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Resistive
opto-isolator
An opto-isolator (also called an optocoupler, photocoupler, or optical isolator) is an electronic component that transfers electrical Signal, signals between two isolated circuits by using light. Opto-isolators prevent high voltages from affecti ...
(RO), also called photoresistive opto-isolator, vactrol (after a
genericized trademark
A generic trademark, also known as a genericized trademark or proprietary eponym, is a trademark or brand name that, because of its popularity or significance, has become the generic term for, or synonymous with, a general class of products or ...
introduced by Vactec, Inc. in the 1960s), analog opto-isolator
[In PerkinElmer literature.] or lamp-coupled photocell, is an
optoelectronic
Optoelectronics (or optronics) is the study and application of electronic devices and systems that find, detect and control light, usually considered a sub-field of photonics. In this context, ''light'' often includes invisible forms of radiatio ...
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
A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (cor ...
(LED), a miniature
incandescent lamp
An incandescent light bulb, incandescent lamp or incandescent light globe is an electric light with a wire filament heated until it glows. The filament is enclosed in a glass bulb with a vacuum or inert gas to protect the filament from oxid ...
, or sometimes a
neon lamp
A neon lamp (also neon glow lamp) is a miniature gas discharge lamp. The lamp typically consists of a small glass capsule that contains a mixture of neon and Penning mixture, other gases at a low pressure and two electrodes (an anode and a cold ...
, whereas the detector is a
semiconductor
A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...
-based
photoresistor
A photoresistor (also known as a photocell, or light-dependent resistor, LDR, or photo-conductive cell) is a passive component that decreases resistance with respect to receiving luminosity (light) on the component's sensitive surface. The elect ...
made of
cadmium selenide
Cadmium selenide is an inorganic compound with the formula Cadmium, CdSelenide, Se. It is a black to red-black solid that is classified as a II-VI semiconductor of the n-type semiconductor, n-type. Much of the current research on this compound i ...
(CdSe) or
cadmium sulfide
Cadmium sulfide is the inorganic compound with the formula CdS. Cadmium sulfide is a yellow solid.Egon Wiberg, Arnold Frederick Holleman (2001''Inorganic Chemistry'' Elsevier It occurs in nature with two different crystal structures as the rare m ...
(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
A photodiode is a light-sensitive semiconductor diode. It produces current when it absorbs photons.
The package of a photodiode allows light (or infrared or ultraviolet radiation, or X-rays) to reach the sensitive part of the device. The packag ...
and
phototransistor
A photodiode is a light-sensitive semiconductor diode. It produces current when it absorbs photons.
The package of a photodiode allows light (or infrared or ultraviolet radiation, or X-rays) to reach the sensitive part of the device. The packa ...
, 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
Parasitic capacitance is an unavoidable and usually unwanted capacitance that exists between the parts of an electronic component or circuit simply because of their proximity to each other. When two electrical conductors at different voltages a ...
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 photoresistors. 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
The photoelectric effect is the emission of electrons when electromagnetic radiation, such as light, hits a material. Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in condensed matter physics, and solid sta ...
in
vacuum tube
A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference has been applied.
The type kn ...
s 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
Lee de Forest (August 26, 1873 – June 30, 1961) was an American inventor and a fundamentally important early pioneer in electronics. He invented the first electronic device for controlling current flow; the three-element "Audion" triode va ...
,
Western Electric
The Western Electric Company was an American electrical engineering and manufacturing company officially founded in 1869. A wholly owned subsidiary of American Telephone & Telegraph for most of its lifespan, it served as the primary equipment ma ...
and
General Electric
General Electric Company (GE) is an American multinational conglomerate founded in 1892, and incorporated in New York state and headquartered in Boston. The company operated in sectors including healthcare, aviation, power, renewable energ ...
produced three competing systems using such photocells.
[ In 1927, the first commercial sound film, '']The Jazz Singer
''The Jazz Singer'' is a 1927 American musical drama film directed by Alan Crosland. It is the first feature-length motion picture with both synchronized recorded music score as well as lip-synchronous singing and speech (in several isolated ...
'', 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 ]loom
A loom is a device used to weave cloth and tapestry. The basic purpose of any loom is to hold the warp threads under tension to facilitate the interweaving of the weft threads. The precise shape of the loom and its mechanics may vary, but th ...
s. Fire alarms with selenium sensors came into mass production in the UK and then in the US. Norbert Wiener
Norbert Wiener (November 26, 1894 – March 18, 1964) was an American mathematician and philosopher. He was a professor of mathematics at the Massachusetts Institute of Technology (MIT). A child prodigy, Wiener later became an early researcher i ...
proposed, and Truman Gray Truman may refer to:
Media
* ''Truman'' (book), a biography of Harry S. Truman by David McCullough
* ''Truman'' (1995 film), 1995 film based on the book by McCullough
* ''Truman'' (2015 film), 2015 Spanish-Argentine film
People
* Truman (surname ...
built an optical scanner for inputting and processing data in analog computers. Kurt Kramer
Kurt is a male given name of Germanic or Turkish origin. ''Kurt'' or ''Curt'' originated as short forms of the Germanic Conrad, depending on geographical usage, with meanings including counselor or advisor.
In Turkish, Kurt means "Wolf" and i ...
introduced a selenium photocell to medical research. In 1940, Glenn Millikan built the first practical selenium-based oximeter
Pulse oximetry is a noninvasive method for monitoring a person's oxygen saturation. Peripheral oxygen saturation (SpO2) readings are typically within 2% accuracy (within 4% accuracy in 95% of cases) of the more accurate (and invasive) reading of ...
to monitor the physical condition of the Royal Air Force
The Royal Air Force (RAF) is the United Kingdom's air and space force. It was formed towards the end of the First World War on 1 April 1918, becoming the first independent air force in the world, by regrouping the Royal Flying Corps (RFC) and ...
pilots. It was a RO where the light source and detector were separated by the ear lobe
The human earlobe (''lobulus auriculae''), the lower portion of the outer ear, is composed of tough areolar and adipose connective tissues, lacking the firmness and elasticity of the rest of the auricle (the external structure of the ear). In ...
of the pilot.[
]
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. After the 1950s selenium in photocells was gradually replaced by CdS and CdSe. 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.[ However, these photoresistors were not adopted in medicine because of their memory effect and rapid aging][ – they required regular recalibration that was not acceptable for medical practice.][
In the early 1960s, ]Gibson
Gibson may refer to:
People
* Gibson (surname)
Businesses
* Gibson Brands, Inc., an American manufacturer of guitars, other musical instruments, and audio equipment
* Gibson Technology, and English automotive and motorsport company based
* Gi ...
and Fender started using ROs to modulate the tremolo effect in guitar amplifiers. Both companies were assembling their ROs from discrete lamps, photoresistors and coupling tubes. 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. Therefore, other producers preferred incandescent lamps for their linearity.
In 1967 Vactec introduced a compact RO branded as Vactrol.[ Unlike the tube-coupled ROs of Fender and Gibson, Vactrols were sealed and sturdy devices. In the early 1970s, Vactec replaced incandescent bulbs with LEDs. This increased the switching speed, but not to the level required for digital devices. Therefore, the introduction of the faster photodiodes and phototransistors in the 1970s pushed out ROs from the market.][ ROs retained narrow application niches in sound equipment and some industrial automation devices that did not require high speeds.][ Vactec did not extend their rights to the trademark Vactrol,][ and it has become a household word in the English language for any RO used in audio equipment, including the ROs of Fender and Gibson.][ 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. Excelitas ended the production of ROs in December 2015.
As of 2022 Vactrol type ROs are still manufactured by successor of Silonex the Advanced Photonix. and at least two factories in Shenzhen, China.
In the European Union, the production and distribution of Cd-based photoresistors is banned since January 1, 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.][ However, in 2009 the European Commission excluded Cd-based ROs used in professional audio equipment from the list of permitted devices.][ "From 2 January 2013, the use of cadmium is permitted in photoresistors for analogue optocouplers applied in professional audio equipment. ... 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."
]
Physical properties
Light sources, detectors and their coupling
Most ROs use CdS or CdSe as light-sensitive material.
The spectral sensitivity of CdS photoresistors peaks for red light (wavelength λ = 640 nm) and extends up to 900 nm. These devices can control a few mA, and have a quasilinear dependence of photocurrent on the light intensity at a constant voltage. Their high dark resistance, reaching tens GOhm, provides a high dynamic range with respect to the light intensity and low signal distortions.[ However, their reaction time to a change in the light intensity is long, about 140 ms at 25 °C.
CdSe photoresistors are 5–100 times more sensitive than CdS devices; their sensitivity peaks in the red to near-infrared region (670–850 nm) and extends up to 1100 nm. They have an inferior dynamic range and linearity than their CdS counterparts, but are faster, with a time constant of less than 20 ms.
The optimal light sources for CdS/CdSe photoresistors are AlGaAs heterostructures (emission wavelength ~660 nm) or GaP LEDs (λ = 697 nm). The luminosity of the LED is nearly proportional to the controlling current. 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.][
For mechanical stability, the LED and photoresistor are glued with a transparent ]epoxy
Epoxy is the family of basic components or cured end products of epoxy resins. Epoxy resins, also known as polyepoxides, are a class of reactive prepolymers and polymers which contain epoxide groups. The epoxide functional group is also coll ...
, glue or an organic polymer. 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.[
]
Transfer characteristic
The transfer characteristic of RO is usually represented as the electrical resistance of the photoresistor as a function of the current through the light source; it is a convolution of three main factors: the dependence of light source intensity on its current, optical coupling and spectral matching between the light source and photoresistor, and the photoresponse of the photoresistor. The first dependence is almost linear and temperature independent for LEDs in the practical range of the controlling current. 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. As to the light detector, its properties depend on temperature, voltage and the history of utilization (memory effect). Therefore, the transfer characteristic takes a range of values.
The equivalent circuit of the photoresistor consists of three components:
* RD – dark resistance, which is determined by the semiconductor and can range from a few MOhm to hundreds GOhm;[
*RRL – residual resistance of illuminated but non-loaded photoresistor, typically between 100 Ohm and 10 kOhm;][
*RI – ideal photoresistance, which is inversely proportional to the light intensity.
Because of the large value of RD, the total resistance is mainly determined by RI.][ The dynamic range of the photoresistor with respect to illumination is equal to the ratio of the critical illumination Φcr to the sensitivity threshold Φth.
RD and RI, but not RRL, decrease with increasing voltage that results in signal distortions.][ At low illumination levels, the resistance of cadmium-based ROs increases by about 1% upon heating by 1 °C.][ At higher light intensities, the thermal coefficient of resistance can change its values and even its sign.
]
Memory effect
Cadmium-based photoresistors exhibit a pronounced memory effect, that is, their resistance depends on the history of illumination. It also shows characteristic overshoots, with the values reaching a temporary minimum or maximum after application of light. These resistivity changes affect the RO temperature, bringing additional instabilities. The stabilization time nonlinearly increases with the light intensity and can vary between hours and days; by convention, it is assumed that the output of an illuminated RO reaches equilibrium within 24 hours.
The memory effect is evaluated using the ratio of Rmax to Rmin (see figure). 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. In some low-resistive RO models this ratio was as high as 5.5, but by 2009 their production has been discontinued.[ High-resistance photoresistors typically have a less pronounced memory effect, are less sensitive to temperature, and have a more linear response, but are also relatively slow. Some devices designed in the 1960s had a negligible memory effect, but exhibited unacceptably high signal distortions at high current levels.][
]
Operating frequencies
The operating frequency range of a RO depends on the input and output characteristics. The highest frequency of the input (controlling) signal is limited by the response of the RO light source to the change in the controlling current and by the response of the photoresistor to light; its typical value ranges between 1 and 250 Hz. The response time of a photoresistor to switching off of the light typically varies between 2.5 and 1000 ms, whereas the response to switching the illumination on is about 10 times faster. As to the light source, its reaction time to a current pulse is in the nanosecond range for an LED, and is therefore neglected. 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 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. A typical value of this capacitance is tens of picofarads that practically limits the output frequency to approximately 100 kHz.
Noise and signal distortions
As for ordinary resistors, the noise of photoresistors consists of thermal, shot and flicker noise; the thermal component dominates at frequencies above 10 kHz and brings a minor contribution at low frequencies. In practice, the noise of a photoresistor is neglected, if the voltage across its terminals is lower than 80 V.
Nonlinear distortions generated by the photoresistor are lower for a higher light intensity and for a lower resistance of the photoresistor.
If the voltage across the photoresistor does not exceed the threshold, which varies between 100 and 300 mV depending on the material, then the coefficient of nonlinear distortions has a value within 0.01%, which is almost independent of the voltage. These distortions are dominated by the second harmonic. Above the voltage threshold, the third harmonic appears, and the amplitude of the distortions increases as the square of the voltage. For a distortion of 0.1% (−80 dB), which is acceptable for high-fidelity sound equipment, the signal voltage should be within 500 mV. The ratio of even and odd harmonics can be controlled by applying a DC bias to the photoresistor.
Degradation
Irreversible degradation of a photoresistor can be induced by exceeding its maximum specified voltage even for a short period. For high-resistivity devices, this voltage is determined by the leakage currents flowing on the semiconductor surface and varies between 100 and 300 V for. For low-resistivity models, the voltage limit is lower and originates from the Joule heating.
The service life of a RO is determined by the lifetime of the light source and the acceptable drift of the parameters of the photoresistor. A typical LED can operate for 10,000 hours, after which its parameters slightly degrade. Its lifetime can be prolonged by limiting the controlling current to half of the maximum value.[ 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.][
Degradation of the photoresistor is gradual and irreversible. If the ]operating temperature
An operating temperature is the allowable temperature range of the local ambient environment at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the de ...
does not exceed the limit (typically 75 °C or less) then for each year of continuous operation, the dark resistance falls by 10%; at higher temperature such changes can occur within minutes. The maximum power dissipated in the photoresistor is usually specified for 25 °C and decreases by 2% for every °C of heating.
Cooling below −25 °C sharply increases the response time of a photoresistor. These changes are reversible unless the cooling induces cracking in the plastic components. Soviet ROs packed in metal cases could withstand even at −60 °C, but at these temperatures their response time reached 4 seconds.
Applications
AC relay
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.
Simple voltage dividers
In the simplest output-limiting circuits, the RO is placed in the top (series connection) or lower (shunt) arm of the voltage divider. The series connection provides a greater controlling range (−80 dB) at DC and low frequencies. The operation is complicated by the nonlinearity of the resistance vs. the controlling current. The narrowing of the dynamic range due to the parasitic capacitance is significant at frequencies as low as hundreds of Hz. The reaction is significantly faster to the increase than decrease in the controlling current.
The shunt connection results in smoother transfer characteristics and lower signal distortions, but also in a lower modulation range (−60 dB). This limitation is lifted by connecting two shunt dividers in series, which keeps the transfer characteristic smooth. 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 frequency response of such circuit is similar to that of the series connection.
Precision voltage dividers
Circuits with defined control voltage of the divider can compensate for the thermal drift of the LED in a RO, but not for the memory effect and thermal drift of the photoresistor. 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. Best compensation is achieved when both photoresistors are formed on the same semiconductor chip. The reference photoresistor is included in a divider of stabilized voltage or in a measuring bridge. The error amplifier compares the voltage at the middle point of the divider with the target value and adjusts the controlling current. In the linear controlling regime the RO becomes an analog multiplier: the current through the photoresistor is proportional to the product of the voltage across the photoresistor and the control voltage.
Automatic control circuits
In the Soviet Union, ROs were used for signal compression in long-distance telephony. 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. Depending on the output voltage, the gain of the circuit varied from 1:1 to 1:10. Similar circuits are still used in professional audio equipment (compressors, limiters and noise suppressors).[
ROs produced by the General Electric are used in AC voltage stabilizers. These stabilizers are based on an autotransformer that is controlled by two sets of ]thyristor
A thyristor () is a solid-state semiconductor device with four layers of alternating P- and N-type materials used for high-power applications. It acts exclusively as a bistable switch (or a latch), conducting when the gate receives a current ...
stacks. The incandescent lamp of RO is protected by a ballast resistor and is connected to the AC output. The lamp averages the output voltage, suppressing spikes and sine distortions originating from the main. The photoresistor of the RO is included in one arm of the measuring bridge, generating the error signal for the feedback loop.[
]
Guitar amplifiers
The first guitar amplifier with a tremolo effect was produced by Fender in 1955.[ In that amplifier, the tremolo generator controlled the bias of an amplifier cascade located near the output circuit, and its harmonics were leaking to the output signal.][ In the early 1960s, Fender and Gibson used a RO as a modulator. Its photoresistor was connected via a blocking capacitor and a controlling potentiometer between the output of the preamplifier and the earth, and shunted the preamplifier when triggered. In this scheme, the control signal did not leak to the output.][ Modulation depth was regulated by a low-impedance potentiometer placed on the front panel. The potentiometer significantly reduced the gain of the previous stage, and thus the preamplifier had to have reserves by amplification.
In their ROs, Gibson used incandescent lamps, which required relatively large currents. Fender replaced them with neon lamps, which increased the modulation frequency and reduced the controlling currents. However, in contrast to the continuous modulation by Gibson, Fender used the on/off switching mode that resulted in less pleasant sound. For this reason, other producers like Univibe preferred incandescent lamps.
By 1967 most producers of guitar amplifiers changed from vacuum tubes to transistors and therefore redesigned their circuits.][ For several years, Gibson continued to use ROs in transistor amplifiers for the tremolo effect.][ In 1973, they designed another RO-based control circuit, where a signal from a pedal or an external generator seamlessly connected a diode-based signal stabilizer.][ However, in the same year they abandoned ROs in favor of ]field-effect transistor
The field-effect transistor (FET) is a type of transistor that uses an electric field to control the flow of current in a semiconductor. FETs (JFETs or MOSFETs) are devices with three terminals: ''source'', ''gate'', and ''drain''. FETs contro ...
s.[
]
Analog synthesizers
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.[ 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.][Synthesizers of the 1970s–1980s often utilized changes in the dynamic resistance or/and inverse capacitance of p-n junctions, in diode-capacitance filters with a regulated positive feedback (Moog, APR). Korg used conventional Sallen-Key filters where ROs were replaced by inversed bipolar transistors.] As of August 2013 RO-based synthesizers are produced by Doepfer
Doepfer Musikelektronik GmbH is a German manufacturer of audio hardware, mostly synthesizer modules (modular synthesizer), based in Gräfelfing, Upper Bavaria and founded by Dieter Döpfer. The product range covers analog circuit, analog modular s ...
(Germany).[ A still popular use for ROs are Lowpass Gates like the 292 by ]Buchla Electronic Musical Instruments
Buchla Electronic Musical Instruments (BEMI) was a manufacturer of synthesizers and unique MIDI controllers. The origins of the company could be found in Buchla & Associates, created in 1963 by synthesizer pioneer Don Buchla of Berkeley, Californi ...
, the Plan B Model 13 and Make Noise MMG.[https://makenoisemusic.com/content/manuals/MMGmanual.pdf ]
Triggers
Series connection of an LED[ROs based on incandescent lamps are unsuitable for relays because of the combination of a high controlling current and high output resistance] and a low-resistance photoresistor make the RO a trigger (memory cell) which can be controlled by current pulses. In transparent ROs, the state of such cell can be monitored visually by the LED emission.[
]
Radio communication
Vactrols have been used as remote-controlled resistors for precise ground termination of Beverage
A drink or beverage is a liquid intended for human consumption. In addition to their basic function of satisfying thirst, drinks play important roles in human culture. Common types of drinks include plain drinking water, milk, juice, smoothies a ...
and Ewe type antenna
Antenna ( antennas or antennae) may refer to:
Science and engineering
* Antenna (radio), also known as an aerial, a transducer designed to transmit or receive electromagnetic (e.g., TV or radio) waves
* Antennae Galaxies, the name of two collid ...
s. In a typical ham radio
Amateur radio, also known as ham radio, is the use of the radio frequency spectrum for purposes of non-commercial exchange of messages, wireless experimentation, self-training, private recreation, radiosport, contesting, and emergency communic ...
setup, the vactrol is placed in the termination box at the farthest point of the antenna. The LDR modifies total resistance between antenna and ground (termination resistance); the operator fine-tunes this resistor from his radio shack
RadioShack, formerly RadioShack Corporation, is an American retailer founded in 1921.
At its peak in 1999, RadioShack operated over 8,000 worldwide stores named RadioShack or Tandy Electronics in the United States, Mexico, United Kingdom, Austra ...
by varying the vactrol's LED or bulb current with a potentiometer
A potentiometer is a three-terminal resistor with a sliding or rotating contact that forms an adjustable voltage divider. If only two terminals are used, one end and the wiper, it acts as a variable resistor or rheostat.
The measuring instrume ...
.[ Tuning with vactrols improves directional ]cardioid
In geometry, a cardioid () is a plane curve traced by a point on the perimeter of a circle that is rolling around a fixed circle of the same radius. It can also be defined as an epicycloid having a single cusp. It is also a type of sinusoidal spi ...
pattern of the antenna. According to Connelly, vactrols are superior to traditional resistors in this function.[ 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 lamp
A neon lamp (also neon glow lamp) is a miniature gas discharge lamp. The lamp typically consists of a small glass capsule that contains a mixture of neon and Penning mixture, other gases at a low pressure and two electrodes (an anode and a cold ...
s acting as gas dischargers.[
]
Notes
References
Bibliography
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{{refend
Optoelectronics
Safety engineering
Electrical safety
Electrical components
Solid state switches
Semiconductor devices
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