[64] The series connection provides a greater controlling range (−80 dB) at DC and low frequencies. Max. [88] 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. However, for an incandescent lamp it is on the order of hundreds of milliseconds, which limits the frequency range of the respective ROs to a few Hz. The incandescent lamp of the RO was connected to the output of the operational amplifier, and the photoresistor was part of a voltage divider in the feedback circuit of a non-inverting amplifier. The frequency response of such circuit is similar to that of the series connection. Cadmium-based photoresistors exhibit a "memory effect": their resistance depends on the illumination history; it also drifts during the illumination and stabilizes within hours,[6] or even weeks for high-sensitivity models. This increased the switching speed, but not to the level required for digital devices. This ratio increases with decreasing light intensity and has a value of 1.5–1.6 at 0.1 lux and 1.05–1.10 at 1000 lux for PerkinElmer devices. [50] It also shows characteristic overshoots, with the values reaching a temporary minimum or maximum after application of light. Depending on the output voltage, the gain of the circuit varied from 1:1 to 1:10. [50] In some low-resistive RO models this ratio was as high as 5.5,[50] but by 2009 their production has been discontinued. [38] Their high dark resistance, reaching tens GOhm,[38] provides a high dynamic range with respect to the light intensity and low signal distortions. [40] At low illumination levels, the resistance of cadmium-based ROs increases by about 1% upon heating by 1 °C. Therefore, ROs were mostly replaced in the 1970s by the faster and more stable photodiodes and photoresistors. However, the exemption is limited in time, since the Commission considers that the research for cadmium-free technology is in progress and substitutes could become available by the end of 2013. “Photoresistor is a variable resistor whose resistance varies inversely with the intensity of light” From our basic knowledge about the relationship between resistivity (ability to resist the flow of electrons) and conductivity (ability to allow the flow of electrons), we know that both are polar opposites of each other. Light resistance: Irradiated by 400-600Lux light for two hours, then test with 10Lux under [25][26], In the early 1960s, Gibson and Fender started using ROs to modulate the tremolo effect in guitar amplifiers. After the 1950s selenium in photocells was gradually replaced by CdS and CdSe. These distortions are dominated by the second harmonic. The operating voltage of this circuit is 6 volts, you can use four 1.5 volts batteries of any size to make this circuit work. Therefore, the introduction of the faster photodiodes and phototransistors in the 1970s pushed out ROs from the market. It was a RO where the light source and detector were separated by the ear lobe of the pilot. [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. The reference photoresistor is included in a divider of stabilized voltage or in a measuring bridge. The maximum output frequency (controlled signal) is limited by the parasitic capacitance of a RO, which originates from the electrodes formed on the surface of the photoresistor and shunts the output circuit. 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. Both companies were assembling their ROs from discrete lamps, photoresistors and coupling tubes. [61], Cooling below −25 °C sharply increases the response time of a photoresistor. [70][71], In the Soviet Union, ROs were used for signal compression in long-distance telephony. [40] 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. [38], CdSe photoresistors are 5–100 times more sensitive than CdS devices;[38] their sensitivity peaks in the red to near-infrared region (670–850 nm) and extends up to 1100 nm. [19] Norbert Wiener proposed, and Truman Gray built an optical scanner for inputting and processing data in analog computers. [74], ROs produced by the General Electric are used in AC voltage stabilizers. This limitation is lifted by connecting two shunt dividers in series, which keeps the transfer characteristic smooth. [39] These devices can control a few mA, and have a quasilinear dependence of photocurrent on the light intensity at a constant voltage. 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. [40] However, their reaction time to a change in the light intensity is long, about 140 ms at 25 °C. The narrowing of the dynamic range due to the parasitic capacitance is significant at frequencies as low as hundreds of Hz. [27] While Gibson used cheap but slow incandescent lamps as light sources, Fender replaced them with neon lamps, which increased the maximum frequency to tens of Hz and reduced controlling currents, but resulted in a nonlinear modulation. In a typical ham radio setup, the vactrol is placed in the termination box at the farthest point of the antenna. [62], Highly resistive ROs can be operated at AC voltages exceeding 200 V and used as low-power AC or DC relays, e.g., to control electroluminescent indicators. 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. [44] 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.[40]. [86][87] Tuning with vactrols improves directional cardioid pattern of the antenna. [9] In 1918, American and German engineers independently suggested the use of vacuum photocells for reading optical phonograms in the film projectors in cinemas,[10] and Lee de Forest, Western Electric and General Electric produced three competing systems using such photocells. [18] Fire alarms with selenium sensors came into mass production in the UK and then in the US. power dissipation at ambient temperature of 25°C.At higher ambient temperature,the maximum power permissible may be lowered. These resistivity changes affect the RO temperature, bringing additional instabilities. [79] However, in the same year they abandoned ROs in favor of field-effect transistors.[80]. The reaction is significantly faster to the increase than decrease in the controlling current. [24][25] ROs retained narrow application niches in sound equipment and some industrial automation devices that did not require high speeds. [84][85], Vactrols have been used as remote-controlled resistors for precise ground termination of Beverage and Ewe type antennae. The emission spectrum depends on the temperature of LED, and thus on the current, but this variation is too small to affect the spectral matching of the LED and photoresistor. Dark resistance: Refer to the resistance ten seconds after the 10Lux light is shut up. Korg used conventional Sallen-Key filters where ROs were replaced by inversed bipolar transistors. [40][47] At higher light intensities, the thermal coefficient of resistance can change its values and even its sign.

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