Resistors are too small to print their value on, so they use coloured stripes instead. Once you know the code, you can read any resistor in a few seconds. In this lesson you learn the colour code, sort the six resistor values in your kit, and build three LEDs with three different resistors to see the difference.
What you'll learn
- The resistor colour code, with a sentence to remember it
- How to read 4-band and 5-band resistors
- Which way round to read a resistor
- What kΩ and MΩ mean
What you need
- UNO R3 board and USB cable (UNO R3 Starter Kit)
- 3 red LEDs
- Resistors: 220 Ω, 560 Ω and 4.7 kΩ
- Breadboard and jumper wires (UNO R3 Starter Kit)
The colour code
| Colour | Number | As the multiplier |
|---|---|---|
| Black | 0 | × 1 |
| Brown | 1 | × 10 |
| Red | 2 | × 100 |
| Orange | 3 | × 1,000 |
| Yellow | 4 | × 10,000 |
| Green | 5 | × 100,000 |
| Blue | 6 | × 1,000,000 |
| Violet | 7 | |
| Grey | 8 | |
| White | 9 | |
| Gold | Tolerance band: ± 5 % | |
| Brown (last band, set apart) | Tolerance band: ± 1 % |
A sentence to remember the order: Big Brown Rabbits Often Yield Great Big Vocal Groans When gingerly snapped: black, brown, red, orange, yellow, green, blue, violet, grey, white.
How to read one
- Turn the resistor so the gold band, or the band that has a bigger gap before it, is on the right. You read from left to right.
- 4 bands (usually on beige resistors): the first two bands are digits, the third is the multiplier. Red, red, brown = 2, 2, × 10 = 220 Ω.
- 5 bands (usually on blue resistors): the first three bands are digits, the fourth is the multiplier. Red, red, black, black = 2, 2, 0, × 1 = 220 Ω.
- The last band is the tolerance: how far from the printed value the real value may be.
Big values use short names: 1,000 Ω is 1 kΩ (kilo-ohm), and 1,000,000 Ω is 1 MΩ (mega-ohm). So 4,700 Ω is written 4.7 kΩ.
The resistors in your kit
| Value | 4 bands | 5 bands |
|---|---|---|
| 220 Ω | red, red, brown, gold | red, red, black, black, brown |
| 560 Ω | green, blue, brown, gold | green, blue, black, black, brown |
| 4.7 kΩ | yellow, violet, red, gold | yellow, violet, black, brown, brown |
| 10 kΩ | brown, black, orange, gold | brown, black, black, red, brown |
| 1 MΩ | brown, black, green, gold | brown, black, black, yellow, brown |
| 10 MΩ | brown, black, blue, gold | brown, black, black, green, brown |
Sort your resistors into six groups and label them. Some colours, like red and orange or brown and red, are hard to tell apart in poor light; use a lamp or the torch on a phone.
Wiring
| From | To |
|---|---|
| UNO 5V | Bottom + rail (red wire) |
| UNO GND | Bottom − rail (black wire) |
| + rail | Columns 14, 28 and 42, row j |
| Resistor 220 Ω | Column 14 (row h) to column 18 (row h), LED 1 in columns 18 and 19, row f |
| Resistor 560 Ω | Column 28 (row h) to column 32 (row h), LED 2 in columns 32 and 33, row f |
| Resistor 4.7 kΩ | Column 42 (row h) to column 46 (row h), LED 3 in columns 46 and 47, row f |
| Columns 19, 33 and 47, row j | − rail |
The long leg (+) of each LED goes in the column with the resistor.
Try it
- Plug in the USB cable. LED 1 is bright, LED 2 a little dimmer and LED 3 is dim.
- Work out each current with Ohm's law: 3 V ÷ 220 Ω = 13.6 mA, 3 V ÷ 560 Ω = 5.4 mA and 3 V ÷ 4,700 Ω = 0.6 mA. Our eyes are very sensitive, so even 0.6 mA is easy to see.
- Swap the 4.7 kΩ resistor for 1 MΩ. The LED goes out, or you can only just see it in the dark: 3 µA (millionths of an amp) is too little.
If you have a multimeter, set it to ohms (Ω) and measure each resistor on its own, not in the circuit. A 220 Ω resistor might read 218 Ω: that's the tolerance. The multimeter guide explains how.
Try this next
- Ask someone to hand you a resistor at random and read it in under 10 seconds.
- Put two 220 Ω resistors one after the other (in series) for LED 1. It gets dimmer: they add up to 440 Ω.
- Look at an old toy or gadget circuit board and try to read some of its resistors.
Troubleshooting
- Nothing lights up: check that the USB cable is plugged in (the UNO's ON LED is lit), that the long leg of the LED goes to the + side, and that every wire sits in the column the table says. Wiggle each wire gently; a wire that isn't pushed all the way in is the most common fault.
- Two LEDs look equally bright: check the stripes again; it's easy to mix up the 220 Ω and 560 Ω resistors at a glance.
- You can't tell which end to start from: look for the gold band, or the band with a bigger gap before it. That one goes on the right.
