UNIT 9    ALARM SYSTEMS

Alarm systems are forms of electric-electronic circuits  designed in conjunction with the detectors or sensors to detect someone entering home illegally with the sound of bell produced to inform everyone.

In this unit, alarm system is a burglar alarm. The block diagram of alarm system  can be represented as follows.

TASK 1: To prevent car crime, the government in England requires three following groups to do the works so that the situation is improved as well as possible.

Group

Action

Motor manufactures

To fit better locks

Car owners

To be more vigillant about car thief

Insurance companies

To offer lower premiums for motorists who install alarm systems

TASK 2:

1.     What does this diagram show a circuit of?

An alarm system                          

2.     How many detection devices does it show? Name them.

Four: door switch, window foil, pressure mat, passive infra-red detector

3.     What warning device does it show?

A bell

4.     Why is the control box switch operated with a key?

So that the alarm system cannot be switched off by burglars.

5.     How does the system work?

When any switch in sense circuit opens, the bell rings.

6.     What problem is there with this circuit?

This circuit has following disadvantages:

First, the controll circuit is provided by the AC mains supply.

Second, if the controll box is put outside the house, it may be destroyed or tampered.

Third, in case of the alarm system is switched on, somone going outside home will make bell ring.

TASK 3: Describe  the detection devices or sensor in the alarm system installed at home such as door switch, window foil, pressure mat, passive infra-red detector in your own knowledge

Door Switch is a magnetic switch including a permanent magnet mounted on the moving part of door and a switch fitted on the frame. When the door closed, the switch is closed, i.e, its contacts complete the sense circuit. Inversely, when the door opens, the switch is in -OFF state, i.e, its contacts disconnect the sense circuit and after that the bell rings to warn a stranger appearing.

Window Foil is classified as break detectors used to detect the break of very thin metal layer glued around the edge of glass in window. Normally, window foil is considered as normally_closed contact and hence it plays a role to complete the sense circuit. When someone hits the glass of window to enter home, the broken glass makes the foil break. In other words, it becomes normally–open contact and this states cause the bell to ring.

Pressure Mat is a form of pressure switch operating under the activation of a force on it. Normally, it’s mounted under a carpet in front of a door on the way to go into a room. It’s a normally-closed switch with the absence of a pressure placed on the carpet. When someon steps onto it, its contacts open, and following that the alarm system is set off. Due to the carpet walked times in time, its contacts because ‘tired’, i.e, its elastic fearture is no longer. In this case, it’s replaced by the same switch.

Passive Infra-red Detector is known as a motion sensor used to sense someone moving in the home in the range of infra-red beam emitted in a limit of distance. If nobody is present in the room, this detection devicd causes its contacts to close. But when somebody passes its infra-red rays, it trips a switch to cut-off the sense circuit to set off the alarm system.

TASK4 : How could the systemshown 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 feartures.

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 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 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 disconects the mains supply.

TASK 5:

Action

è

consequence

(device)

A burglar disconnects the supply         

The arlam continues to operate on batteries.

The glass is broken

The foil breaks too.

A door is opened

A magnet on the door trips a switch on the frame.

A window is opened

A magnet on the moving part trips a switch.

You’re not there to reset the system

The alarm stops after the set time.

A burglar tries to force the alarm open

Tamper sensors trigger the alarm.

Someone steps on them

Two thin metal plates come in contact.

If someone moves in a room under the infra-red rays scanned by the infra-red detector, this sensor makes the sense circuit open and the alarm system triggers the audio warning to operate in a set time.

In case of a burglar entering home illegally by disconnecting the wires of the mains power supply, the battery back up operates to connect the alarm system to the secondary batteries available, i.e, the power supply provides for the alarm system continuously in time.

TASK 6: Complete these sentences with a suitable action or consequence:

1.     If pressure mats are constantly walked on, their contact points made of two thin plates are no longer elastic, i.e, they become ‘tired’ very quickly or in other words, they cannot come in good contact when someone steps upon the mats.

2.     If you fit an exit delay, someone inside home or room going out makes the alarm system without enough time to operate.

3.     If your system doesn’t have an automatic cut-off, the audio warning operates continuosly and this causes noise pollution for everyone in home.

4.     If a burglar walks in front of a motion sensor, the switch in the sensing circuit opens and triggers the alarm system so that the audio warning produces sound enough large heard by everyone surrounding home.

5.     The vibration sensor will respond if there’s a shock in the glass window hit by a burglar to enter home.

6.     Tamper sensors will trigger the alarm if the control box isn’t opened by a key, i.e, someone tries to open it by tools suchas screwwdrivers, hammers, etc

7.     A magnet on the moving part trips a switch if the door in closing state is opened at any moment and without re-closing at once.

8.     The alarm stops after a set time if an automatic cut-off is installed to prevent the audio warning from operating long-lastly.

TASK 8: Word study                           

1.     integrate circuit

2.     circuit diagram

3.     alternating current

4.     primary cell

5.     zenner diode

6.     remote control

7.     reed switch

8.     surface wave

9.     vibration sensor

10.                        reverse bias

11.                        mains supply

TASK 9:

1.      

Sensing device

Used to detect

LDR

Light

Thermistor

Heat

Microphone

Sound

What’s LDR?

LDR is a sensing device used to detect light means of its resistance decreasing under the effect of light falling on it.

2.     What effect does light have on an LDR?

Light has the effect to cause LDR to reduce its resistance from high to low values

3.     What’s the purpose of RV1 in Fig. 2 on page 53?

The rheostat RV1 in Fig. 2 is used to adjust the sensitivity of LDR after a long _lasting.

4.     Use words from the text to complete the following table:

Term                        

Opposite

Cut-off

Saturation

Fixed resistor

Variable resistor

Increases

Decreases

Energize

De-energize

Slow

Rapid

To cause

To prevent

Forward bias

Reverse bias

5.     How is the transistor in Fig. 2 protected from a large back EMF?

The transistor in Fig. 2 is protected against the very large back EMF  produced by transistor from ON to OFF states by mean of connecting a diode in reverse bias in parallel to the coil of relay.

TASK 10:

A reed switch is a magnetic switch constructed by two contacts controlled by a permanent magnet, they are made of ferromagnetic material so that they can be magnetized and de-magnetize easily. To protect them against the dust or explosive medium, a glass envelope is used to contain them. In addition, to avoid the oxidation in process of opening and closing the contacts, the nitrogen is filled inside the envelope. The operation of reed switch depends on the distance between it and a magnet. In particular, when a magnet approaches the switch, the contacts make due to magnetization. Inversely, when a magnet moves far away from the switch, the contacts break beacause of de-magnetization.

TASK 11:

A door-alarm circuit is a very common and simple electric circuit used to detect a burglar entering home through a main door. It consists of two permanent magnets, a nine-volt battery, an alarm bell or a buzzer a single pole single throw switch and reed switch. These components are installed as follows. A movable magnet is mounted in the door frame next to the reed switch also placed there. The circiut includes a reed switch, an alarm bell or a buzzer, a battery and a switch connected inseries together. To begin the operation of circuit, the switch is closed by hand. When the door is closed, the movable magnet comes near the fixed magnet, due to the arrangement with the oppsite north and south poles between two magnets, the magnetic fields produces by them remove or cancel each other. As a result, the contacts of reed switch break and the alarm circuit is de-energized. In other words, no sound is emitted by an alarm bell.

In case of the door opened by a stranger, the distance between two magnet is large enough so that their magnetizm cannot interact together. As a consequence, under the effect of the fixed magnet, the contacts of reed switch make. At once, the alarm circuit is energized and at the same time the alarm bell produces. Sound as a signal to remind everyone inside home ready to react the situation.