Reading Circuit Diagrams and Resistor Codes

Introduction

A schematic is the map of a circuit. It shows how parts connect, not what they look like. Once you can read one, you can build and understand circuits from anywhere in the world.

How to read a schematic

• Each part is drawn as a symbol, with a label such as R1 for the first resistor, C1 for a capacitor, and D1 for a diode.
• Lines are wires. A dot where lines cross means they connect. Lines crossing without a dot do not connect.
• Values appear next to parts, such as 150 Ω or 100 µF.
• Ground is the 0 V reference point, usually the battery's negative side.
• Current is usually drawn from the positive supply at the top, through parts, to ground at the bottom.

To follow a circuit: start at the positive terminal, trace the wire through each part in order, and end at the negative terminal. Ask at each junction: does the current split (parallel) or continue (series)?

In the last lesson you will build an LED circuit. In its diagram, start at the 5 V battery, go through the resistor, then the LED, then back to the battery. It is a series circuit.

Resistor colour code

Small resistors are too tiny to print numbers on, so they use coloured bands.

The resistor colour code. Digits 0 to 9 are black, brown, red, orange, yellow, green, blue, violet, grey and white.

For a four-band resistor:

1. Band 1: first digit
2. Band 2: second digit
3. Band 3: multiplier (how many zeros to add, or ×10 to the power)
4. Band 4: tolerance. Gold is ±5%, silver is ±10%.

Example: brown, green, brown, gold means 1, 5, ×10, ±5%, which is 150 Ω.

Another: red, red, orange, gold means 2, 2, ×1,000, which is 22,000 Ω, written 22 kΩ.

Tips: read from the end where the bands are closer together, and with a multimeter you can check the value for certain.

Writing values

• Below 1,000: write ohms, such as 470 Ω.
• 1,000 and up: kilohms, such as 4.7 kΩ = 4,700 Ω.
• 1,000,000 and up: megohms, such as 1 MΩ.

Resistors come in standard values, such as 100, 150, 220, 330, 470 and 1 kΩ. You pick the nearest standard value to what you calculate.

Try it yourself

Decode these: (1) orange, orange, brown, gold. (2) brown, black, red, gold. (3) green, blue, orange, gold. Check your answers: 330 Ω, 1 kΩ and 56 kΩ.

Summary

• A schematic shows connections, using symbols, labelled parts, dots for joined wires and a ground reference.
• Resistor bands give two digits, a multiplier and a tolerance.
• Use standard resistor values, and a multimeter to confirm.