Resistors

Resistors are the least glamorous component in your kit and quite possibly the most important. They do not light up, spin or beep. What they do is slow the flow of electricity, and in doing so they protect LEDs from burning out, hold input pins at sensible levels and let sensors produce readable voltages.

What resistance actually means

Go back to the water picture: if voltage is pressure and current is flow, resistance is a narrow section of pipe. Squeeze the pipe and less water gets through for the same pressure. Resistance is measured in ohms (Ω), and you will meet values from a few ohms up to millions. In Arduino projects three values cover most situations:

• 220Ω — the LED resistor. Almost every LED in every project uses one.
• 10kΩ — the pull-up or pull-down resistor for buttons and sensors.
• 1kΩ — a general-purpose in-between value, handy for driving transistors.

Reading the colour bands

Resistors are too small to print numbers on, so their value is written in coloured stripes. Each colour is a digit: black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9. On a four-band resistor, the first two bands are digits, the third says how many zeros to add, and the fourth (gold or silver, set slightly apart) is the tolerance.

So red-red-brown means 2, 2, add one zero — 220 ohms. Brown-black-orange means 1, 0, add three zeros — 10,000 ohms, which everybody calls 10k. Learning just those two patterns will cover most of your building, and a cheap multimeter will tell you the rest instantly.

Resistors do not care which way round

Unlike LEDs and chips, a resistor has no direction. Either way round works. This is a small relief when you are wiring a busy breadboard at speed.

The voltage divider

Put two resistors in a line between 5V and ground, and the point between them sits at a voltage somewhere in the middle — the exact value depends on the ratio of the two resistances. This little arrangement is called a voltage divider and it is everywhere.

It becomes really useful when one of the two resistors is a sensor. A photoresistor changes with light, so the middle point moves up and down as the room brightens and darkens. Connect that middle point to A0, and suddenly the Arduino can see the light. A potentiometer is the same idea in one package: turning the knob slides the middle connection along a strip of resistive material.

Choosing an LED resistor

The formula is R = (supply voltage − LED voltage) ÷ desired current. For a red LED on 5V: (5 − 2) ÷ 0.02 = 150Ω. A 220Ω resistor is the nearest common value, gives a slightly gentler current and a longer LED life, which is exactly why it is the one in every kit.