Transistor Circuits

Types of Transistor

A Transistor is a very important component in Electronics, without it the miniturisation of computers would not have been possible. A Transistor is simply a Voltage-dependent switch. When a large enough Voltage is applied to the Transistor, the component 'turns on' and allows current to flow in the connected circuit.

There are two types of Transistor covered in the National 5 course:-

1. NPN Transistor - switch on Voltage ~ 0.7 V.

2. MOSFET Transistor - switch on Voltage ~ 2 V.


The NPN Transistor

The diagram below shows the symbol for an NPN Transistor:-

The Transistor acts in a circuit as a processing device. If the Voltage at the Base is over 0.7 Volts, a Current can flow between the Emitter and Collector.


The MOSFET Transistor

The diagram below shows the symbol for a MOSFET Transistor:-

The Transistor acts in a circuit as a processing device. If the Voltage at the gate is over 2 Volts, a Current can flow between the Source and Drain.


Transistor Circuits

The diagram below shows a full Transistor circuit:-

The red dashed lines above split the circuit into its three constituent parts :-

A - Voltage Divider (circuit input)

B - Transistor (circuit process)

C - Motor (circuit output)


In order to fully explain the function of the above circuit, each section must be taken in sequence.

As seen in a previous section, a Voltage divider can be used as an input for another circuit. In the circuit above a Thermistor is connected in series with a fixed resistor. A Thermistor's Resistance varies with Temperature, as the Temperature increases, its Resistance decreases.


If the System starts at a high temperature:-

1. The Resistance of the Thermistor is small.

2. The Voltage share across the Thermistor is small (< 0.7 V).

3. The Transistor is off.


If the Temperature starts to decrease:-

1. The Resistance of the Thermistor increases.

2. The Voltage share across the Thermistor increases.

3. Voltage share across the Thermistor reaches 0.7 V, the Transistor 'turns on', allowing Current to flow in the final section, causing the motor to run.


Note - The above circuit's function is to start a motor when the Temperature reaches a set low value. By making changes to the components, this type of 'sensor circuit' can be used for a variety of different jobs.


Sensor Circuit Examples

The diagrams below show several different sensor circuits and their function:-


High Temperature sensor which activates a motor:-

High light level sensor which activates a lamp:-

Low light level sensor which activates an LED:-

Note - rule to identify type of sensor circuit:-

Variable Resistor 'high up' in circuit - high level sensor.

Variable Resistor 'low down' in circuit - low level sensor.