Ir diode
IR
LED
transm
buffer amplifier
itter
_ audio oscillator
Fig. 7
5 To obtain these pulsed signals, electrical pulses are first generated at a frequency in the upper audio range by the audio oscillator in the transmitter. They are then amplified by the buffer amplifier to enable them to drive the light-emitting diode (LED). Finally, the electrical pulses are converted by the LED into pulsed infra-red radiation which
Ю is directed at the receiver. Almost no visible light is emitted from the LED.
Now fill in the gaps in this diagram with the help of the text which follows.
Components Function
IC1
R1,R2 and C2 and amplifier
The circuit diagram of a remote control transmitter is shown in Fig. 2.
Fig. 2
In this circuit, both the oscillator and amplifier are combined in a single chip integrated circuit (IC1). The frequency of the oscillator is 15 set by the external timing components R1, R2, and C2. The current output of the amplifier is controlled by the resistor R3 which is connected in series with the LED. Decoupling of the DC supply from the pulsed output of the 1С is provided by the electrolytic capacitor C1
Task 1 Listen to this radio news item and complete the table below.
The Government's three-pronged attack on car crime aims to persuade:
Group
motor manufacturers
car owners
insurance companies
Action
to fit
to be more vigilant about
to offer lower premiums for motorists who install
Task 2 Study the diagram at the top of the following page. Try to answer these questions:
1 What does this diagram show a circuit of?
2 How many detection devices does it show? Name them.
What warning device does it show?
Why is the control box switch operated with a key?
How does the system work?
What problem is there with this circuit?
Task
3 Work
in groups of four. Find out how one of these devices works by
reading the
appropriate paragraph in this text. Your teacher will tell you which device to read about. Then explain briefly to your group how the device works.
door switch
window foil
pressure mat
passive infra-red detector
Detection devices Magnetic switches
These are used on windows and doors. A magnet mounted on the moving part of the window or door trips a switch mounted on the frame when the window or door is opened.
Break detectors
These are fitted on the inside surface of glass in windows and doors. 5 Some use a thin metal foil which is glued around the edge of the glass: if the glass is broken the foil breaks too. Others are vibration sensors, and respond to the shock of the glass being broken.
Pressure mats
These are fitted under the carpet-at the bottom of the stairs, for example. The pressure of someone stepping on them causes two thin 10 metal plates inside to come in contact, setting off the alarm. Because they're constantly being walked on, pressure mats can get 'tired' quite quickly, and should be regularly tested and replaced if necessary.
Motion sensors
These may use passive infra-red, ultrasonic, or microwave energy to detect movement within their range.
Task 4
How could the system shown in Task 2 be improved? Note your ideas. Now read this text to check which of your ideas are described.
We can make the simple alarm circuit more effective by including some of these features.
Entry and exit delays These mean you won't set off the alarm when leaving or returning to the house. On the better systems, the delays 5 are adjustable. An audible warning during the delay period is a useful reminder.
Automatic cut-off This will stop the alarm after it has sounded for a set time, so that the noise doesn't go on for hours if you're not there to reset the system. In better systems, the alarm automatically resets at 10 the end of the alarm time.
Tamper protection The control panel incorporates sensors which will trigger the alarm if a burglar tries to force the box open.
Battery backup This means the alarm will continue to work in a power cut, or if an intruder disconnects the mains supply.
Task 5 Match each action with its consequence. Then identify the device or feature
described. For example:
Action: Someone moves within its range. Consequence: It detects the movement. Device = motion sensor
Action
A burglar disconnects the supply.
The glass is broken.
A door is opened.
A window is opened.
You're not there to reset the system.
A burglar tries to force the alarm open.
Someone steps on them.
Consequence
a A magnet on the moving part
trips a switch, b Tamper sensors trigger the alarm, с The alarm continues to operate
on batteries, d Two thin metal plates come in
contact, e The foil breaks too. f A magnet on the door trips a
switch on the frame, g The alarm stops after a set time.
Language study If-sentences
Study this action and its consequence:
Action: A burglar tries to force the alarm open.
Consequence: Sensors trigger the alarm.
We can link action and consequence like this:
If a burglar tries to force the alarm open, sensors trigger the alarm.
If a burglar tries to force the alarm open, sensors will trigger the alarm.
Sensors will trigger the alarm if a burglar tries to force it open.
Task 6 Complete these sentences with a suitable action or consequence.
If pressure mats are constantly walked on,
If you fit an exit delay,
If your system doesn't have an automatic cut-off,