Reading: Electronicalarmcircuits

Ex. 1 Work in pairs. Make a list of electronical arms in column A. Note what triggers the alarm in column B. Two examples are completed for you.

Ex.2 Now read the description of how the device works. Then put the events in the correct sequence. The first and last events are entered for you.

a There lay is activated

b Current flows through the collector-emitter circuit____________.

сThe voltage in the base-emitter circuit rises above0.7 volts

d The resistance of R1 rises

e The warning device is switched on.

F The temperature falls. I

g Small differences in voltage are amplified by the amplifier__________________

A circuit diagram

The diagram shows a simple frost alarm. It can be used to warn drivers that roads may be icy or to warn gardeners and fruit farmers to protect their crops. It can also be used to switch on heaters. It is triggered by a fall in temperature. When the temperature falls to 0° C or any temperatures elected, the alarm operates.

The principal component is the thermistor, Rl. As the temperature falls, the resistance of Rl rises. At 25°C it has a resistance of 15 k Q. At 0°C the resistance is 45 k Q. The 100 k Q potentiometer, R2, can be adjusted to allow the circuit to trigger at other temperatures.

The higher the resistance of Rl, the smaller the voltage is flowing to the amplifier, UA741. This is a very sensitive amplifier which amplifies small differences in voltage. The output from the amplifier is fed to the base of the transistor, BC107B. This acts like a switch. When the voltage in the base-emitter circuit rises above 0.7 volts, current flows through the collector-emitter circuit, activating the relay. The diode, Dl, across the relay prevents sparking. R5 helps to ensure the relay changes smoothly when the trigger temperature is reached. The relay can trigger a warning device such as a buzzer or light, or switch on a heater.

Grammar: Complex sentence review

We can use complex sentences of two or more clauses to describe how a series of events relate to each other.

Study these ways of forming complex sentences:

Using time clauses to link actions

1. When the temperature falls, the alarm is switched on.

2. A capacitor charges until it is full.

3. As the capacitor charges, the voltage rises.

4. Using if-c lauses to link a cause and effect

5. If excess current is passed, the transistor will overheat.

Using relative clauses to make definitions

6. Diodes are electronic devices which allow current to pass in one direction only.

Using relative clauses to add information. We use commas to show this is extra but not essential information.

7. Diodes, which are made of silicon or germanium, have many uses in electronics

We can replace which or who in a relative clause followed by a Continuous or Simple verb with an active participle, the -ing part of the verb.

8. This completes a circuit, which generates a series of pulses.

OR

This completes a circuit, generating a series of pulses.

Now note how we can link these events in the frost alarm circuit.

1. The temperature falls to zero.

2. The transistor is switched on.

3. This activates the relay.

When the temperature falls to zero, the transistor is switched on.

The transistor is switched on, which activates the relay.

When the temperature falls to zero, the transistor is switched on, which activates the relay.

OR

When the temperature falls to zero, the transistor is switched on, activating the relay.

Electronics

Make sentences by matching in columns A-C and then linking it together using a relative clause with which or who.

Example: Silicon which comes from sand is an important component of some semiconductors.