Environmental Engineering Reference
In-Depth Information
and wastewater utilities, these devices are often used to link remote moni-
toring stations (e.g., SCADA components) or portable computers to computer
networks without using physical wiring connections.
Electronic Controllers
An electronic controller is a piece of electronic equipment that receives
incoming electric signals and uses preprogrammed logic to generate elec-
tronic output signals based on the incoming signals. Electronic controllers
can be implemented for any application that involves inputs and outputs
(such as to control equipment in a factory), but in a security application they
essentially act as the system's brain and can respond to specific security-
related inputs with preprogrammed output responses. These systems com-
bine the control of electronic circuitry with a logic function such that circuits
are opened and closed (and equipment is turned on and off) through some
preprogrammed logic. The basic principle behind the operation of an electri-
cal controller is that it receives electronic inputs from sensors or any device
generating an electrical signal (e.g., electrical signals from motion sensors)
and then uses its preprogrammed logic to produce electrical outputs (e.g.,
these outputs could turn on power to a surveillance camera or an audible
alarm). Thus, these systems automatically generate a preprogrammed, logi-
cal response to a preprogrammed input scenario.
The three major types of electronic controllers are timers, electrome-
chanical relays, and programmable logic controllers (PLCs), which are
often called
digital relays
. Timers use internal signals or inputs (in contrast
to externally generated inputs) to generate electronic output signals at cer-
tain times. More specifically, timers control electric current flow to any
application to which they are connected and can turn the current on or
off on a schedule prespecified by the user. Typical timer range (amount of
time that can be programmed to elapse before the timer activates linked
equipment) is from 0.2 seconds to 10 hours, although some of the more
advanced timers have ranges of up to 60 hours. Timers are useful in fixed
applications that do not require frequent schedule changes; for example, a
timer can be used to turn on the lights in a room or a building at a certain
time every day. Timers are usually connected to their own power supply
(usually 120 to 240 V).
In contrast to timers, which have internal triggers operating on a regular
schedule, electromechanical relays and PLCs have both external inputs and
external outputs; however, PLCs are more flexible and more powerful than
electromechanical relays and are the predominant technology for security-
related electronic control applications. Electromechanical relays are simple
devices that use a magnetic field to control a switch. Voltage applied to the
input coil of the relay creates a magnetic field that attracts an internal metal
switch. This causes the contacts of the relay to touch, thus closing the switch
