Resistors and colour codes

A resistor limits current. It protects LEDs, sets signal levels and turns heat into a useful safety margin. The value is printed as coloured bands because the part is too small for numbers.

Reading four bands

1. Hold the resistor so the gold or silver band is on the right.
2. Band 1 is the first digit and band 2 is the second digit.
3. Band 3 is how many zeros to add.
4. Band 4 is the tolerance. Gold means ±5%.

0 black · 1 brown · 2 red · 3 orange · 4 yellow · 5 green · 6 blue · 7 violet · 8 grey · 9 white

Three you will use often

• 220 Ω: red, red, brown. Used with LEDs.
• 1 kΩ: brown, black, red. Used for signals and dividers.
• 10 kΩ: brown, black, orange. Used for pull-up and pull-down.

Power rating

Common resistors handle 0.25 W. Check with this formula, and stay well under the rating:

P = V² ÷ R

5 V across 220 Ω is 0.11 W, which is safe. 12 V across 100 Ω is 1.44 W, and that small resistor would burn.

The voltage divider

Two resistors in series can lower a voltage. Many sensors in our projects give 5 V, but an ESP32 pin must not get more than 3.3 V. A 1 kΩ and 2 kΩ pair brings 5 V down to about 3.3 V.

Use the resistor colour code tool in Tools to check any resistor.