The big idea: A circuit diagram shows what is connected to what, using standard symbols — not what the board looks like.
The symbols are fixed by standard so that a circuit drawn in one country can be read in another, by somebody who does not speak the language.
Every symbol the guide names, grouped the way an exam groups them — then a complete circuit built from them.
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| Group | How to recognise them |
|---|---|
| Passive | A plain box is a fixed resistor; an arrow across it makes it variable. Two parallel plates are a capacitor, and a curved second plate means it is polarised. A gap with a lever is a switch. A coil drawn beside a separate pair of contacts is a relay |
| Active | A triangle pointing at a bar means current one way only — a diode. Arrows leaving it mean light comes out, so it is an LED. A circle containing a bar and two angled legs is a transistor. A triangle with + and − inputs and one output is an op-amp |
| Inputs | Most sensors are a resistor symbol inside a circle with a mark saying what changes it: arrows IN for light, t° for temperature. That is a reminder that they behave as resistors in the circuit |
| Outputs | A circle with a letter names it — M for a motor, B for a buzzer. A cross in a circle is a lamp, and a rectangle opening into a cone is a loudspeaker |
A dot means a junction: Wires that cross with a dot are connected. Wires that cross without one are not.
Missing that is the commonest way a student misreads a circuit they otherwise understand.
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A light-operated switch, read in four steps
Find the supply rails
The two horizontal lines, positive at the top and 0 V at the bottom. Everything sits between them.
Find the sensing chain
An LDR and a variable resistor in series across the rails: a voltage divider, with the signal at their junction.
Follow the signal
From that junction to the base of a transistor. As it gets dark the junction voltage rises and the transistor turns on.
Find the load
The lamp between the positive rail and the transistor, so the transistor completes its circuit to 0 V. The variable resistor sets how dark it must be first.
Read a circuit the way you read a system: Input, process, output — in that order, on the drawing.
The sensing chain is the input, the transistor or chip is the process, and whatever the current finally does work in is the output. Every circuit in this course reads that way.
How this is tested — identifying the symbols used in a circuit diagram. It comes up two ways:
Paper 1 — multiple choice
- Name the component a given symbol represents.
- Identify which two symbols are connected in a drawing.
Paper 2 — analysing a product
- Identify the components in a circuit and explain the function of each.
- Explain what a given circuit does, following the signal through it.
The trap: Naming symbols without following the circuit. The marks in Paper 2 are for saying what the circuit DOES, which means tracing the signal from the sensor to the load.
A circuit contains a thermistor and a variable resistor in series across a 9 V supply, with the junction connected to a transistor base, and a relay coil with a diode across it in the transistor's collector circuit. Explain what this circuit does.
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