In the following equation, R is resistance; VSUPPLY is the voltage supplied from the power source (this is 5V for a standard Arduino digital pin, but could be more or less if the VIN pin is used); VFORWARD is the voltage required by the component, and I is the current required by the component:
R = (VSUPPLY - VFORWARD) / I
Here is an example for powering an LED:
(5V – 2V) / 0.03A = 100Ω
After you’ve determined which resistor you need, the next task is to find it. Fixed-value resistors use colored bands to indicate the value of the resistor. To find the value you can use a multimeter on the ohms (Ω) setting, but if you don’t have a multimeter handy, use the following table to find the value instead. For example, a resistor with brown, black, brown, and gold bands is a 100Ω resistor with a 5% tolerance.
In addition, the Circuit Playground by Adafruit has resistor color codes as well as a variety of other useful tools for building circuits.
| Color | Value | Multiplier | Tolerance |
|---|---|---|---|
| Black | 0 | x100 | - |
| Brown | 1 | x101 | ±1% |
| Red | 2 | x102 | ±2% |
| Orange | 3 | x103 | - |
| Yellow | 4 | x104 | ±5% |
| Green | 5 | x105 | ±0.5% |
| Blue | 6 | x106 | ±0.25% |
| Violet | 7 | x107 | ±0.1% |
| Grey | 8 | x108 | ±0.05% |
| White | 9 | x109 | - |
| Gold | - | x10-1 | ±5% |
| Silver | - | x10-2 | ±10% |
| None | - | - | ±20% |