Keeping Watch

Most monitoring systems consist of radio transmitters and receivers. Some systems employ lasers at IR or visible wavelengths. A few employ wire or cable links. Some have mechanical hardware, such as robots, that are controlled by the signals reaching the receiver. Here are a few specific examples of monitoring systems.

Baby Monitor

A short-range AM or FM radio transmitter and receiver can be used to listen at a distance to the sounds in an infant’s room. The transmitter contains a sensitive microphone, a whip antenna, and a power supply. The receiver is battery-powered and portable. It has a short antenna, similar to the antennas on cordless telephone sets. The receiver can pick up signals from the transmitter at distances of up to about 50 m (165 ft). The signals pass easily through the walls, ceilings, and floors in frame houses.
 
A so-called baby monitor is subject to interference from other units that might be operating nearby on the same channel. Some baby monitors have multiple, selectable channels to help combat this problem. If interference occurs, the channel can be changed. Communications privacy and security are not a concern.

Smoke Detector

Smoke and fire change the characteristics of the atmosphere. Smoke consists of solid particles, and fire burns away oxygen and produces other gases such as carbon dioxide, carbon monoxide, and sulfur dioxide. These changes can be sensed, and alarms set off if the changes exceed certain limits.
 
A smoke detector senses changes in two characteristics of the air: the dielectric constant (an expression of the extent to which pollutants increase the capacitance of the air) and the ionization potential (an expression of the extent to which pollutants change the voltage necessary to produce a spark that jumps through the air). Smoke and fire almost always affect one or both of these parameters.
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Simplified functional diagram of a smoke detector
Above figure is a functional diagram of a smoke detector that operates by sensing changes in the dielectric constant of the air. Two electrically charged plates are positioned a small distance apart. The plates, and the air between them, form a capacitor. A source of dc is connected to the plates. Normally, the plates retain a constant charge, and the current in the circuit is zero. If the dielectric constant of the air increases, the capacitance changes, causing a small, momentary electric current to flow. This current can be detected, and the resulting signal sets off an alarm. The signal can also actuate a robotic system, such as a group of water sprinklers.

Quality Control

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A method of quality control (QC) for manufactured items
Lasers are useful in industrial monitoring and control applications. An example is the quality control (QC) checking of bottles for height as they move along an assembly line. A laser/robot combination can find and remove bottles that are not of the correct height. The principle is shown in above figure. If a bottle is too short, both laser beams reach the photodetectors. If a bottle is too tall, neither laser beam reaches the photodetectors. In either of these situations, a robot arm, equipped with a gripper, picks the faulty bottle off the line and discards it. Only when a bottle is within a narrow range of heights (the acceptable range) does the top laser reach its photodetector while the bottom laser is blocked. Then the bottle is allowed to pass.
 
Of course, proper operation of the QC machine shown in above figure depends on the reliability of the lasers and photodetectors. If, for example, the lower laser burns out and there is no way for the system operator to know about it, the machine will pass all bottles, whether or not they are of the correct height.

Tracking People

Suppose that a person is sentenced to house arrest. Compliance can be monitored by having the person carry a conventional beeper (pager). A police or probation officer can page the person at random times; the person must then call the officer within a couple of minutes. The call can be traced, and the location of the telephone verified. This is a simple method of electronically tracking the whereabouts of a person.
 
A more secure method of ascertaining that a person is at a certain place, at a certain time, is by means of a short-range radio transmitter and receiver. The person wears the transmitting unit. Tamper-proof receiving units are placed at the convict’s home, in the car, and at the place of work. The transmitter range is similar to that of a baby monitor. Receiver signals are sent to a central monitoring point. The signals are encoded so the monitoring personnel (or computers) know whether the person is at home, in the car, or at work. Any deviation from normal patterns can be detected.
 
Radiolocation provides another way to keep track of people. A transponder can be carried or worn by the person to be tracked; continuous signals can be sent to the unit asking for a position fix, and the unit can respond through a wireless network such as the cellular telephone system.

Electronic Bug

An electronic bug consists of a tiny radio transmitter that can be hidden in a room, placed in a shirt pocket, or planted in a car. The antenna is a length of thin, almost invisible wire. A receiver can be located nearby. The device operates at a low RF power level (on the order of a few milliwatts) to con serve battery energy. If the transmitter is near a wireless repeater that connects into a larger system, eavesdropping can be done anywhere within the coverage of the wireless system. With the advent of low-earth-orbit (LEO) satellite systems, it is theoretically possible to bug a room on the other side of the world.
 
Wireless electronic bugs can be detected by means of a device called an RF field-strength meter. This instrument consists of a microammeter connected to a short whip antenna through a semiconductor diode. The diode rectifies the RF signal, producing dc that shows up as an indication on the meter. When the meter is close to the bug, the current rises. When the bug is within a few centimeters of the meter, the needle may go to full-scale.
 
The presence of RF fields in your house does not necessarily mean you are being bugged. Many appliances and electronic devices produce RF fields, including computers, radio-operated remotecontrol units, cell phones, cordless phone sets, and certain medical devices. Even radio receivers and TV sets emit some RF energy.

Electric Eye

The simplest device for detecting an unwanted visitor is an electric eye. Narrow beams of IR or visible light are shone across all reasonable points of entry, such as doorways and window openings. A photodetector receives energy from each beam. If, for any reason, the photodetector stops receiving its assigned beam, an alarm is actuated. A person breaking into a property cannot avoid interrupting at least one beam if every possible point of entry has a large enough number of electric eyes spaced at suitable intervals.

IR Motion Detector

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An IR motion detector
A common intrusion alarm device employs an IR motion detector. Two or three wide-angle IR pulses are transmitted at regular intervals; these pulses cover most of the room in which the device is installed. A receiving transducer picks up the returned IR energy, normally reflected from the walls, floor, ceiling, and furniture. The intensity of the received pulses is noted by a microprocessor. If anything in the room changes position, there is a change in the intensity of the received energy. The microprocessor detects this change and triggers an alarm (above figure). This type of device consumes very little power in regular operation, so batteries can serve as the power source.

Radiant Heat Detector

Certain devices can detect changes in the indoor environment by directly sensing the IR energy (often called radiant heat) emanating from objects. Humans, and all warm-blooded animals, emit IR. So does fire. A simple IR sensor, in conjunction with a microprocessor, can detect rapid or large increases in the amount of radiant heat in a room. The time threshold can be set so that gradual or small changes will not trigger the alarm, while significant changes, such as are caused by a person entering the room, will trigger it. The temperature-change threshold can be set so that a small animal will not actuate the alarm, while a full-grown person will. This type of device, like the IR motion detector, can operate from batteries.
 
The main limitation of radiant-heat detectors is the fact that they can be fooled. False alarms are a risk. The sun, coming out on an overcast day, might suddenly shine directly on the sensor and trigger the alarm. It is also possible that a person clad in a winter jacket, thermal pants, insulated boots, hood, and face mask, entering from a cold outdoor environment, will fail to set off the alarm. For this reason, radiant-heat sensors are used more often as fire-alarm actuators than as intrusion detectors.

Ultrasonic Motion Detector

Motion in a room can be detected by sensing the changes in the relative phase of acoustic waves. An ultrasonic motion detector employs a set of transducers that emit acoustic waves at frequencies above the range of human hearing (higher than 20 kHz). Another set of transducers picks up the reflected acoustic waves, whose wavelength is on the order of a few millimeters. If anything in the room changes position, the relative phase of the waves, as received by the various acoustic pickups, will change. This data is sent to a microprocessor, which can trigger an alarm and/or notify the police.