Cat's whisker diode - AbsoluteAstronomy.com

A cat’s whisker detector (sometimes called a crystal detector) is an antique electronic component

Electronic component

An electronic component is a basic electronic element and may be available in a discrete form having two or more electrical terminals . These are intended to be connected together, usually by soldering to a printed circuit board, in order to create an electronic circuit with a particular function...

consisting of a thin wire

Wire

A wire is a single, usually cylindrical, flexible strand or rod of metal. Wires are used to bear mechanical loads and to carry electricity and telecommunications signals. Wire is commonly formed by drawing the metal through a hole in a die or draw plate. Standard sizes are determined by various...

that lightly touches a crystal of semiconducting

Semiconductor

A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...

mineral to make a crude point-contact rectifier

Rectifier

A rectifier is an electrical device that converts alternating current , which periodically reverses direction, to direct current , which flows in only one direction. The process is known as rectification...

. Developed by early radio researchers Jagadish Chandra Bose, G. W. Pickard

Greenleaf Whittier Pickard

Greenleaf Whittier Pickard was a United States radio pioneer. Pickard was a researcher in the early days of wireless. He experimented with crystal detectors, antennas, wave propagation, and noise suppression...

and others, this device was used as the detector

Detector (radio)

A detector is a device that recovers information of interest contained in a modulated wave. The term dates from the early days of radio when all transmissions were in Morse code, and it was only necessary to detect the presence of a radio wave using a device such as a coherer without necessarily...

in early crystal radios, from about 1906 through the Second World War

World War II

World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

. It gave this type of radio receiver its name. It was the first type of semiconductor diode, and in fact the first semiconductor electronic device

Semiconductor device

Semiconductor devices are electronic components that exploit the electronic properties of semiconductor materials, principally silicon, germanium, and gallium arsenide, as well as organic semiconductors. Semiconductor devices have replaced thermionic devices in most applications...

. The term cat's whisker was also sometimes used to describe the crystal receiver itself. Cat's whisker detectors are obsolete and are now only used in antique or antique-reproduction radios.

Description

The wire touching the surface of the crystal formed a crude and unstable metal

Metal

A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

–semiconductor

Semiconductor

or p-n point-contact junction, forming a Schottky barrier diode

Schottky diode

The Schottky diode is a semiconductor diode with a low forward voltage drop and a very fast switching action...

. This junction conducts electric current

Electric current

Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...

in only one direction and resists current flowing in the other direction. In a crystal radio, its function was to rectify

Rectification

Rectification has the following technical meanings:* Rectification, in astrology* Rectification , a concept found in biology and industrial chemistry* Chinese history: see Cheng Feng...

the radio signal, converting it from alternating current

Alternating current

In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....

to a pulsing direct current

Direct current

Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...

, to extract the audio signal

Audio signal

An audio signal is an analog representation of sound, typically as an electrical voltage. Audio signals may be synthesized directly, or may originate at a transducer such as a microphone, musical instrument pickup, phonograph cartridge, or tape head. Loudspeakers or headphones convert an electrical...

(modulation

Modulation

In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...

) from the radio frequency

Radio frequency

Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...

carrier wave

Carrier wave

In telecommunications, a carrier wave or carrier is a waveform that is modulated with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal...

.

Only certain sites on the crystal surface functioned as rectifying junctions

P-n junction

A p–n junction is formed at the boundary between a P-type and N-type semiconductor created in a single crystal of semiconductor by doping, for example by ion implantation, diffusion of dopants, or by epitaxy .If two separate pieces of material were used, this would...

. Thus, the device was very sensitive to the exact geometry and pressure of contact between wire and crystal. Therefore it was made adjustable, and a usable point of contact was found by trial and error before each use. The wire was suspended from a moveable arm, and was dragged across the crystal face by the operator until the device began functioning. In a crystal radio, the operator would tune the radio to a strong local station if possible, and then adjust the cat's whisker until the station or static sounds was heard in the radio's earphones. This required some skill and a great deal of patience; even then a good contact could easily be lost by the slightest vibration. An alternate method of adjustment was to use a battery-operated buzzer

Buzzer

A buzzer or beeper is an audio signaling device, which may be mechanical, electromechanical, or piezoelectric. Typical uses of buzzers and beepers include alarm devices, timers and confirmation of user input such as a mouse click or keystroke....

to generate a test signal. The spark at the buzzer's contacts functioned as a weak radio transmitter, so when the crystal began functioning the buzz could be heard in the earphones, and the buzzer was turned off. The temperamental, unreliable action of the crystal detector was a barrier to its acceptance as a standard component in commercial radio equipment, and was one reason for its rapid replacement by vacuum tube

Vacuum tube

In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...

s after 1920. Frederick Seitz, a later semiconductor researcher, wrote:

Such variability, bordering on what seemed the mystical, plagued the early history of crystal detectors and caused many of the vacuum tube experts of a later generation to regard the art of crystal rectification as being close to disreputable.

Crystal

A natural mineral

Mineral

A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...

crystal

Crystal

A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...

forms the semiconductor side of the junction. The most common crystal used was galena

Galena

Galena is the natural mineral form of lead sulfide. It is the most important lead ore mineral.Galena is one of the most abundant and widely distributed sulfide minerals. It crystallizes in the cubic crystal system often showing octahedral forms...

(PbS, lead sulfide

Lead sulfide

Lead sulfide is an ionic compound of lead and sulfur, having two possible proportions:*Lead sulfide, the ionic compound containing lead in the +2 oxidation state*Lead sulfide, the ionic compound containing lead in the +4 oxidation state...

), a naturally occurring ore of lead

Lead

Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...

. Galena is a semiconductor with a small bandgap of about 0.4 eV, and is used without treatment directly as it is mined. Galena with good detecting properties is rare and has no reliable visual characteristics distinguishing it from galena samples with poor detecting properties. A rough pebble of detecting mineral about the size of a pea was mounted in a metal cup, which formed one side of the circuit. The electrical contact between the cup and the crystal had to be good, because this contact must not act as a second rectifying junction. To make good contact with the crystal, it was either clamped with setscrews or mounted in solder

Solder

Solder is a fusible metal alloy used to join together metal workpieces and having a melting point below that of the workpiece.Soft solder is what is most often thought of when solder or soldering are mentioned and it typically has a melting range of . It is commonly used in electronics and...

. Because the relatively high temperature of tin-lead solder

Solder

can damage many crystals, a low melting point (well under 200°F) alloy such as Wood's metal

Wood's metal

Wood's metal, also known as Lipowitz's alloy or by the commercial names cerrobend, bendalloy, pewtalloy or MCP 158, is a eutectic, fusible alloy with a melting point of approximately . It is a eutectic alloy of 50% bismuth, 26.7% lead, 13.3% tin, and 10% cadmium by weight. It...

was used. One surface was left exposed to allow contact with the cat's whisker wire.

Whisker

The "cat's whisker", a springy piece of thin metal wire, formed the metal side of the junction. Phosphor bronze

Phosphor bronze

Phosphor bronze is an alloy of copper with 3.5 to 10% of tin and a significant phosphorus content of up to 1%. The phosphorus is added as deoxidizing agent during melting....

wire of about 30 gauge

Wire gauge

Wire gauge is a measurement of how large a wire is, either in diameter or cross sectional area. This determines the amount of electric current a wire can safely carry, as well as its electrical resistance and weight per unit of length...

was commonly used because it had the right amount of springiness. It was mounted on an adjustable arm with an insulated handle so that the entire exposed surface of the crystal could be probed from many directions to try to find the most sensitive spot. Cat's whiskers in simple detectors were straight or curved, but most professional cat's whiskers had a coiled section in the middle that served as a spring. The crystal required just the right gentle pressure by the wire; too much pressure caused the device to conduct in both directions. Precision detectors often used a metal needle instead of a cat's whisker, mounted on a thumbscrew-operated leaf spring to adjust the pressure applied.

Types

Historically, many other minerals and compounds besides galena were used for the crystal, the most important being iron pyrite ("fool's gold", iron disulfide), silicon

Silicon

Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...

, molybdenite

Molybdenite

Molybdenite is a mineral of molybdenum disulfide, MoS2. Similar in appearance and feel to graphite, molybdenite has a lubricating effect that is a consequence of its layered structure. The atomic structure consists of a sheet of molybdenum atoms sandwiched between sheets of sulfur atoms...

(MoS₂), and silicon carbide

Silicon carbide

Silicon carbide , also known as carborundum, is a compound of silicon and carbon with chemical formula SiC. It occurs in nature as the extremely rare mineral moissanite. Silicon carbide powder has been mass-produced since 1893 for use as an abrasive...

(carborundum, SiC). Some were used with gold or graphite

Graphite

The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...

"cat's whiskers". Another type had a crystal-to-crystal junction instead of a "cat's whisker", with two crystals mounted facing each other. One crystal was moved forward on an adjustable mount until the crystal faces touched. The most common of these was a zincite

Zincite

Zincite is the mineral form of zinc oxide . Its crystal form is rare in nature; a notable exception to this is at the Franklin and Sterling Hill Mines in New Jersey, an area also famed for its many fluorescent minerals. It has a hexagonal crystal structure and a color that depends on the presence...

-bornite

Bornite

Bornite is a sulfide mineral with chemical composition Cu5FeS4 that crystallizes in the orthorhombic system .-Appearance:Bornite has a brown to copper-red color on fresh surfaces that tarnishes to various iridescent shades of blue to purple in places...

(ZnO-Cu₅FeS₄) junction trade-named Perikon, but zincite-chalcopyrite, silicon-arsenic and silicon-antimony junctions were also used. The goal of researchers was to find junctions that were not as sensitive to vibration and unreliable as galena and pyrite. Some of these other junctions, particularly carborundum, were stable enough that they used a more permanent spring-loaded contact rather than a "cat's whisker". For this reason carborundum detectors were preferred in large commercial wireless stations, and military and shipboard stations which were subject to vibration from waves and gunnery exercises. Another quality desired was the ability to withstand high currents without damage, because in wireless stations the fragile detector junction could be "burned out" by atmospheric electric charge from the antenna, or high radio frequency current leaking into the receiver from the powerful spark-gap transmitter

Spark-gap transmitter

A spark-gap transmitter is a device for generating radio frequency electromagnetic waves using a spark gap.These devices served as the transmitters for most wireless telegraphy systems for the first three decades of radio and the first demonstrations of practical radio were carried out using them...

during transmissions. Carborundum detectors, which used large area contacts, were also particularly robust in this regard.

To increase sensitivity, some of these junctions such as silicon carbide were "biased" by connecting a battery and potentiometer

Potentiometer

A potentiometer , informally, a pot, is a three-terminal resistor with a sliding contact that forms an adjustable voltage divider. If only two terminals are used , it acts as a variable resistor or rheostat. Potentiometers are commonly used to control electrical devices such as volume controls on...

across them to provide a small constant forward voltage across the junction.

The oxide layers that form on many ordinary metal surfaces have semiconducting properties, and detectors for crystal radios have been improvised from a variety of everyday objects such as rusty needles and corroded pennies. The foxhole radio was a crystal radio receiver improvised by soldiers during World War II

World War II

without access to conventional sets. It used a razor blade and a safety pin or lead pencil to form a demodulating junction. Much patience was required to find an active detecting site on the blade. Stray rectifying junctions between metal parts of radio transmitter installations are still a source for interference, because they can radiate harmonics of the transmitter frequency.

History

Unlike modern radio stations that transmit sound, the first radio transmitters during the first three decades of radio transmitted information by telegraphy

Telegraphy

Telegraphy is the long-distance transmission of messages via some form of signalling technology. Telegraphy requires messages to be converted to a code which is known to both sender and receiver...

, turning the transmitter on and off with a switch called a telegraph key

Telegraph key

Telegraph key is a general term for any switching device used primarily to send Morse code. Similar keys are used for all forms of manual telegraphy, such as in electrical telegraph and radio telegraphy.- Types of keys :...

to spell out messages in Morse code

Morse code

Morse code is a method of transmitting textual information as a series of on-off tones, lights, or clicks that can be directly understood by a skilled listener or observer without special equipment...

, consisting of "dots" and "dashes". So early radio receiving apparatus merely had to detect the presence or absence of the radio signal, not convert it into audio. The device that did this was called a detector

Detector (radio)

. The crystal detector was the most successful of many detector devices that were used in the early days of radio. It replaced electrolytic, magnetic, and particularly coherer

Coherer

The coherer was a primitive form of radio signal detector used in the first radio receivers during the wireless telegraphy era at the beginning of the twentieth century. Invented around 1890 by French scientist Édouard Branly, it consisted of a tube or capsule containing two electrodes spaced a...

detectors in radio receivers around 1906. Later, when AM

Amplitude modulation

Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...

radio transmission was developed to transmit sound, around World War 1, crystal detectors proved able to receive this as well.

The "unilateral conduction" of crystals, as it was then called, was discovered by Ferdinand Braun, a German physicist in 1874 at the University of Würzburg

University of Würzburg

The University of Würzburg is a university in Würzburg, Germany, founded in 1402. The university is a member of the distinguished Coimbra Group.-Name:...

, before radio had been invented. Based on this work G.W. Pickard

Greenleaf Whittier Pickard

developed the cat's whisker diode using a silicon crystal, which was patented in 1906. However, Bengali scientist Jagadish Chandra Bose was the first to use a crystal to detect radio waves, in his experiments with microwaves in 1894, applying for a patent on a galena detector in 1901.

When these devices were in common use, more advanced proprietary versions of "permanent" detectors were developed, many of them by G. W. Pickard, who tested more than 30,000 combinations of crystal and wire contacts. One consisted of various combinations of pairs of different crystals such as Zincite

Zincite

touching Bornite

Bornite

or Chalcopyrite

Chalcopyrite

Chalcopyrite is a copper iron sulfide mineral that crystallizes in the tetragonal system. It has the chemical composition CuFeS2. It has a brassy to golden yellow color and a hardness of 3.5 to 4 on the Mohs scale. Its streak is diagnostic as green tinged black.On exposure to air, chalcopyrite...

, in fairly heavily spring-loaded contact. Pickard named this the Perikon detector, from "PERfect pIcKard cONtact". Other detectors patented by Pickard included the common crystal iron pyrite. Pickard has the distinction of having brought silicon into use as a detector, patenting it in 1906. At nearly the same time, General Henry Harrison Chase Dunwoody patented the use of the silicon carbide

Silicon carbide

(carborundum) detector, an artificial substance created accidentally during attempts by Edward Acheson to create diamonds.

Unamplified

Amplifier

Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...

radio receivers, most of which were crystal radios, were the only way to receive radio signals during most of the wireless telegraphy

Wireless telegraphy

Wireless telegraphy is a historical term used today to apply to early radio telegraph communications techniques and practices, particularly those used during the first three decades of radio before the term radio came into use....

era, which ended around 1920. Mineral

Mineral

detectors were largely superseded by vacuum tube

Vacuum tube

s, invented in 1906, although the expense of tube receivers meant that full replacement took several decades. By the 1920s crystal radios were relegated to use by hobbyists and youth groups.

The point-contact semiconductor detector was subsequently resurrected around World War II

World War II

because of the military requirement for microwave radar

Radar

Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...

detectors. Vacuum tube detectors do not work at microwave frequencies. The small area of the point contact minimized minority carrier storage and capacitance, making these diodes fast enough to function at radar frequencies. Silicon and germanium point contact diodes were developed. Wartime research on PN junctions in crystals paved the way for the invention of the transistor

Transistor

A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...

in 1956.(Transistor#History 1947? Silicon transistor 1954?) The first transistors also used cat's whisker contacts.

The germanium diodes that became widely available after the war proved to be as sensitive as galena and didn't require any adjustment, so they replaced cat's whisker detectors in the few crystal radios still being made, largely putting an end to the manufacture of this antique radio component. Although cat's whisker detectors are obsolete, modern point-contact silicon detectors are still commercially produced. Thus the point contact method used to make these first semiconductor diodes 100 years ago is still being used today.

Copies of original cat's-whisker detectors are still manufactured and sold, for antique radio hobbyists.

External links

Crystal and Solid Contact Rectifiers 1909 publication describes experiments to determine the means of rectification (PDF file)
Radio Detector Development from 1917 The Electrical Experimenter
The Crystal Experimenters Handbook 1922 London publication devoted to point contact diode detectors. (PDF file courtesy of Lorne Clark via earlywireless.com)

Patents

- Means for receiving intelligence communicated by electric waves (silicon detector), Greenleaf Whittier Pickard, 1906 - Wireless telegraph system (silicon carbide detector), Henry H.C. Dunwoody, 1906 - Oscillation detector (multiple metallic sulfide detectors), Clifford D. Babcock, 1908 - Oscillation detector and rectifier ("plated" silicon carbide detector with DC bias), G.W. Pickard, 1909 - Oscillation receiver (fractured surface red zinc oxide (zincite) detector), G.W. Pickard, 1909 - Oscillation device (iron pyrite detector), G.W. Pickard, 1909 - Oscillation detectors (paired dissimilar minerals), G.W. Pickard, 1914

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.

Description

Crystal

Whisker

Types

History

See also

External links

Patents