Process Components (1)

A process component is one which carries out some function in response to an input stimulus. For example, imagine I want to sound an alarm if the headlights of a carĀ are left on when the engine is turned off.

A sensor or switch could be used to electronically indicate whether or not the engine is running. Either will produce an electronic signal which can be used to trigger the process component.

The process component is responsible for producing the output voltage which can be used to sound a buzzer or speaker system (output components). Lets briefly look at this in block diagram form.

Basic Electronic System Design

Process Components

The process components you will need to know about in this unit are listed below. You need to be able to identify each component, describe its function and demonstrate an application

  • Transistor (NPN and PNP)
  • Darlington Pair
  • Thyristor
  • Field Effect Transistor
  • 555 Timer in Monostable and Astable modes
  • Operational Amplifiers
  • Peripheral Interface Controller (PIC)

NPN Transistor


For the purposes of this unit we will be using the ztx689b transistor. The following slide contains a physical image and a labelled circuit symbol.



Transistors appear in all but the simplest types of circuit. They carry out 3 main functions:

  • Transistor Switch
  • Voltage Amplifier
  • Current Amplifier

Learning and understanding the function of a transistor as a switch will suffice for this unit. It is useful to know that they can be used for other purposes but learning and understanding the function of a transistor amplifier requires significant further study.

The Transistor Switch

The following diagram demonstrates the basic set up required to make an NPN transistor switch on.

Transistor 1

Now we will demonstrate how a transistor can be used as a switch in a real circuit using the next 2 slides.

Transistor 2

Transistor 3

PNP Transistors

To keep things simple I would advise you to use an NPN transistor for your switching component. However I will also briefly cover the PNP here as well.

Basically the PNP transistor operates in exactly the same way as the NPN except it has an inverse action, this means that the emitter needs to be approximately 0.6 volts positive with respect to the base and that current flows from the emitter to the base. You can find an excellent tutorial on the subject at here.

The following slide explains the basic differences between the two transistor switches.

Transistor 4

Darlington Pair

This is the name give to a special configuration of two transistors when more current (power) is required to run the electrical load for example motors or heaters

Darlington Pair

Silicon Controlled Rectifier (Thyristor) Switch

A Thyristor switch operates in a similar way to a transistor switch with some important differences. Firstly the thyristor switch can be made to switch both ac and dc currents although the ac switching only happens on positive swings of the sine wave. The thyristor switch can also handle large currents (depending on type and manufacture) however the biggest difference is that once a thyristor is triggered on it remains switched on even when the triggering signal has stopped. Thyristors are extremely fast switching devices which come in very useful for controlling ac motors, large currents to dc loads and regulating power for given devices.

The following resource explains the construction and operation of a thyristor and its associated circuit symbol.

The following slide explains one application of a Thyristor (there are many) which could be useful in completing this unit.





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