The Basics...

Electronics can be the most difficult concept to understand because, unlike other areas of the syllabus, it is impossible to see how it works. To try to make it easier I’m going to go back to basics.
Every circuit requires a power source to make it function. You can have all the resistors, capacitors, transistors etc. in the world but, until you connect them to a power source, nothing will happen.
The simplest source of power is a battery. These come in various sizes and can be rechargeable. Batteries contain toxic chemicals (a car battery contains lead and sulphuric acid!)and you ought to be aware that as such they can have an environmental impact during manufacture and when disposed. Rechargeable batteries are much better for the environment but are more expensive and cannot be recharged indefinitely. The desks in TE3 have built-in power supplies and take their power from the mains. Mains electricity is produced at power stations around the country by burning fossil fuels - coal, gas and oil - to heat water, creating steam which, in turn, spins a turbine (a giant dynamo). Some electricity is produced in Nuclear power stations and a small percentage in more environmentally friendly ways such as Hydro-electric and wind turbines, but the end result is the same.
Each one of these power supplies provides a certain voltage. Batteries usually provide 1.5 Volts (V), although the rectangular ones kick out 9V and car batteries 12V. Mains electricity is 240V, the third rail on the train-line is 600V and the overhead power lines carry 25,000V.
When you connect something to a power supplies, a voltage will exist across it. If you put your fingers onto the terminals of a 1.5V calculator battery, there would be a voltage difference of 1.5Volts. If you put your fingers into a mains socket, the voltage would be 240Volts.
Everything has a resistance to electricity and that resistance is measured in Ohms (Ω). Metals are excellent conductors of electricity because they have extremely low resistance. Humans are poor conductors because they have a high resistance. Materials such as ceramics and plastics have extremely high resistance and in most circumstances can be used as insulators.

So far we have discussed voltage and resistance but there is one other related term that we need to consider - current.

To explain how voltage, resistance and current are related to each other, I am going to use water as an analogy.

In the illustration on the left, the height of the lake above sea level represents the voltage.

The flow of water down the hill represents the current.

The rocks and debris in the stream represent the resistance.

In this illustration, the rocky stream is replaced by a smooth pipe which will have a lower resistance. The water (current) will flow much faster. So, a lower resistance will result in a higher current.

In this illustration, we're back with the rocky old stream but this time the lake is much higher than it was (we've increased the voltage)

The water will flow faster than it did in the first illustration. So, increasing the voltage, increases the current.

The current flowing through an object (I), measured in Amps, is related to the voltage (V) across an object and its resistance (R) as shown below.

Let’s think carefully what this means. If you took a lump of metal with a resistance of, say, 6Ω and placed it across the terminals of a car battery with a voltage of 12V then the current flowing through that lump of metal will be 2A.
If you, with a resistance of, say, 20000Ω went up to the same 12V battery and put your fingers across the terminal the current flowing through your body will be .0006A.
This is important because it shows that you can’t control the current flowing through an object directly. You can control the voltage and the current will then depend upon the resistance of the object connected to it. This is why you can buy a 1.5 Volt battery but you could never by a 3 Amp battery.
It’s worth pointing out at this stage that drawing a large current would drain most batteries very quickly. You get an audible warning of this happening if you connect a wire across the power supply on one of the benches in TE3.
If you’ve ever wondered why electrical appliances are so dangerous in the bathroom when they seem to be perfectly okay in the bedroom, it’s because when you’re wet your resistance falls dramatically. Therefore contact with 240V will mean a much greater current flows through your body giving you a potentially fatal electric shock.

Here are some of the electrical symbols used in circuits.

Calculating Current Flow and Potential (Voltage) in a Circuit

Circuit A shows a 6V battery connected to a 3Ω resistor. The current flowing through the resistor = V/R = 2A.

Circuit B shows a 6V battery connected to two 3Ω resistors. Total resistance of the circuit is 6Ω. Current flowing through the circuit = V/R = 1A.

We can now use V=IR to calculate the potential difference across each reistor. V = 1x3 = 3V. And not surprisingly, with a 3V potential difference across each resistor, the total potential difference across the circuit is 6V.

Potential Dividers

Circuit C makes things a bit more interesting. The total resistance is still 6Ω and the current flowing through the circuit will still be 1A, but the potential difference across the 2Ω resistor (using V=IR) will be 2V, whilst the potential difference across the 4Ω resistor will be 4V. The voltage at point X will be 4V.

We have effectively divided the voltage. We have created a potential divider.

The formula for calculating the potential at X is shown below.

Power

Think back to the illustrations showing the water falling down from the lake along a stream. Imagine, for one moment, that you were standing in the stream. The force or power of the water that you would feel would depend on two things. The height from which the water was falling down and the speed that it was travelling at. Remember that these are the analogies for voltage and current.

To calcuate the power (P) measured in Watts (W) in a circuit, use the following formula.

Resistors in Parallel

Circuits D and E have their resistors arranged in parallel. The potential difference across each of the resistors is the same, 6V. The current in this example divides. To calculate the total resistance of each circuit you need to use the following formula. (It will be given to you in an exam and I have yet to see a paper in which it was required but you never know).

Therefore the total resistance for circuit D = 1.5Ω and the total resistance for circuit E = 1.33Ω
The total current flowing in circuit D (using I=V/R) = 4A, whilst the total current flowing in circuit E = 4.5A.

Use I=V/R to calculate the current flowing through each of the resistors in circuit D and circuit E and then add them to confirm that the sum of the currents flowing in each branch of the circuit equals the total current flowing through the circuit.

Analogue Components and LEDs

You are probably getting bored of resistors now so let’s introduce some new components.

The LED is a component which emits light when a current is passed through it. A diode will only allow current to pass through in one direction. An LED will light up as the current passes through. It’s important, therefore, to ensure that you’ve connected it up correctly; otherwise no light. LED’s are cheap and available in a variety of colours. (White LED’s however are relatively expensive). LEDs need a very small current to operate (approx. 20mA) compared to a conventional bulb (200mA). This means that they consume much less power which in turn means that a battery will last longer.
The variable resistor is like a conventional resistor but can be adjusted. Such a resistor usually carries a mark such as 10K which means that the resistance can be adjusted between 0-10000Ω.
The light dependent resistor (LDR) is a component whose resistance varies according to the light shining upon it. Usually when the light level is low the resistance is high. As more light falls on the LDR the resistance drops.
The thermistor is a component whose resistance varies according to the temperature. Usually when the teperature is low the resistance is high. As the temperature rises the resistance drops.

Circuit F is a modification of circuit C. I have swapped one of the resistors with an LDR. Let’s say that when the LDR is covered its resistance is 8Ω. The total resistance of the circuit will be 12Ω and the current flowing through the circuit will be 0.5A. The potential difference across the LDR (using V=IR) will be 4V. The potential at point X therefore will be 2V.

Let’s say that when the LDR is exposed to light its resistance falls to 2Ω. The total resistance of the circuit will now be 6Ω and the current flowing through the circuit will be 1A. The potential difference across the LDR (using V=IR) will be 2V. The potential at point X therefore will be 4V.

The key thing is this. As the light falling on the LDR increases, the potential at X rises from 2V to 4V. Remember this because it will be significant later.

Transistors

The buzzer and the switches are fairly obvious but the most important and useful component is the Transistor. It was the invention of this that kick-started the boom in electronic products in the 1960’s. Transistors also form the basis of the chips found in computers. Miniaturisation has meant that it’s possible to fit thousands of transistors onto a single chip.
So what is a transistor? Quite simply an electronic switch. When a small current flows from the base to the emitter, it allows a much larger current to flow from the collector to the emitter.

Look at the circuit on the left. As it stands at the moment, no current is flowing between the base and the emitter so no current can pass between the collector and the emitter. The LED will therefore not light up.
If you were to place one finger on A and another finger on B a tiny current would flow through your body, to the base and through the emitter. This tiny current flowing between base and emitter would allow a much larger current to flow from the collector to the emitter and that in turn would cause the LED to illuminate.
It doesn’t matter whether the LED is situated before or after the transistor. Because if the current isn’t flowing from the positive to the negative terminal, nothing will work.

Transistors and Potential Dividers Combined

So how can we use the transistor then? Well, remember this from earlier on....

'The key thing is this. As the light falling on the LDR increases, the potential at X rises from 2V to 4V. Remember this because it will be significant later.'

....if we now conect our transistor circuit to circuit F such that the base of the transistor is connected to X we will have made a device which illuminates an LED when the LDR is exposed to light, as shown below.

The rise in voltage at X causes a current to flow between the base and the emitter and will allow a much larger current to flow through the collector and the emitter. RT is a resistor which stops too much current passing through the base of the transistor whilst RL is a resistor which prevents too much current passing through the LED.

This is significant because it is a circuit that responds to something happening (light levels changing) and does something as a result (turns an LED on).

If you have answered the homework questions relating to potential dividers you will have noticed that by reversing the LDR and the fixed resistor you alter the change required to cause the voltage to increase, ie. when the LDR is covered the LED will illuminate.

We can change the input and output devices whilst still retaining the basic potential divider/transistor circuit, ie. replace an LDR with a thermistor and the LED with a buzzer so that a change in temperature causes an alarm to sound.

The circuits below show the different combinations and what they will do. Note that in these cases I have the components connected to a power supply rather than a battery. The outcome is no different but this is the way it’s more likely to appear on an exam paper.

In each of the circuits the arrow points to the base of the transistor which I have left out.

Output Devices

We have dealt with LED’s as outputs and it is possible to replace these with light bulbs or buzzers. Below I have shown two other output devices that can be connected to transistors although you need to take certain precautions.

A motor is a fairly obvious device. It spins, at about 2000rpm, when a current passes through it. It can be connected up to gears or pulleys and in its larger form is the basis of most electrical machines such as drills and lathes.

A relay consists of a coil which becomes magnetised when a current passes through it. The magnetic field is used to bring two contacts together which completes a seperate circuit.
Why is this useful? Well, a transistor on its own can allow a maximum voltage across it of 20V and a maximum current of 100mA. If you wanted to use a transistor circuit so that a low temperature caused a mains electric heater to turn on, the transistor on its own wouldn’t be able to handle it. If the low voltage, low current circuit required for the transistor could be used to energise the magnetic coil of the relay then the 240V high current mains power could be supplied independently to the heater through the contacts of the relay.

There is one slight problem caused by the wire coils which are common to both of these devices. When the current to these devices is cut off they generate a voltage ‘spike’ of approximately 200V for a very brief instant. To prevent this a diode must be connected to these devices as shown below.