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-clauses 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

1 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.

A	B	C
Subject	Additional information.	Important information
Silicon	It is short for binary digit.	He was one of three inventors of the transistor.
1 Digital electronics	He worked at Bell Laboratories.	They are remembered in the basic units of electricity.
2 Walter Brattain	This means light-emitting diodes.	They are used in watches and many electronic displays.
3 A bit	It is used in everything from watches to computers.	They can provide a higher current than other batteries.
4 Lithium batteries	They were pioneers in the study of electricity.	It is an important component of some semiconductors
5 LEDs	It comes from sand.	It is concerned with electrical systems made up of a series of switchers
6 Ohm,Volta and Ampere	They a often used in cameras.	It is a single unit of information