Potentiometer with Arduino Uno — analogRead, volts and PWM dimming

A potentiometer is a knob that gives out any voltage between its two ends. It is the first analog input most people use: it sets a brightness, a motor speed, a servo angle or a threshold.

On the Uno, put the outer legs on GND and 5V and the middle leg (the wiper) on A0. The Uno’s 10-bit ADC turns 0–5 V into a number from 0 to 1023.

The sketch prints the raw reading, the voltage and a percentage. It also passes the value to PWM on D9, so an LED there dims as you turn the knob. That is the same analogRead → map → analogWrite pattern you will use for fans and motors later.

Wiring

Board quirks

• Use a 10 kΩ pot. The Uno’s ADC wants a source of 10 kΩ or less; a 100 kΩ or 1 MΩ pot gives readings that wander and lag.
• The Uno’s ADC is 10-bit: 0–1023 for 0–5 V, about 4.9 mV per step.
• The voltage shown assumes exactly 5.0 V. On USB the 5V pin is often 4.7–5.1 V, so the volts can be a little off — but the raw value and the percent stay right, because the pot and the ADC use the same supply.
• analogWrite() only works on the PWM pins (3, 5, 6, 9, 10, 11). The sketch uses D9; add an LED with a 220 Ω resistor from D9 to GND to see it dim.

Code

// Potentiometer on an Arduino Uno R3: Pin 1 → GND, Wiper → A0, Pin 3 → 5V.
// Prints the raw 10-bit reading, the voltage and a percentage, and sets the
// PWM duty on D9 (connect an LED + 220 Ω resistor there to see it dim).
const uint8_t POT_PIN = A0;
const uint8_t PWM_PIN = 9;

void setup() {
  pinMode(PWM_PIN, OUTPUT);
  Serial.begin(9600);
  Serial.println(F("Turn the knob"));
}

void loop() {
  int raw = analogRead(POT_PIN);    // 0..1023
  float volts = raw * 5.0 / 1023.0; // assumes 5.0 V - USB is often 4.7-5.1 V
  int pct = map(raw, 0, 1023, 0, 100);
  analogWrite(PWM_PIN, raw / 4);    // 0..255
  Serial.print(F("Raw "));
  Serial.print(raw);
  Serial.print(F("  "));
  Serial.print(volts, 2);
  Serial.print(F(" V  "));
  Serial.print(pct);
  Serial.println(F(" %"));
  delay(200);
}

// Potentiometer on an Arduino Uno R3: Pin 1 → GND, Wiper → A0, Pin 3 → 5V.
// Prints the raw 10-bit reading, the voltage and a percentage, and sets the
// PWM duty on D9 (connect an LED + 220 Ω resistor there to see it dim).
const uint8_t POT_PIN = A0;
const uint8_t PWM_PIN = 9;

void setup() {
  pinMode(PWM_PIN, OUTPUT);
  Serial.begin(9600);
  Serial.println(F("Turn the knob"));
}

void loop() {
  int raw = analogRead(POT_PIN);    // 0..1023
  float volts = raw * 5.0 / 1023.0; // assumes 5.0 V - USB is often 4.7-5.1 V
  int pct = map(raw, 0, 1023, 0, 100);
  analogWrite(PWM_PIN, raw / 4);    // 0..255
  Serial.print(F("Raw "));
  Serial.print(raw);
  Serial.print(F("  "));
  Serial.print(volts, 2);
  Serial.print(F(" V  "));
  Serial.print(pct);
  Serial.println(F(" %"));
  delay(200);
}

Expected serial output

Turn the knob
Raw 0  0.00 V  0 %
Raw 512  2.50 V  50 %
Raw 1023  5.00 V  100 %

Turn the knob
Raw 0  0.00 V  0 %
Raw 512  2.50 V  50 %
Raw 1023  5.00 V  100 %

Common errors