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Beginners guide to electronic circuits and circuit design

Simple electronic switch circuit

To get started with the basics of electronics we are going to look at a very basic circuit based around a switch that can turn a light on and off. Okay this may not be exactly what you are hoping to achieve in terms of creating the latest computer controlled electronic gadget, but that will come a bit later. For the moment we need to look at the basics and learn to walk before we can run.

First here are some basics about electricity and how it works. I've kept this to the very minimum so that we can get on to creating our first circuit.

Electricity, electric current and electrons

We all know of electricity as the energy that makes our lights shine, powers the TV and for which the energy companies like to charge us lots of money for using, but to understand electronics we need to look at what electricity is. Essentially electricity is a flow of charge. Negatively charged electrons normally orbit an atom, those in the outer shell are known as valence electrons. With and appropraite force applied some of these can break free creating electrical charge. These electrons flrow around the circuit. The electrons always flow in a full circuit needing to get back to where they started (eg the battery or other electricity source).

This is oversimplified, but from the point of view of designing electronic circuits (rather than designing the components themselves) you don't need to go any deeper at the moment.

Conductors vs. insulators

Electrons (ie. electricity) can move through some materials much more easily than others. The wires connecting the mains electricity to a mains appliance is normally made of copper as this allows the electrons to pass very easily, but to save you getting electrocuted every time you touch the power lead the copper wire is covered in a plastic coating which resists the flow of electrons.

Materials that allow the electrons to move easily are called conductors, whereas those that prevent electrons from flowing are called insulators. It is these properties that allow us to control where electricity is allowed to pass and to be able to turn devices on and off. The insulating properties of a material will differ depending upon the material and the thickness, so an appropriate insulator should always be used when dealing with electricity especially with mains electricity (see electrical safety section).

Some common conductors and insulators are listed in the following table:

Copper wireMost plastics
Other metalsDry wood
Tap / rain water*Glass and ceramics

Note that I specifically mention tap and rain water rather than just water. Pure distilled water is actually an insulator, but the impurities in most water turns it into a conductor. Never operate live mains equipment near water or outside in the rain, unless the equipment is specifically designed for that purpose (see electrical safety section).

There is another type of material called a semi-conductor whose properties can change between an insulator and a conductor under certain conditions. Semiconductors will be covered in more details later when we get on to active components.

Complete circuit

This is an example of a real circuit used in battery operated torches.

For any electronic circuit to work there must be a complete circuit. This means that there must be a connection made out of conducting material that goes in a circle from one terminal of the battery through the equipment and then back to the other terminal of the battery. If there is a gap at any point then we have air which is a bad conductor and as a result nothing will happen.

This is how a switch works.

When the switch is in the open position then it creates a break in the circuit and the light is off. When the switch is closed the metal contacts inside the switch join and complete the circuit.

The circuit diagram (schematic)

The picture that you can see is known as a circuit diagram or a schematic diagram. This is the standard way of showing an electronic circuit so that you can see how the circuit should work. Each component has its own symbol which indicates what it's function is. There are a number of different electronic component circuit symbols in the electronics reference section. The symbol on the left is for a battery, at the top there is the symbol for a switch and at the right the circle with a cross in it is the symbol for an lamp (or indicator lamp).

Note that when a circuit is created there is often a component layout diagram which shows how the components are installed onto the circuit board. This is useful if you are creating a replica of a circuit that has already been designed, but it is the schematic (circuit diagram) that is most useful for understanding how and why a circuit works as it does.

Unfortunately circuit diagrams / schematics do not always look exactly the same as there are differences in the circuit symbols used depending upon region and preference. For example the resistor in the IEC circuit symbols is shown as a rectangle, but in the US a resistor is normally shown as a zig-zag line. In most cases the differences are only small and it's still possible to recognise the symbol even if it is not the one you are familiar with, but in the worst case it usually means there are a couple of extra symbols to remember / look-up.

The circuit diagram is normally static. The switch symbol would not normally change to the closed position, or the lamp change colour, but these are included to show how the electronic circuit works.

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