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Electronic Components


Aerial. Used to help receive radio waves.


Ammeter. A device used to measure the current flowing through a certain point in a circuit.

See Measuring Circuits for information on how to use an ammeter.


Battery. A collection of cells.


Capacitor. A device with two plates separated by an insulting layer. One simple kind is aluminium foil separated by waxed paper. The plates build up a charge when electricity flows, but when the plates are fully charged the capacitor prevents any further flow of electricity.

polarised capacitor

Some capacitors are polarised, and have to be connected the right way around.

variable capacitor

Some capacitors are variable.


Cell. A single unit of a battery. The positive terminal is the longer line. The negative terminal is normally drawn thicker than the positive (unless drawn by hand).


Diode. A semiconductor device that lets current through in one direction but not the other. Also see light emitting diode.


Light emitting diodes (LEDs) are a special kind of diode that emit light when operating. Diodes have a higher forward voltage than standard silicon diodes, and the shorter the wavelength of light they produce, the more voltage they require to make light. Hence, blue (and white) LEDs need more voltage than red LEDs. Also see LEDs.


Earth. Also called ground. The symbol saves having to draw in the negative terminal or rail of a circuit, meaning many diagrams can be drawn much simpler.

eacth circuit

These two circuits are identical. Car wiring diagrams in particular use this method, since the whole body of the car is the earth and is connected straight to the negative terminal of the car battery with a thick wire.


Fuse. A wire (normally) which will burn out if too much current is passed through it. Each fuse has a rated current, which if exceeded will melt, breaking the circuit. There are several ways of drawing fuses.


Inductor. Also called a coil. If a current starts to flow it tries to resist its flow. If a current stops flowing it tries to keep it going. Thus it tries to maintain the status quo.


Lamp – indicator. A light bulb simply used and an indicator if something is working.

lamp  lamp

Lamp – lighting. A light bulb used to give illumination.


LED. Acronym for light emitting diode, a particular kind of diode.


Loudspeaker. A device which creates sound from an input electrical signal.


Ohmmeter. A device used to measure the resistance of a component (in isolation).

See Measuring Circuits for information on how to use an ohmmeter.


Relay. A kind of switch operated by an electromagnet. Contacts on the switch side are normally open, common, and normally closed.

relay     relay

A relay can switch just one thing on, or switch between two things – called a crossover relay.


A relay can switch between lots of things at the same time. This one is a double pole double throw relay.


Resistor. Something that resists the flow of electrons. This resistance is dissipated as heat. A light bulb is a resistor that gets so hot it glows. Also, the resistance of a light bulb increases as it gets hotter.

The traditional symbol was a zig-zag line, which dates back to the days when a resistor was made from a long piece of wire wrapped in such a manner as to not produce inductance (which would have made it a coil, or inductor).  


Wirewound resistors are now used only in high-power applications, with smaller resistors being cast from carbon composition (a mixture of carbon and filler) or fabricated as an insulating tube or chip coated with a metal film. To illustrate this, European circuit diagrams have replaced the zig-zag symbol by a simple oblong, sometimes with the value in ohms written inside.  

variable resistor

A resistor can be variable. There are different ways of drawing this, with either two or three connections.


A three connection variable resistor is called a potentiometer.


A light dependent resistor is sensitive to light. Its resistance decreases when light falls on it because the light triggers the release of electrons, which carry current. The more light, the more electrons are available for carrying current, hence the lower its resistance.


Solenoid. A long thin inductor (coil) which uses the magnetic field to move an iron rod in the middle of it. They can be used to steer radio controlled cars or ring doorbells, and lots of other things. Compare with a transformer or a relay.

momentary switch

Switch. A mechanical device to turn power on and off. There are lots of different sorts of switches. The simplest are push button (momentary) and on-off (single pole, single throw) switches.

dpdt switch

An example of a more complex switch is a double pole, double throw, centre off switch, which I used at one point on my caving headlamp. It switches two things at the same time, which each have two on positions and one off position.


Thermistor. A resistor which changes resistance depending on temperature. Ordinary resistors are designed to change resistance as little as possible with changes in temperature.

A thermistor may have a positive temperature coefficient (PTC) or a negative temperature coefficient (NTC). NTC thermistors are the most commonly used in the Cambridge Exams syllabus. Their resistance decreases as they get hotter.


Transformer. Two coils wrapped around an iron rod or core. A current through one coil causes a current to flow through the other due to the magnetic field created by the first current. Different numbers of loops in the two coils mean that a different voltage can be obtained in the second coil.

transistor   transistor

Transistor. A semiconductor device with three leads. It allows a large current to flow across two of its leads when a small current flows across a different two leads (one shared, obviously).

They are very useful for amplifying small signals into large signals, for example in a radio receiver amplifying the radio signal or in an audio amplifier making a sound signal louder.

Transistors are either NPN (with the arrow pointing out) or PNP (with the arrow pointing in). The arrow helps show which way current flows through the transistor.


Voltmeter. A device used to measure the potential difference (voltage) between two points in a circuit.

See Measuring Circuits for information on how to use a voltmeter.