained narrow application niches in sound equipment and some industrial automation devices that did not require high speeds.

[30][31] Vactec did not extend their rights to the trademark Vactrol,[29] and it has become a household word in the English language for any RO used in audio equipment,[32] including the ROs of Fender and Gibson.

[33] As of 2010, the Vactrol ROs were produced by PerkinElmer, the successor of Vactec until its illumination and detection solutions businesses were divested as an independent company Excelitas Technologies in November 2010.

The initial version of the EU Directive on the restriction of hazardous substances (RoHS), adopted in 2003, allowed the use of cadmium in the devices that had no Cd-free counterparts.

[40] However, in 2009 the European Commission excluded Cd-based ROs used in professional audio equipment from the list of permitted devices.

[49] The glue also functions as a diffuser spreading the light beam – if it would fall near the border of the semiconductor and electrical contacts then a slight shift of the LED position might significantly alter the RO response.

On the contrary, for incandescent lamps the light-current curve is not linear, and the emission spectrum varies with temperature, and thus with the input current.

The equivalent circuit of the photoresistor consists of three components: Because of the large value of RD, the total resistance is mainly determined by RI.

[45] ROs based on incandescent lamps typically fail after about 20,000 hours, due to the burnout of the spiral, and are more prone to overheating.

[69] In the simplest output-limiting circuits, the RO is placed in the top (series connection) or lower (shunt) arm of the voltage divider.

The narrowing of the dynamic range due to the parasitic capacitance is significant at frequencies as low as hundreds of Hz.

[72] The best combination of a smooth transfer characteristic, low distortion, wide range of adjustment, and nearly equal rates of increase and decrease of the transmission coefficient is achieved in a series-parallel circuit composed of two ROs and a series resistor.

The latter compensation requires a second (reference) photoresistor, which is illuminated by the same light intensity, at the same temperature as the main (modulating) device.

The error amplifier compares the voltage at the middle point of the divider with the target value and adjusts the controlling current.

[86] RO is a simple and convenient tool for tuning the frequency of oscillators, filters and amplifiers in analog synthesizers.

Particularly simple is their implementation in the voltage-controlled RC filters in a Sallen-Key topology, where the RO provides a nearly exponential dependence of the cutoff frequency on the controlling current, without using feedback by the modulation signal.

[87] However, due to the slow response of ROs, most synthesizer developers of the 1970s and 1980s, such as ARP, Korg, Moog and Roland, preferred other elements.

[88] A still popular use for ROs are Lowpass Gates like the 292 by Buchla Electronic Musical Instruments, the Plan B Model 13 [89] and Make Noise MMG.

[90] Series connection of an LED[notes 4] and a low-resistance photoresistor make the RO a trigger (memory cell) which can be controlled by current pulses.

[95] In this simple layout, the LED or bulb of the vactrol is prone to damage by voltage surges induced by lightning, and must be protected by a pair of neon lamps acting as gas dischargers.