part · Level 2 · Sound · input
How a potentiometer works
10 kΩ potentiometer — A resistor with a knob — twist to change the speed bump size.
What it is
A potentiometer — a pot — is a resistor you can adjust. Inside is a curved strip of resistive material with the full 10 kΩ between its two ends, and a metal contact called the wiper that slides along the strip when you turn the shaft.
Three legs: the two outer ones are the ends of the strip, always 10 kΩ apart. The middle leg is the wiper. Turn the knob fully one way and the wiper touches one end (0 Ω to that leg, 10 kΩ to the other); turn it the other way and the numbers swap. Everything in between is in between.
Use two legs and it is a variable resistor — a dial that sets current. Use all three and it is a voltage divider — a dial that sets voltage.
How it works
As a variable resistor (rheostat). Wiper and one outer leg in series with a 220 Ω and a red LED on 4.5 V:
- Knob at 0 Ω: total 220 Ω → 2.5 ÷ 220 ≈ 11 mA, full brightness.
- Knob at 5 kΩ: total 5220 Ω → 2.5 ÷ 5220 ≈ 0.48 mA, a dim glow.
- Knob at 10 kΩ: total 10 220 Ω → 0.24 mA, effectively off.
The 220 Ω stays because at the 0 Ω end the pot alone would let the LED draw whatever it likes. That is Dial-a-Brightness.
As a voltage divider. Outer legs across the pack, wiper as the output: V_out = 4.5 V × (resistance from wiper to −) ÷ 10 kΩ. Middle of travel: 4.5 × 5 ÷ 10 = 2.25 V. The strip itself always passes 4.5 ÷ 10 000 = 0.45 mA, wherever the knob is. This is how a pot feeds an analog pin on an Arduino later.
Volume knob. In series with an active buzzer (≈ 150 Ω): at 1 kΩ, 4.5 ÷ 1150 ≈ 3.9 mA (quiet); at 10 kΩ, 0.44 mA (silent). The Whisper Alarm.
Most hobby pots are linear (resistance changes evenly with angle). Audio pots are logarithmic, to match how ears hear loudness — the divider maths is the same, the feel is different.
The numbers
| Total resistance | 10 kΩ — outer leg to outer leg, always |
|---|---|
| Wiper range | 0 – 10 kΩ — to either outer leg |
| Divider output at mid-travel | 2.25 V — 4.5 × 5 ÷ 10 |
| Current through the strip (as divider) | 0.45 mA — 4.5 ÷ 10 000 |
| Power rating (typical) | ≈ 0.1–0.25 W — never stressed at 4.5 V |
See it working
The Dial-a-Brightness circuit, simulated live. Open it on the Workbench to change anything.
simulating…
Mistakes everyone makes
- Using the two outer legs only. Turning the knob does nothing — it is a fixed 10 kΩ. Fix: one outer leg plus the middle wiper.
- Pot straight in series with an LED, no fixed resistor. At the 0 Ω end the LED gets unlimited current. Fix: keep the 220 Ω in the lane.
- Wiper as the only path from + to − with the outer legs unused. Nothing flows. Fix: the divider needs both ends on the supply.
- Legs too fat for the breadboard. Panel pots have solder lugs. Fix: buy breadboard-friendly (PCB-pin) pots, or use short wires.
Questions people ask
- How do you wire a potentiometer as a variable resistor?
- Use the middle (wiper) leg and one outer leg; leave the other outer leg unconnected. Turning the knob varies the resistance between those two from 0 to 10 kΩ.
- What does the middle pin of a potentiometer do?
- It is the wiper — the sliding contact. With the outer pins across the battery, the middle pin gives a voltage from 0 V to 4.5 V depending on the knob position.
- Can a potentiometer dim an LED?
- Yes, in series with a fixed 220 Ω. On 4.5 V the current runs from about 11 mA (knob at 0 Ω) down to 0.24 mA (knob at 10 kΩ).
Projects that use this part
- 🎛️ Dial-a-Brightness — Twist a knob and watch the LED fade up and down — your first "how much" input. L2 · 20 min
- 📻 Dial-a-Buzz — Dial-a-Brightness for sound — one knob rides the buzzer between full alarm and library silence, with no button in the way. L2 · 15 min
- 🔈 Whisper Alarm — A doorbell with a volume knob — the pot lives in the buzzer lane, so twisting it changes the beep without touching the light. L2 · 25 min
- 📖 Reading Light — The Automatic Night Light with a knob on top — turn it to dial in how dark the room has to get before the lamp fires. Your first tunable threshold. L3 · 30 min
- 🎚️ Dial-a-Spin — Dial-a-Brightness with a motor instead of an LED. Twist the knob and watch the fan speed slide from full spin to a slow crawl. L4 · 20 min
verified against: Arduino — Potentiometer Basics · SparkFun — Voltage Dividers · SparkFun — Resistors