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Heat Detector

Detector

In general, something that reacts to stimuli in a set manner, and is either part of a living being, or made by a living being, for the purpose of doing such reacting. Such reactions include sending a signal on for further processing, or activating a reaction that has either evolved, or been designed, to aid a living being. In telecommunications, the term detector has the following meanings: #A device that is responsive to the presence or absence of a stimulus. #In an AM radio receiver, a circuit or device that recovers the signal of interest from the modulated wave. #In FM reception, a circuit called a discriminator is used to convert frequency variations to amplitude variations. #In an optical communications receiver, a device that converts the received optical signal to another form. ''Note:'' Currently, this conversion is from optical to electrical power; however, optical-to-optical techniques are under development. :Source: from Federal Standard 1037C In astronomy, the detecting devices generally used is charge-coupled devices (CCD), although, before the 1990s, photographic plates were the most common. In experimental particle physics, a particle detector is a device used to track and identify elementary particles. Category:Detectors Category:Communication circuits Category:Radio electronics

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Metal Detector

Metal detectors use electromagnetic induction to detect metal. In 1881, Alexander Graham Bell constructed a crude metal detector in an attempt to find an assassin's bullet in President James Garfield. Gerhard Fischar patented a portable version in 1931. Upright "archway" detectors are used at entrances to secured buildings, such as courthouses or airports, to detect metallic weapons which may be brought in. Small portable "wand" detectors are used by security staff to frisk persons for the same. Larger portable metal detectors are used by treasure hunters to locate metallic items, such as jewelry or coins, buried shallowly underground. There are three types of metal detectors: beat frequency oscillator, induction balance, and pulse induction.

In a beat frequency oscillator detector, a coil is used as an inductor in an oscillator, whose frequency changes when metal causes its inductance to change. Another oscillator produces a close frequency, and audible beats between them signal metal.

In an induction balance detector, there are two coils, usually gibbous with about 10% overlap, and a sine wave is transmitted with one coil and received with the other. The coils are adjusted so that there is no signal in the receive coil when there is no metal nearby.

In a pulse induction detector, a pulse is generated (usually by cutting off an inductor) and sent through a coil and the detector listens for echoes.

Metal detectors and archaeology

The use of metal detectors to search for archaeological finds is practised both by archaeologists and hobbyists. In some European countries including France and Sweden the use of a metal detector is forbidden by law, unless one has special permission. This is intended to protect archaeological sites but rarely means that illicit metal detecting ('nighthawking') does not take place and has the effect that new sites found by metal detector are never publicised or investigated fully. Instead they are slowly plundered

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Bullshit Detector

''Bullshit Detector'' was the name of a series of compilation LPs put together by the punk band Crass and released on their Crass Records label. Three editions were released between 1980 and 1984, consisting of demo tapes, rough recordings and artwork that had been sent to the band. The sound quality of the ''Bullshit Detector'' series was mixed, and was often very basic or poor as Crass would master the tapes directly to record without any additional production or enhancement. For Crass, the expectation of a polished performance was missing the point of the DIY punk ethic: :''"Don't expect music when the melody is anger, when the message sings defiance, three chords are frustration when the words are from the heart"'' - Sleeve notes from ''Bullshit Detector Volume 3'' The title ''Bullshit Detector'' was derived as a reference to the Clash song "Garageland" from their first LP ''The Clash''. That reference, in turn, may be taken from a quotation attributed to Ernest Hemingway: "The most essential gift for a good writer is a built-in, shock-proof, bullshit detector." A fourth ''Bullshit Detector'', this time an international compilation, was released by the Swiss independent anarcho-punk record label Resistance Productions in 1994. See also: Cassette culture

Category:Crass albums Category:1980 albums Category:1982 albums Category:1984 albums Category:1994 albums Category:Anarcho-punk

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Smoke Detector

A smoke detector (sometimes called a "smoke alarm") is a safety device that detects airborne smoke and issues an audible alarm, thereby alerting nearby people to the danger of fire. Most smoke detectors work either by optical detection or by ionization, but some of them use both detection methods to increase sensitivity to smoke.

History

In 1902 George Andrew Darby, an electrical engineer of 211 Bloomsbury Street, Birmingham, England, patented the electrical Heat-Indicator and Fire Alarm. The device indicated any change of temperature in the apartment where it was fixed. The device operated by closing an electrical circuit to sound an alarm if the temperature raised above the safe limit. The contact was made by bridging a gap with a conductor, or allowing one plate to fall on another: this movement was caused simply by a block of butter which melted as the temperature raised. This early device subsequently gave way to more modern fire and eventually smoke alarms.

Optical detector

thumb|250px|Optical Smoke Detector
1: optical chamber
2: cover
3: case moulding
4: photodiode (detector)
5: infrared LEDThis type of detector includes a light source, a lens to collimate the light into a beam, and a photodiode or other photoelectric sensor at right-angles to the beam. In the absence of smoke, the light passes in front of the detector but does not fall on it. When visible smoke enters the beam, some light is scattered by the smoke particles, and some of the scattered light is detected by the sensor. An increased output from the sensor sets off the alarm.

Ionization detector

This type of detector is cheaper than the optical detector, and can detect particles of smoke that are too small to be visible. It includes a tiny mass of radioactive americium-241, which is a source of alpha radiation. The radiation passes through an ionization chamber, which is an air-filled space between two electrodes, and permits

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Product Detector

A product detector is a type of demodulator used for AM and SSB signals. Rather than converting the envelope of the signal into the decoded waveform like an envelope detector, the product detector takes the product of the modulated signal and a local oscillator, hence the name. Product detectors can be designed to accept either IF or RF frequency inputs. A product detector which accepts an IF signal would be used as a demodulator block in a superheterodyne receiver, and a detector designed for RF can be combined with an RF amplifier and a low-pass filter into a direct-conversion receiver.

A simple product detector

The simplest form of product detector multiplies an incoming signal by its carrier to produce a copy of the original message and another AM signal at twice the original carrier frequency. This high-frequency component can then be filtered out leaving the original signal.

Mathematical model of the simple product detector

If ''m''(''t'') is the original message, the AM signal can be shown by :

x(t)=(C+m(t))\cos(\omega t).

Multiplying the AM signal ''x''(''t'') by an oscillator at the same frequency as and in phase with the carrier yields :

y(t)=(C+m(t))\cos(\omega t)\cos(\omega t),

which can be re-written as :

y(t)=(C+m(t))(1/2 + 1/2\cos(2\omega t)).

After filtering out the high-frequency component based around cos(2ω''t'') and the DC component C, the original message will be recovered.

Drawbacks of the simple product detector

Although this simple detector works, it has two major drawbacks:

The frequency of the local oscillator must be the same as the frequency of the carrier, or else the output message will fade in and out in the case of AM, or be frequency shifted in the case of SSB

Once the frequency is matched, the phase of carrier must be obtained, or else the demodulated message will be attenuated, but the noise will not be. Frequency of an AM carrier

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Heat Detector