ELECTRONIC SYSTEMS Note: There is no compulsory question set on Section 25 of the syllabus. Questions set on topics within Section 25 are always set as an alternative within a question.

TRANSISTORS A transistor is an electronic component with three terminals or connections. The diagrams below shows some examples together with the circuit symbol.

The three terminal are called the collector (c), base (b) and emitter (e). In the circuit a transistor acts as a device for controlling the flow of current. The transistor is connected so that there is a p.d. of 6V between the collector and emitter. A current wants to flow from the collector to the emitter. The arrow indicates the direction of current flow. The transistor has a very high resistance, so no current flows. However, if a small current flows into the base this greatly reduces the transistors resistance, and so large current flows from the collector to emitter. Hence a small base current Ib permits a large current Ic to flow into the collector. The two current join together and flow out through the emitter. We can show this as a formula: base current + collector current = emitter current.

Hence a small current (Ib) controls a large current (Ic) Hence a small current (Ib) controls a large current (Ic). When no current flows through the transistor, we say that it is OFF. When a current flows, it is ON. So the transistor acts as a switch, controlled by the base current.

A PRACTICAL SWITCHING CIRCUIT WITH THE HELP OF TRANSISTOR Now we will look at an example of a circuit in which sufficient current can flow through a transistor to operate buzzer as shown below. This circuit sounds an alarm when the temperature rises – it could be part of fire alarm system. The input to the transistor is a potential divider circuit, consisting of a fixed resistor and a thermistor, connected in series. The resistance of the thermistor decreases if it gets hot.

To switch the transistor ON, we need a current to flow into the base To switch the transistor ON, we need a current to flow into the base. For this to happen, the voltage at point X must be high. This will happen when the resistance of the thermistor is low, that is, when it is hot. So here is how the circuit works. When the thermistor is cold, its resistance is high. The voltage across point X is low, and so the base current Ib flowing into the transistor is too small to cause a current to flow though the transistor. The transistor is OFF. When the thermistor is hot, its resistance is low. The voltage at point X will be close to 6V, and this voltage makes a small current flow into the base of the transistor. (the resistor is there to make sure the current does not get too big). This allows a large current Ic to flow through the transistor, which is ON. The current also flows through the buzzer, which buzzes.

LOGIC GATES Rather than working with individual transistors, electronic engineers prefer to use logic gates. Each logic gates has a specific function. And many can be combined together to produce complex effects. However, inside each logic gate there are number of transistors working a switches, together with other components. A logic gate is a device that receives one or more electrical input signals and produces an output signals depends on those input signals. These signals are voltages. a high voltage is referred to as ON(high), and is represent by symbol 1. a low voltage is referred to as OFF (low), and is represent by symbol 0.

There are different types of logic gates these include: AND, OR, NOT, NOR and NAND. AND gate This gate has two inputs (input 1 and 2) and output as shown below. An AND gate function like this: Its output is ON (high) if both input 1 and input 2 are ON (high). The truth table for this gate is shown below.

OR gate This gate has two inputs (input 1 and 2) and output as shown below. An OR gate function like this: Its output is ON (high) if either input 1 or input 2 or both is ON (high). The truth table for this gate is shown below.

NOT gate This gate has one input and one output as shown below. This gate is also called an inverter. An NOT gate function like this: Its output is ON (high) if its input is not ON (high). The truth table for this gate is shown below.

NAND gate This gate has two inputs (input 1 and 2) and output as shown below. An NAND gate function like this: Its output is ON (high) if both input 1 and input 2 are not ON (high). The truth table for this gate is shown below.

NOR gate This gate has two inputs (input 1 and 2) and output as shown below. An NOR gate function like this: Its output is ON (high) if neither input 1 nor input 2 is ON (high). The truth table for this gate is shown below.