Analog Inputs & PWM on Arduino

Analog vs Digital

Digital signals are either fully ON or fully OFF. Analog signals vary continuously ߀” like the position of a potentiometer, the light level hitting a photoresistor, or a temperature sensor's voltage output.

The Arduino Uno has a 10-bit ADC (Analog-to-Digital Converter) on pins A0߀“A5. It converts voltages between 0 V and 5 V into numbers from 0 to 1023.

Reading an Analog Pin ߀” analogRead()

int value = analogRead(A0);  // Returns 0߀“1023

No pinMode() call is needed for analog inputs ߀” they are always configured as inputs.

Converting to Voltage

int raw = analogRead(A0);
float voltage = raw * (5.0 / 1023.0);

Reading a Potentiometer

A potentiometer (pot) is a variable resistor. Wire its outer legs to 5 V and GND, and the middle wiper to A0.

void setup() {
  Serial.begin(9600);
}

void loop() {
  int pot = analogRead(A0);
  Serial.println(pot);   // Print raw value (0߀“1023)
  delay(100);
}

Open Tools ߆’ Serial Monitor to see the values change as you turn the knob.

PWM ߀” Simulating Analog Output

The Arduino Uno cannot output a true analog voltage, but it can simulate one using PWM (Pulse Width Modulation). PWM rapidly switches a pin on and off; the ratio of on-time to off-time is the duty cycle.

Only pins marked with ~ support PWM on the Uno: 3, 5, 6, 9, 10, 11.

Writing PWM ߀” analogWrite()

analogWrite(pin, value);  // value: 0 (always off) to 255 (always on)

analogWrite(9, 0);    // LED fully off
analogWrite(9, 128);  // LED at ~50% brightness
analogWrite(9, 255);  // LED fully on

LED Dimmer with a Potentiometer

Map the 0߀“1023 pot reading to the 0߀“255 PWM range using map():

void setup() {
  pinMode(9, OUTPUT);
}

void loop() {
  int pot = analogRead(A0);
  int brightness = map(pot, 0, 1023, 0, 255);
  analogWrite(9, brightness);
}

map(value, fromLow, fromHigh, toLow, toHigh) rescales a number from one range to another.

The map() Function

long map(long x, long in_min, long in_max, long out_min, long out_max);

Examples:

map(512, 0, 1023, 0, 255)   // ߆’ 127
map(0,   0, 1023, 0, 255)   // ߆’ 0
map(1023,0, 1023, 0, 255)   // ߆’ 255

Common Sensors

Sensor	Reads	Typical range
Potentiometer	Rotation	0߀“1023
LDR (photoresistor)	Light level	0߀“1023
TMP36	Temperature	Voltage ߉? 0.5 V at 0 ?°C
Soil moisture	Moisture	0߀“1023

Reading Temperature with TMP36

void setup() {
  Serial.begin(9600);
}

void loop() {
  int raw = analogRead(A0);
  float voltage = raw * (5.0 / 1023.0);
  float tempC = (voltage - 0.5) * 100.0;   // TMP36 formula
  Serial.print("Temp: ");
  Serial.print(tempC);
  Serial.println(" C");
  delay(1000);
}