4 If a burglar walks in front of a motion sensor,

5. Vibration sensors will respond if

6. Tamper sensors will trigger the alarm if

7 A magnet on the moving part trips a switch if

8 The alarm stops after a set time if —

Task 7

Speaking practice

Work in pairs, A and B. Find out from your partner how to perform the tasks you have been set. Explain to your partner how to perform his/her tasks with the help of the diagrams provided.

Example:

Task: Operating a mercury switch.

Useful language:

How do you operate a mercury switch? You tilt it. How does a mercury switch work? By tilting it.

Student A: Your tasks are on page 175. Student B: Your tasks are on page 182.

Task 8

Word study Word pairs, 1

Each word in column A often goes before one word from column B. For example, integrated circuit (If). Find the other word pairs.

	A		В
1	integrated	a	sensor
2	circuit	b	cell
3	alternating	с	switch
4	primary	d	supply
5	zener	e	diode
6	remote	f	circuit
7	reed	g	current
8	surface	h	bias
9	vibration	i	control
10	reverse	J	diagram
11	mains	к	wave

Technical reading Alarm systems

Task 9 Use information from the text below to complete the tables/answer the

questions.

1 Complete this table.

Sensing device Used to detect

LDR

heat sound

2. What effect does light have on an LDR?

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

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

Term Opposite

cut-off saturation

fixed resistor

increases

energize

slow

to cause

forward bias

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

The three stages of a simple alarm system are shown in Fig. 1.

sensing device		electronic switch		output transducer
	w		—W

Fig. 1

The first stage is a sensing device that changes its resistance when it detects a particular form of energy. For example, a microphone may be used to detect sound, a thermistor to detect heat, or an LDR (light-5 dependent resistor) to detect light.

The second stage is an electronic switch. In its simplest form, this could be a single transistor. The transistor switches between cut-off and saturation as the input resistance changes.

The third stage is an output transducer which is switched off and on by 10 the electronic switch. The output transducer could be a buzzer, a light, or a relay which operates a more powerful circuit.

An example of a simple alarm circuit is shown in Fig. 2.

2N3053 r

BFY51

(j)0RP12 @] ^relaV

R8 1КП

_€)^2N_°

RFV

_RV1 v~ 500O.

г sets trigger level

6 + 6V

bell q-

Fig.2

The LDR forms a potential divider with the variable resistor RV1. When light falls on the LDR, its resistance decreases. This causes the base 15 voltage of the transistor and the bias current to increase. The

transistor switches on and there is a rapid rise in the collector current until the transistor goes into saturation. The increased current causes the relay to operate and switch on the output circuit. The sensitivity of the input can be adjusted using RV1.

20 In a similar way, the relay is de-energized when the light source is removed from the LDR. A large back EMF, which would destroy the transistor, could be generated across the relay. To prevent this, a diode is connected in reverse bias across the relay.

Writing Explanations

Task 10 Explanations provide answers to Why? and How? questions. Try to answer

these questions about the diagram below.

1. What does the diagram show?

2. Why are the reeds sealed in a glass envelope?

3. Why does the envelope contain nitrogen?

4. How does it operate?