LIBRARYResistors.

A resistor is a part built to have a chosen, stable resistance, and almost every board is covered in them. What they do, the standard values you can actually buy, and how to read one off a schematic, is most of what you need.

What does a resistor do?

A resistor limits current, sets a voltage when paired with another resistor as a divider, pulls a signal line to a known level, or senses a current by the small voltage it drops. Each use is the same part chosen for a different value.

The standard values: the E-series

You cannot buy just any resistance. Manufacturers make a fixed set of values in each decade, spaced so their tolerance bands overlap, and this set is the E-series defined by IEC 60063. The E24 series has 24 values per decade for 5 percent parts; the E12 series has 12 for 10 percent parts. So you compute the value you want, then pick the nearest standard one.

Tolerance and power rating

Tolerance is how close the real value sits to the printed one, commonly 1 or 5 percent. The power rating is how much heat the part can shed before it drifts or fails, so size it above the power it dissipates. On a small board these are surface-mount chips in 0402 and 0603 sizes.

A One Thousand Drones board's bill of materials is mostly 0402 and 0603 chip resistors at E24 values, 5.1 kΩ, 10 kΩ, 330 Ω, chosen because they are standard and in stock.