The reed switch is an electrical switch operated by an applied magnetic field. The basic reed switch consists of two identical flattened ferromagnetic reeds, sealed in a dry inert-gas
atmosphere within a glass capsule, thereby protecting the contact from contamination. The reeds are sealed in the capsule in such a way that their free ends overlap and are separated by a small air gap.The contacts may be normally open, closing when a magnetic field is present, or normally closed and opening when a magnetic field is applied.A magnetic field from an
electromagnet or a permanent magnet will cause the contacts to pull together, thus completing an electrical circuit. The stiffness of the reeds causes them to separate, and open the circuit, when the magnetic field ceases. Good electrical contact is assured by plating a thin layer of precious metal over the flat contact portions of the reeds. Since the contacts of the reed switch are sealed away from the atmosphere, they are protected against atmospheric corrosion. The hermetic sealing of a reed switch makes them suitable for use in explosive atmospheres where tiny sparks from conventional switches would constitute a hazard. A reed sensor is a device built using a reed switch with additional functionality like ability to withstand higher shock, easier mounting, additional intelligent circuitry, etc.In production, a metal reed is inserted in each end of a glass tube and the end of the tube heated so that it seals around a shank portion on the reed.
Infrared-62 absorbing glass is used, so an infrared heat source can concentrate the heat in the small sealing zone of the glass tube. The thermal coefficient of expansion of the glass material and metal parts must be similar to prevent breaking the glass-to-metal seal. The glass used must have a high electrical resistance and must not contain volatile components such as lead oxide and fluorides.
The leads of the switch must be handled carefully to prevent breaking the glass envelope.
4. IR Sensor:
INTRODUCTION TO IR
One of the advantages of infrared remote is that there is no radio signal for crooks to monitor and record for use against you later on. Instead, there is a beam of invisible infrared light which comes from a standard hand-held remote control unit. So from that point of view, it is pretty secure. There is, though, an enormous variety of tasks to which you could put the unit. Just think of the myriad of things in your home these days which use infrared remote to turn things on and off, change levels, open and close.
WHAT IS INFRARED?
InfraRed is a energy radiation with a frequency below our eyes sensitivity, so we can not see it. Even that we can not "see" sound frequencies, we know that it exist, we can listen them.
Even that we can not see or hear infrared, we can feel it at our skin temperature sensors.
When you approach your hand to fire or warm element, you will "feel" the heat, but you can't see it. You can see the fire because it emits other types of radiation, visible to your eyes, but it also emits lots of infrared that you can only feel in your skin.
63 INFRARED IN ELECTRONICS
Infra-Red is interesting, because it is easily generated and doesn't suffer electromagnetic interference, so it is nicely used to communication and control, but it is not perfect, some other light emissions could contains infrared as well, and that can interfere in this communication. The sun is an example, since it emits a wide spectrum or radiation. The adventure of using lots of infra-red in TV/VCR remote controls and other applications, brought infra-red diodes (emitter and receivers) at very low cost at the market.
From now on you should think as infrared as just a "red" light. This light can means something to the receiver, the "on or off" radiation can transmit different meanings. Lots of things can generate infrared, anything that radiate heat do it, including out body, lamps, stove, oven, friction your hands together, even the hot water at the faucet.
To allow a good communication using infra-red, and avoid those "fake" signals, it is imperative to use a "key" that can tell the receiver what is the real data transmitted and what is fake. As an analogy, looking eye naked to the night sky you can see hundreds of stars, but you can spot easily a far away airplane just by its flashing strobe light. That strobe light is the "key", the "coding" element that alerts us.
Similar to the airplane at the night sky, our TV room may have hundreds of tinny IR sources, our body, the lamps around, even the hot cup of tea. A way to avoid all those other sources, is generating a key, like the flashing airplane. So, remote controls use to pulsate its infrared in a certain frequency. The IR receiver module at the TV, VCR or stereo "tunes" to this certain frequency and ignores all other IR received. The best frequency for the job is between 30 and 60kHz, the most used is around 36kHz
So, remote controls use the 36kHz (or around) to transmit information. InfraRed light emitted by IR Diodes is pulsated at 36 thousand times per second, when transmitting logic level "1" and silence for "0".
IR GENERATION
64 To generate a 36kHz pulsating infrared is quite easy, more difficult is to receive and identify this frequency. This is why some companies produce infrared receives, that contains the filters, decoding circuits and the output shaper, that delivers a square wave, meaning the existence or not of the 36kHz incoming pulsating infrared.
It means that those 3 dollars small units, have an output pin that goes high (+5V) when there is a pulsating 36kHz infrared in front of it, and zero volts when there is not this radiation.
A square wave of approximately 27uS (microseconds) injected at the base of a transistor, can drive an infrared LED to transmit this pulsating light wave. Upon its presence, the commercial receiver will switch its output to high level (+5V).
If you can turn on and off this frequency at the transmitter, your receiver's output will indicate when the transmitter is on or off.
Those IR demodulators have inverted logic at its output, when a burst of IR is sensed it drives its output to low level, meaning logic level = 1.
The TV, VCR, and Audio equipment manufacturers for long use infra-red at their remote
65 controls. To avoid a Philips remote control to change channels in a Panasonic TV, they use different codification at the infrared, even that all of them use basically the same transmitted frequency, from 36 to 50kHz. So, all of them use a different combination of bits or how to code the transmitted data to avoid interference.
RC-5
Various remote control systems are used in electronic equipment today. The RC5 control protocol is one of the most popular and is widely used to control numerous home appliances, entertainment systems and some industrial applications including utility consumption remote meter reading, contact-less apparatus control, telemetry data transmission, and car security systems. Philips originally invented this protocol and virtually all Philips‘ remotes use this protocol. Following is a description of the RC5. When the
user pushes a button on the hand-held remote, the device is activated and sends modulated infrared light to transmit the command. The remote separates command data into packets. Each data packet consists of a 14-bit data word, which is repeated if the user continues to push the remote button. The data packet structure is as follows:
� 2 start bits, remote button and is intended to differentiate situations when the user continues to hold the same button or presses it again. The next 5 bits are the address bits and select the destination device. A number of devices can use RC5 at the same time. To exclude possible interference, each must use a different address. The 6 command bits describe the actual command. As a result, a RC5 transmitter can send the 2048 unique commands. The transmitter shifts the data word, applies Manchester encoding and passes the created one-bit sequence to a control carrier frequency signal amplitude modulator. The amplitude modulated carrier signal is sent to the optical
66 transmitter, which radiates the infrared light. In RC5 systems the carrier frequency has been set to 36 kHz. Figure below displays the RC5 protocol
The receiver performs the reverse function. The photo detector converts optical transmission into electric signals, filters it and executes amplitude demodulation. The receiver output bit stream can be used to decode the RC5 data word. This operation is done by the microprocessor typically, but complete hardware implementations are present on the market as well. Single-die optical receivers are being mass produced by a number of companies such as Siemens, Temic, Sharp, Xiamen Hualian, Japanese Electric and others. Please note that the receiver output is inverted (log. 1 corresponds to illumination absence).
IR Receiver:
Description:
The TSOP17 – Series are miniaturized receivers for infrared remote control systems. PIN diode and preamplifier are assembled on lead frame, the epoxy package is designed as IR filter. The demodulated output signal can directly be decoded by a microprocessor.
TSOP1738 is the standard IR remote control receiver series, supporting all major transmission codes.
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Features
_ Photo detector and preamplifier in one package _ Internal filter for PCM frequency
_ Improved shielding against electrical field disturbance _ TTL and CMOS compatibility
_ Output active low
_ Low power consumption
_ High immunity against ambient light
_ Continuous data transmission possible (up to 2400 bps) _ Suitable burst length .10 cycles/burst
68 Introduction
Photo detectors are used primarily as an optical receiver to convert light into electricity. The principle that applies to photo detectors is the photoelectric effect, which is the effect on a circuit due to light. Max Planck In 1900 discovered that energy is radiated in small discrete units called quanta; he also discovered a universal constant of nature which is known as the Planck‘s constant. Planck‘s discoveries lead to a new form of physics known as quantum mechanics and the photoelectric effect E = hv which is Planck constant multiplied by the frequency of radiation. The photo electric effect is the effect of light on a surface of metal in a vacuum, the result is electrons being ejected from the surface this explains the principle theory of light energy that allows photo detectors to operate. Photo detectors are commonly used as safety devices in homes in the form of a smoke detector, also in conjunction with other optical devices to form security systems.
Photo detector:
A photo detector operates by converting light signals that hit the junction to a voltage or current.
The junction uses an illumination window with an anti-reflect coating to absorb the light photons. The result of the absorption of photons is the creation of electron-hole pairs in the depletion region. Examples of photo detectors are photodiodes and phototransistors. Other optical devices similar to photo detectors are solar cells which also absorb light and turn it into energy. A similar but different optical device is the LED which is basically the inverse of a photodiode, instead of converting light to a voltage or current, it converts a voltage or current to light.
Photodiodes:
A commonly used photo detector is the photodiode. A photodiode is based on a junction of oppositely doped regions (pn junction) in a sample of semiconductor. This creates a region depleted of charge carriers that results in high impedance. The high impedance allows the construction of detectors using silicon and germanium to operatewith high sensitivity at low temperatures. The photodiode functions using an illumination window (Figure 1), which allows
69 the use of light as an external input. Since light is used as an input, the diode is operated under reverse bias conditions. Under the reverse bias condition the current through the junction is zero when no light is present, this allows the diode to be used as a switch or relay when sufficient light is present.
Photodiodes are mainly made from gallium arsenide instead of silicon because silicon creates crystal lattice vibrations called phonons when photons are absorbed in order to create electron-hole pairs. Gallium arsenide can produce electron-electron-hole pairs without the slowly moving phonons; this allows faster switching between on and off states and GaAsalso is more sensitive to the light intensity. Once charge carriers are produced in the diode material, the carriers reach the junction by diffusion. Important parameters for the photodiode include quantum efficiency, current and capacitance which will be covered in the equations section.
PIN Photodiode
Another type of photodiode is the PIN photodiode; this photodiode includes an intrinsic layer in between the P and N type materials. The PIN must be reverse bias due to the high resistivity of the intrinsic layer; the PIN has a larger depletion region which allows more electron-hole pairs to develop at a lower capacitance. The illumination window for a PIN is on the P-side of the diode because the mobility of electrons is greater than holes which results in better frequency response.
The larger breakdown voltage in comparison to the PN photodiode allows it to be used with a biased voltage of approximately 100 which results in a fast response time by the equation below.
70 Avalanche Photodiode
An Avalanche photodiode is operated at reverse bias close to the breakdown, which causes photo excited charge carriers to accelerate in the depletion region and produce additional carriers by avalanching. The avalanche photodiodes are good for fiber optic systems that require low light levels with quantum efficiency larger than 100%.
Phototransistor
Phototransistor is similar to the photodiode except an additional n-type region is added to the photodiode configuration. The phototransistor includes a photodiode with an internal gain. A phototransistor can be represented as a bipolar transistor that is enclosed in a transparent case so that photons can reach the base-collector junction. The electrons that are generated by photons in the base-collector junction are injected into the base, and the current is then amplified. Since phototransistor detection is on the order of the photodiode they can not detect light any better than a photodiode. The draw back of a phototransistor is the slower response time in comparison to a photodiode. The figure below shows the relationship between a photodiode and phototransistor.
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