Electronics for Building Things · Lesson 8 · Components in Depth
Resistors in Depth
Read any resistor's value, pick the right one, and meet the voltage divider.
You met the resistor in Lesson 1 as a "narrow pipe" that
limits current. Now we go deeper: how to read a resistor, how to choose one, and one
new trick — the voltage divider — that you'll use constantly to read sensors.
The one win
Pick up any resistor and know its value and power rating — and understand how two resistors can turn
a big voltage into a smaller one.
Reading the color bands
Through-hole resistors are marked with colored bands. The common 4-band scheme reads
like this (SparkFun: Resistors):
Band 1 & 2 — digits. Each color is a digit 0–9.
Band 3 — multiplier. How many zeros to add (×10, ×100, ×1000…).
Band 4 — tolerance. Usually gold (±5%) — how close to the marked value it really is.
Example: yellow, violet, red → 4, 7, then ×100 → 4,700 Ω = 4.7 kΩ.
Rule of thumb (don't memorize)
Nobody memorizes the color code. Use a phone app, a printed chart, or just measure with a
multimeter's resistance setting. Knowing how the bands work is enough; let a tool do the lookup.
You'll keep meeting the same handful of "standard" values — 220, 330, 1k, 4.7k, 10k, 100k. A small
assortment kit covers almost every hobby project.
Power rating: can it take the heat?
Resistors turn the energy they block into heat. Each has a power rating —
typically ⅛ W to 1 W for the ones you'll use. Exceed it and the resistor overheats,
discolors, and can fail.
Rule of thumb
For signal-level work (LEDs, pull-ups, dividers on a 3.3–5V board), the standard ¼ W
resistor is plenty. Only worry about wattage when a resistor carries real current — e.g. dropping
voltage for a power load.
The voltage divider
Put two resistors in series across a voltage, and the point between them sits at a smaller
voltage — a voltage divider. It's how you scale a voltage down, and how a
board reads many sensors (SparkFun: Voltage Dividers).
The output between R1 and R2 is a fraction of Vin, set by the ratio of the two resistors.
The intuition, no algebra needed: the output is Vin scaled by R2's share of the total
resistance. Equal resistors → half the voltage. Make R2 small → small output; make R2 large →
output close to Vin.
Why you'll use it
Read resistive sensors (photoresistor, thermistor): pair the sensor with a
fixed resistor as a divider, and the middle voltage changes with light/heat — read it with an
analog pin (next lesson on potentiometers uses this exact idea).
Scale a voltage down to fit a 3.3V ESP32 input (recall the 3.3V vs 5V trap from
Lesson 4).
Heads-up: a divider is for signals, not for powering things — drawing real current
from the midpoint breaks the ratio. For that you use a regulator.