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A security alarm[1] is a system designed to detect intrusion unauthorized entry into a building or area. Security alarms are used in residential, commercial, industrial, and military properties for protection against burglary (theft) or property damage, as well as personal protection against intruders. Car alarms likewise protect vehicles and their contents. Prisons also use security systems for control of inmates.
Some alarm systems serve a single purpose of burglary protection; combination systems provide both fire and intrusion protection. Intrusion alarm systems may also be combined with closed-circuit television surveillance systems to automatically record the activities of intruders, and may interface to access control systems for electrically locked doors. Systems range from small, self-contained noisemakers, to complicated, multi-area systems with computer monitoring and control.
The most basic alarm consists of one or more sensors to detect intruders, and an alerting device to indicate the intrusion. However, a typical premises security alarm employs the following components:
In addition to the system itself, security alarms are often coupled with a monitoring service. In the event of an alarm, the premises control unit contacts a central monitoring station. Operators at the station see the signal and take appropriate action, such as contacting property owners, notifying police, or dispatching private security forces. Such signals may be transmitted via dedicated alarm circuits, telephone lines, or Internet.
The passive infrared (PIR) motion detector is one of the most common sensors found in household and small business environments. It offers affordable and reliable functionality. The term passive refers to the fact that the detector does not generate or radiate its own energy; it works entirely by detecting the heat energy given off by other objects.
Strictly speaking, PIR sensors do not detect motion; rather, they detect abrupt changes in temperature at a given point. As an intruder walks in front of the sensor, the temperature at that point will rise from room temperature to body temperature, and then back again. This quick change triggers the detection.
PIR sensors may be designed to be wall or ceiling mounted, and come in various fields of view, from narrow "point" detectors to 360 degree fields. PIRs require a power supply in addition to the detection signalling circuit.
Using frequencies between 15kHz and 75kHz, these active detectors transmit ultrasonic sound waves that are inaudible to humans. The Doppler shift principle is the underlying method of operation, in which a change in frequency is detected due to object motion. This is caused when the object must cause a change in the ultrasonic frequency to the receiver relative to the transmitting frequency.
The ultrasonic detector operates by the transmitter emitting an ultrasonic signal into the area to be protected. The sound waves are reflected by solid objects (such as the surrounding floor, walls and ceiling) and then detected by the receiver. Because ultrasonic waves are transmitted through air, then hard-surfaced objects tend to reflect most of the ultrasonic energy, while soft surfaces tend to absorb most energy.
When the surfaces are stationary, the frequency of the waves detected by the receiver will be equal to the transmitted frequency. However, a change in frequency will occur as a result of the Doppler principle, when a person or object is moving towards or away from the detector. Such an event initiates an alarm signal. This technology is considered obsolete by many alarm professionals, and is not actively installed.
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Security alarm - Wikipedia, the free encyclopedia
Q. I've been dutifully checking my smoke detectors with the "push to test" buttons for many years. I'm intrigued by the idea of testing them with actual smoke. I tried it with a match once, but the alarm didn't go off. Should it have, or do I need much more smoke and/or heat?
A. I tried using a smoky flame some time ago, wrote about it, and received an angry note instructing me never, ever to use an open flame to test a smoke detector. The writer suggested I buy a little gadget that spews out carbon dioxide. You could do the same. It sounds as if your alarms are old, so I suggest you buy new ones, both smoke and carbon monoxide.
Regarding testing, the Massachusetts Department of Fire Services says not to use a lighted match and that the following steps are sufficient:
-- Testing your smoke detectors monthly.
-- Replacing alkaline batteries twice a year, when you change your clocks, even if your detectors are hard-wired.
-- Replacing your smoke detector every 10 years (the sensors wear out) and your carbon monoxide ones every five to seven years.
Q. Would you like to read how I solved a very serious situation: the leaky foundation of our house, particularly after it rains? Our house is on a slope going down from front to back. Water started seeping through a few places in the stone foundation into the basement on the front during heavy rains. Three years ago, I saw puddles against the foundation in the front. Years of raking leaves away from the house had put a depression in the ground. I put earth and compost against the foundation to create a berm.
A. You solved a significant problem with a simple answer. I don't think you made a berm, but rather an apron that diverts rainwater away from the foundation so that it seeps into the ground harmlessly.
Write to Peter Hotton at photton@globe.com.
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Handyman: Real flame to test smoke detector? Don't try it
Resurfacing of Center Street in Grayslake and the second stage of Peterson Road reconstruction are slated to move forward in the next construction season after the Lake County Board approved funding for the projects last week.
Center Street, a 1.4-mile stretch between Routes 45 and 83, part of which runs past the former Lake County Fairgrounds, will be resurfaced with paved shoulders in areas without curb and gutter. With the $2.23 million appropriation, approved by the board at its Nov. 11 meeting, the project is now cleared to head out for bids.
Grayslake is also eyeing Center Street for improvements east of the villages business district, with a resurfacing project from Route 83 to the Canadian National Railway tracks now in the design phase.
The board also approved a total of $22.8 million in appropriations including $16.2 million from the countys quarter-percent sales tax for transportation improvements for the ongoing widening of Peterson Road near Libertyville and Grayslake.
Work began earlier this year on a widening of Peterson from Route 45 to a point just west of Route 83, a project that includes intersection improvements at not only Route 83 but Midlothian Road. The funds approved last week are projected to cover the reconstruction and widening of Peterson from Route 83 to Allegheny Road.
The scope of that second phase also includes installation of a bike path on the north side of Peterson and resurfacing of nearly the entire length of Allegheny a 2.2 mile stretch from Peterson north to Hojem Lane, near Belvidere Road. As with the Center Street project, the Peterson project is now set to take bids.
In other transportation business at its Nov. 11 meeting, the board approved the following measures:
Appropriation of $1.8 million for the reconstruction and widening of Petite Lake Road from Fairfield Road to Route 59 near Lake Villa.
Appropriation of $295,000 in county transportation sales-tax funds to improve Metras Grayslake North Central station platform in preparation for the Washington Street underpass project just to the south. That overall $35 million project, which includes widening of Washington to four lanes west of Lake Street, is scheduled to start next spring and continue for two full construction seasons.
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Center Street, Peterson Road lead list of county road projects
