Reading Resistor Colour Codes

Learning Goals 5 min

By the end of this lesson you will be able to:

1. Read any 4-band resistor by starting from the tolerance band and decoding the colours into a number with units.
2. Use a multimeter set to ohms (Ω) to measure an actual resistor and compare the reading to its rated value.
3. From your kit, physically identify a 220 Ω, a 1 kΩ and a 10 kΩ resistor by colour alone — no multimeter peeking.

Warm-Up 10 min

Last lesson we used Ohm's Law to pick the right resistor for a 5 V circuit. Today we learn how to actually find that resistor in your kit — because every resistor looks the same at a glance.

Quick-fire puzzle

Mia opens her starter kit and tips out a bag of resistors. They all look like tiny beige tubes with coloured stripes. Without measuring anything:

1. How can two resistors that look almost identical have completely different values?
2. Why does the kit use coloured stripes instead of just printing the number on the side?
3. If a resistor has 4 stripes — red, red, brown, gold — which end do you start reading from?

New Concept 20 min

The big idea — a barcode for ohms

A 4-band resistor's stripes are a tiny barcode. Each colour stands for a digit. Read them in order and you get the resistance value in ohms.

What each band means

On a 4-band resistor, reading from the tolerance band inwards:

• Band 1 — first digit of the value
• Band 2 — second digit of the value
• Band 3 — the multiplier: how many zeros to add after the two digits
• Band 4 — tolerance: how accurate the resistor is (almost always gold = ±5% in your kit)

Anatomy of a resistor

The colour-to-digit chart

This is the chart you will use for the rest of the course. Keep it open in front of you.

A trick to remember the colour order

The first ten colours go in the order of the rainbow, with black at the start and grey/white at the end. A short Spanish-and-English memory phrase used in workshops in Tokyo:

"Big Bats Race On Yellow Ground, Blue Vans Go West."

Black, Brown, Red, Orange, Yellow, Green, Blue, Violet, Grey, White. Make up your own — the sillier, the better it sticks.

Decode one for yourself

Pick up a real resistor from your kit. Turn it so the gold band is on the right. Now set the three left-hand pickers below to the colours you can actually see, and read off what they mean.

If the value that comes out is not one of the five in your kit — 220 Ω, 560 Ω, 1 kΩ, 4.7 kΩ or 10 kΩ — you have almost certainly read the resistor from the wrong end. Turn it round and try again.

Recording a decoded value in a sketch

Just like the last lesson, we record each decoding as a comment in an Arduino sketch. The chip ignores comments — humans read them.

// Resistor decoding worksheet — Mia, Year 8
// Bands: red, red, brown, gold
// Digits: 2, 2  →  22
// Multiplier: brown = x10
// Value: 22 x 10 = 220 ohms  (±5% tolerance)

void setup() {
}

void loop() {
}

Why it matters

Every external component in this course needs the right resistor. Pick the wrong one and your LED is either invisible or fried. Reading the colour code is the very first habit of a good electronics builder — faster than reaching for a multimeter every single time.

Worked Example 20 min

Goal: decode three resistors from the starter kit and verify each with a multimeter.

Step 1 — orient the resistor

Pick up the first resistor. Turn it so the gold band is on the right. Now you are reading left to right.

Step 2 — read the first three bands

Suppose you see: brown, black, red, gap, gold.

• Band 1 — brown = 1
• Band 2 — black = 0
• Band 3 — red = multiplier ×100
• Band 4 — gold = ±5% tolerance

Step 3 — combine

• Digits: 10
• Value: 10 × 100 = 1 000 Ω = 1 kΩ

So that resistor is a 1 kΩ. The ±5% tolerance means a multimeter reading anywhere from 950 Ω to 1 050 Ω is normal and fine.

Step 4 — verify with a multimeter

1. Set the multimeter dial to the Ω (ohms) region. If it has ranges, pick 2 kΩ or 20 kΩ — anything bigger than the expected value.
2. Touch one probe to each leg of the resistor. Don't pinch the metal legs with your fingers — your body's resistance will throw the reading off.
3. Read the screen. Expect roughly the rated value, within ±5%.

Step 5 — decode two more and record everything in a sketch

Repeat for two more resistors. For each, write down the colours, the decoded value, and the multimeter reading.

// Three resistors decoded and measured — Felix, Year 8
// 1) brown, black, red, gold     -> 10 x 100 = 1 000 ohm = 1 kohm
//    Multimeter reading: 0.98 kohm  -> within 5%, OK

// 2) red, red, brown, gold       -> 22 x 10 = 220 ohm
//    Multimeter reading: 218 ohm    -> within 5%, OK

// 3) brown, black, orange, gold  -> 10 x 1k = 10 000 ohm = 10 kohm
//    Multimeter reading: 9.91 kohm  -> within 5%, OK

void setup() {
}

void loop() {
}

What changed since last lesson? In L01-05 you calculated which resistor you needed. Today you can actually find that resistor on your bench in seconds — without measuring every one.

Basic 7 min

For each task, decode the bands on paper, then verify with a multimeter. Save your final notes as comments in a sketch named resistor-codes.

// Task: yellow, violet, brown, gold
// Digits: ____, ____  ->  ____
// Multiplier: brown = x____
// Value: ____ x ____ = ____ ohm
// Multimeter reading: ____ ohm

void setup() {
}

void loop() {
}

Challenge 1 7 min

// Three mystery resistors from my kit
// A) bands: ____, ____, ____, gold  -> ____ ohm
// B) bands: ____, ____, ____, gold  -> ____ ohm
// C) bands: ____, ____, ____, gold  -> ____ ohm

void setup() {
}

void loop() {
}

Challenge 2 6 min

// Task: find a 4.7 kohm resistor by its bands
// 4.7 kohm = 4 700 ohm = 47 x 100
// Band 1 (digit 4): ________
// Band 2 (digit 7): ________
// Band 3 (x100):    ________
// Band 4 (5%):      gold
// Multimeter reading: ____ ohm

void setup() {
}

void loop() {
}

Challenge 3 15 min

The wrong-resistor detective

A classmate is getting ready for next week's LED lesson. They planned four LED circuits on a 5V supply. Each LED drops about 2 V, so each resistor sees 3 V. The safe, visible range is 5 mA to 25 mA. They wrote down the bands like this:

Your task:

1. Decode each resistor. One of them was written down from the wrong end — spot it and read it the right way.
2. Use I = V ÷ R with V = 3 V to find each current in milliamps.
3. Give each circuit a verdict: too much, too little or OK.
4. Find the resistor in your kit that gives closest to 15 mA. Pick it out by its bands alone.
5. Check it with a multimeter.

It works if all four verdicts match the maths, and your multimeter reads within 5% of the value you picked.

Recap 5 min

A 4-band resistor's value is a colour barcode. Read from the tolerance band (gold) inwards: two digits, a multiplier, and the tolerance. Combine the digits, multiply, and you have the resistance in ohms — usually within ±5% of the printed value.

Resistor

A small component that limits how much current can flow. Measured in ohms (Ω). The kit holds 220 Ω, 560 Ω, 1 kΩ, 4.7 kΩ and 10 kΩ values.

Band

A coloured stripe painted around a resistor. 4-band resistors have two digit bands, one multiplier band and one tolerance band.

Multiplier

The third band's value, telling you how many times to multiply the two digits. Brown = ×10, red = ×100, orange = ×1 000.

Tolerance

How accurate the resistor's value is. Gold = ±5% — almost every resistor in your starter kit.

Multimeter

A handheld instrument that measures resistance, voltage and current. Set the dial to Ω to check a resistor.

Extra Mission 5 min

Part 1 — Design a light code for numbers

A resistor's bands are a code for three numbers. Could one blinking LED say those numbers too? On paper, design a light code that lets the on-board LED "speak" a resistor's three band digits.

Your design must include:

• How a digit is shown, such as one quick flash for each count.
• How a zero is shown, since zero flashes would be invisible.
• The gap between two digits, and the longer gap before the code repeats, in milliseconds.
• Your code written out for 4.7 kΩ (yellow, violet, red: 4, 7, 2).

Part 2 — Make it

Write and upload a sketch that makes the on-board LED blink out the 4.7 kΩ code exactly as you designed it. Use only pinMode, digitalWrite and delay. Add a comment above each group of flashes naming the band and its digit. Then pick the 4.7 kΩ resistor out of your kit by its bands and check it with a multimeter.

Bring back next class: your light-code design, the saved .ino file, and your multimeter reading.