Design & Technology


There are many different types of electrical and electronic components, including resistors, capacitors and diodes. Each of these has a specific use in a circuit.


Electronic components can be divided into two groups: discrete electronic components and integrated circuits (ICs).

Discrete electronic components

A selection of electrical circuit board components

Discrete (meaning separate) electronic components can be selected individually and put together to make a circuit. Examples of discrete components include resistorsresistor: An electrical component that restricts the flow of electrical current. Fixed-value resistors do not change their resistance, but with variable resistors it is possible to vary the resistance., capacitors [capacitor: Circuit component which stores and discharges electrical current. They are made from two parallel metal plates separated by an insulator (called a dielectric). ], diodes and transistors [transistor: Components which do not conduct electricity unless they are turned on by a (different) electrical current. This means they can be used as switches, amplifiers and in other ways. ].

Discrete components can also be used as components in circuits that include an integrated circuit. For example, a 555 astable [astable: An astable circuit gives a pulsed digital output. For example, it could be used to make an LED flash. ] integrated circuit requires two discrete resistors and a discrete capacitor to make it work.

Integrated circuits

These are miniature circuits etched onto a piece of silicon, often called a chip. These chips are fitted inside a protective plastic package and are manufactured in large quantities.

A dual-in-line surface mount package integrated circuit resting on a finger-tip

The circuits inside the package are arranged in different configurations depending upon the type of chip and its function. The most common type of configuration is called the dual-in-line or DIL package, which has two rows of connecting 'legs', one on each side.

You don't need to understand how the circuit inside a silicon chip works. It's best to think of ICs simply in terms of their function: eg as timers, counters, logic gateslogic gate: Digital electronic devices that allow an electronic system to make a decision based on a number of inputs. or operational amplifiers [operational amplifier: A device which takes a relatively weak signal as an input and produces a much stronger signal as an output. ] (op-amps).

Input output process block

Normally they are one of the process blocks in an electronic system, as in the image here:

When using ICs you need to know:

  • which pins have to be connected
  • the function of each pin
  • how the IC is connected to the power supply

A circuit diagram that includes one or more ICs should show the pin numbers and how the pins are connected to the rest of the circuit.


Resistors restrict or limit the flow of current in a circuit. The ability of a material or component to resist current flow is measured in ohms [ohm: Units of electrical resistance (R). 1 volt will force a current of 1 amp through a resistance of 1 ohm. ]. There are three main types of resistor:

  • fixed resistors
  • variable resistors
  • special resistors, such as thermistors and light-dependent resistors (LDRs)

Fixed resistors

These are the most common type of resistor. They have three important uses:

  • protecting components
  • dividing voltage between different parts of a circuit
  • controlling a time delay

Protecting components

Resistors can be used to protect other components (such as an LED [LED: Stands for Light-Emitting Diode. LEDs glow when current passes through them. ]) from damage by too much current.

Potential dividers

A capacitor in series with a resistor.

In a potential divider, a fixed resistor is used to split voltage between different parts of the circuit. Potential dividers (or voltage dividers) are used, for example, with LDRs in circuits which detect changes in light.

Timing applications

A fixed resistor can be used in series with a capacitor to control a time delay.

Variable resistors or potentiometers

There are two types of variable resistor:

  • Variable resistors, which are altered continually as they work: eg for volume control in a radio.
  • Pre-set potentiometers, which have a resistance control that is adjusted and then fixed. These resistors would normally be adjusted once only.

The main difference between the two types of variable resistor is their size. Pre-set potentiometers tend to be smaller and are usually adjusted with a screwdriver. A variable resistor has a long spindle with an operating knob attached.

Special resistors

Rod type thermistor -  thermally sensitive negative temperature coefficient resistor

Thermistors change resistance as temperatures change. Most thermistors have a negative temperature coefficient, meaning their resistance falls as temperature increases. Thermistors are used in temperature-sensing circuits.

Light-dependent resistors (LDRs) change resistance as light levels change. The light levels are detected by a photo-sensitive plate on the resistor. Most LDRs have a negative light coefficient, meaning that their resistance falls as the amount of light falling on them increases. LDRs are used in light-detection circuits.

Ohms and resistance values


The ohm is the unit of resistance. Larger values are measured in kilo-ohms (1000 ohms) and mega-ohms (1,000,000 ohms). Resistors are marked, using a code specified in British Standard 1852, as follows:

  • The letter R means ohm. Numbers coming before the R indicate a value more than one. So 1R (or 1R0) = 1 ohm; 47R = 47 ohms; and 4R7 = 4.7 ohms. Numbers coming after the R indicate a value less than one: so R56 = 0.56 ohms.
  • The letter K means kilo-ohm. Numbers coming before the K indicate a value more than one, while numbers coming after the K indicate a value less than one. So 1K8 = 1.8 kilo-ohms and 5K6 = 5.6 kilo-ohms.
  • The letter M means mega-ohm. Numbers coming before the M indicate a value of more than one, while numbers after the M indicate a value less than one. So 2M = 2 mega-ohms, and 2M2 = 2.2 mega-ohms.

Resistance values

Resistance value is shown by a series of coloured bands, read from left to right.

Graphical table summarises the colour coding found on the four bands on a resistor.

  • The first band denotes tens, and the second band units. Each colour stands for a different unit: black is zero; brown is one; red is two; orange is three; yellow is four; green is five; blue is six; violet is seven; grey is eight; white is nine. So the sequence yellow through to violet denotes the value 47.
  • The third band is the multiplier: black denotes a multiplier of one; brown 10; red 100; orange 1000 and so on. So the sequence yellow to violet to red denotes a value of 47 x 100, or 4.7 kilo-ohms.
  • The fourth band is the tolerance. Manufacturers of resistors cannot guarantee the exact resistance figure shown by the first three bands, so they give a percentage value by which the resistance may be higher or lower than the resistance quoted. A red band denotes a tolerance of 2%; gold a tolerance of 5%; and silver a tolerance of 10%. So a 100 ohm resistor of 10% tolerance has an exact resistance value falling somewhere between 90 ohms and 110 ohms.

Potential dividers

  • A circuit diagram of two fixed resistors in series.

    Fixed resistors in series

    Potential dividers divide up the voltage within a circuit, so that parts of a circuit only receive the voltage they require. Potential dividers usually consist of two or more resistors arranged in series across a power supply.

  • A circuit diagram of a fixed resistor in series with a light-dependant resistor

    Fixed resistor in series with an LDR

    Potential dividers form an important part of sensor circuits. For example, an LDR [LDR: A Light Dependent Resistor, or LDR, is a type of resistor which is affected by changes in light levels. A cadmium sulphide layer causes a decrease in resistance in the light and increase in the dark. ] or thermistorthermistor: An electrical device whose resistance decreases as its temperature increases. can be used in place of one of the resistors, with the output voltage signal being directed to an op-amp [operational amplifier: A device which takes a relatively weak signal as an input and produces a much stronger signal as an output. ] or IC.

  • A circuit diagram of a variable resistor in series with a thermistor

    Variable resistor in series with a thermistor

    Instead of using a fixed resistor, a variable resistor allows the output voltage to be adjusted. A low-value fixed resistor should be placed in series with the variable resistor to prevent the full power of the circuit from being routed down the output voltage path if the variable resistor is accidentally moved to a low resistance.

Common uses of potential dividers

Potential dividers are important in both transistor-switching circuits and op-amp comparator circuits [Comparator circuits: Circuit with a comparator - a component which compares two voltages and gives either a high or low output depending on the result of the comparison. ]. The diagram shows a darkness-sensor circuit with a transistor [transistor: Components which do not conduct electricity unless they are turned on by a (different) electrical current. This means they can be used as switches, amplifiers and in other ways. ] used as a switch. When the LDR senses a drop in light, the LED is switched on.

  • When the LDR has light falling on it, its resistance is low, usually around 400 ohms.
  • When the LDR is covered up, the resistance increases (often to many kilo-ohms).

When the resistance of the LDR is small, its share of the voltage supply is small too; this means that the output voltage (Vout) from the voltage divider is small and the transistor is switched off.

In the dark the large resistance of the LDR takes a large share of the voltage supply so Vout is large and the transistor and LED both switch on.


A selection of electrical capacitors

A capacitor is a discrete component that can store an electrical charge. The larger the capacitance the more charge it can store.

The unit of measurement of capacitance is the farad. Often you will see capacitors of much less than a farad. These will be measured in microfarads (one millionth of a farad or 1/1,000,000) or picofarads (one million-millionth of a farad or 1/1,000,000,000,000).

There are two types of capacitor:

  • polarised or electrolytic capacitors
  • non-polarised or non-electrolytic capacitors

Polarised capacitors

These generally have larger capacitance values. Polarised capacitors have a positive pole and a negative pole, so they must be connected to a circuit the correct way round.

Mounting of polarised capacitors

Polarised capacitors may be either axially mounted (on their side, connected at each end) or radially mounted (upright with both connections at the bottom).

A capacitor is marked with the polarity of the wires and the capacitor value.

Radially mounted capacitors have both poles on the bottom. Axially mounted capacitors have the poles coming out of the sides.

Non-polarised capacitors

These are usually much smaller than the polarised type, and have smaller capacitance values. These might range from a few picofarads to a few microfarads. They don't have positive or negative poles so they can be connected to a circuit either way round.

Applications of capacitors

Capacitors are used to smooth rectified alternating-current voltages into steady direct-current voltages. They can also be used to filter out fluctuations in a signal.

Capacitors are often used in series with resistors to achieve a time delay. The time it takes for the capacitor to become charged is related to the size of the capacitor and the value of the regulating resistor.


An electrical diode

A diode is a discrete component that allows current to flow in one direction only. It is a polarised component with two leads, called the cathode and the anode. The cathode is normally marked with a silver or coloured band or the symbol '-'.

If the anode is connected to a higher voltage than the cathode, current will flow from anode to cathode. This is called forward bias.

If the diode is put in the circuit back to front, so that the voltage at the cathode is higher than the voltage at the anode, the diode will not conduct electricity. This is called reverse bias.

Using diodes to prevent damage to circuits

Diodes are normally used to prevent damage to other polarised components in circuits: eg a diode can protect against current flowing the wrong way if the battery is put in back to front.

Forward and reverse bias

Diagram of forward/reverse bias

Diodes and EMF

An important use of diodes is to prevent circuit damage due to back electromotive force (known as EMF). This is a momentary change in the direction of the flow of electricity when components such as motors, solenoids or relays are switched off. The diode is connected in parallel to the component, which generates the back EMF in reverse bias to the 'normal' direction of flow of electricity in the circuit. A diode used in this way is called a clamping diode.

Reverse biased diode protecting transistor

Reverse biased diode protecting transistor

Light-emitting diode (LED)

A white LED light

A light-emitting diode (LED) is a special kind of diode that glows when electricity passes through it. Most LEDs are made from a semi-conducting material called gallium arsenide phosphide.

LEDs can be bought in a range of colours. They can also be bought in forms that will switch between two colours (bi-colour), three colours (tri-colour) or emit infra-red light.

In common with all diodes, the LED will only allow current to pass in one direction. The cathode is normally indicated by a flat side on the casing and the anode is normally indicated by a slightly longer leg. The current required to power an LED is usually around 20 mA.

Seven-segment LED displays

Seven-segment LCD alarm clock

A seven-segment LED is a special type of LED display used in digital clocks, video recorders and microwave ovens.

Other output devices

  • Like LEDs and seven-segment displays, lamps also convert electricity into light.
  • Piezo sounders, buzzers, bells, loudspeakers and sirens are used to convert electricity into sound.
  • Microphones convert sound into electricity.
  • Solenoids are used to convert electricity into linear movement.
  • Motors convert electricity into rotary movement.

Power supplies

Batteries are the most common source of power used for electrical circuits in schools. They come in a variety of sizes, which can be combined in series battery holders. This allows the voltage produced to be equal to the sum of the batteries used: eg 3 x 1.5 V AA batteries would produce a voltage of 4.5V.

Batteries and Voltage


When choosing a battery, the following should be considered:

  • The power requirements of the circuit.
  • The battery life needed, measured in milliamp hours.
  • The size, shape and weight of the battery.
  • The cost of the battery.
  • Sustainability: could rechargeable batteries be used?
  • Battery disposal: how will the battery be disposed of at the end of its working life?

As an alternative to batteries, external power supplies can be used with low-voltage regulators (such as L7805 and 78L05). Their use is often limited by cost, the need to be attached to the external power supply and the power requirements of the circuit.

Standard symbols guide

The standard symbols for the key components used in electronic circuits are shown in the table below.

Electronic circuit symbols

The cell symbol looks like a letter T with a bigger, upside-down letter T on top.The symbol for an LED looks like a diode with two arrows pointing away from it.Push-to-make switch circuit symbolThe diode symbol is triangular, pointint towards the cathode. A vertical bar is on the triangle's point next to the cathode.The bell symbol looks like a letter D.Voltage rails circuit symbol
CellLight-emitting diode (LED)Push-to-make switchDiodeBellVoltage rails
The battery symbol is made from several cells connected in series.The flashing LED symbol looks like a diode with an opposite-facing triangle inside, and arrows leading away.Push-to-break switch circuit symbolThe thyristor symbol is a downwards-pointing triangle with a horizontal bar on its point, and another line coming in from the leftThe symbol for a microphone looks like a circle touching a vertical line.The Earth symbol is three horizontal lines tapering to a downwards point
BatteryFlashing LEDPush-to-break switchThyristorMicrophoneEarth
The symbol for a resistor is a vertical rectangle on a vertical line.The bi-colour LED symbol looks like a rectangle with two opposite-facing diodes inside, with arrows pointing away from the box.The symbol for a switch is a line moving away from the vertical, showing the switch in an open position.The symbol for a transistor looks like a sideways table. The Collector is on the top, the Emitter on the bottom. The Base is on the left.The buzzer symbol looks like a backwards letter D.A box with eight numbered pins leading out.
ResistorBi-colour LEDSingle-pole single-throw switchNPN transistorBuzzer555 timer IC
The symbol for a variable resistor looks like a resistor, with a diagonal arrow over it.The symbol for a tri-colour LED looks like two LED symbols facing each other, with a line coming out from where they joinA switch has two positions, with a line extending to the currently connected top path.A FET is shown by three wires from the right terminating near a perpendicular line which leads off to the left.The loudspeaker symbol looks like an inverted cone.Operational Amplifier circuit symbol
Variable resistorTri-colour LEDSingle-pole double-throw switchField effect transistor (FET)Loud-speakerOperational amplifier (op-amp)
The sybol for a potentiometer looks like a resistor with an arrow pointing at it on the side.The symbol for a photo transistor looks like an NPN transistor with arrow pointing towards it.The capacitor symbol looks like two letter Ts facing each other.The AND gate has two inputs and one output. The symbol looks like a letter D.The lamp symbol looks like a circle with an X going through it.The voltage regulator symbol is a rectangle with lines coming out from three of its sides.
Potent-iometerPhoto transistorCapacitorAND gateLampVoltage regulator
The symbol for a thermistor looks like a resistor, with a diagonal line over it, ending with a short vertical section.The symbol for an opto-isolator looks like an LED next to a photo transistor.The symbol for a polarised capacitor looks like a normal capacitor, but with polarity symbols.The OR gate has two inputs and one output. The symbol looks like a letter D where the left line of the D is bent inThe symbol for a motor is the letter M inside a circle.Two wires crossing look like a plus sign.
ThermistorOpto-isolatorElectrolytic capacitorOR gateMotorCrossing of conductors
The symbol for an LDR looks like two arrows pointing at a rectangle.The Ammeter symbol is an A in a circle.The symbol for a piezo crystal oscillator looks like two letter Ts facing each other with a square between them.A NOT gate has a single input and one output. The symbol is a triangle with a circle on the point leading to the outputThe symbol for a voltmeter is the letter V inside a circle.Wires joining can look either like two lines touching, or a circle showing where the join is.
Light-dependent resistor (LDR)AmmeterPiezo crystal oscillatorNOT gateVoltmeterJoined conductors

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