What is a sensor?

Learn

What you’ll learn

• What a sensor does
• The difference between digital and analog signals
• How an Arduino reads each one

Sensors turn the world into numbers

A sensor measures something (light, distance, touch, tilt, temperature) and turns it into an electrical signal the controller can read. It’s the sense part of sense, think, act.

Digital signals: on or off

A digital signal has two states: HIGH (about 5 V on an UNO) or LOW (0 V). Buttons, bump switches and many obstacle modules are digital.

int state = digitalRead(2);   // HIGH or LOW

Analog signals: anything in between

An analog signal can be any voltage in a range, like 0 to 5 V. Knobs (potentiometers), light sensors and some distance sensors are analog. The UNO’s analog pins A0–A5 measure the voltage and give a number from 0 to 1023:

Voltage on A0	analogRead(A0)
0 V	0
2.5 V	about 512
5 V	1023

int raw = analogRead(A0);            // 0–1023
float volts = raw * 5.0 / 1023.0;    // back to volts

Common robot sensors

Sensor	Measures	Signal
Push button, bump switch	Touch	Digital
Potentiometer	Knob position	Analog
LDR (light-dependent resistor)	Brightness	Analog
IR line sensor	Dark or light floor	Digital or analog
Ultrasonic (HC-SR04)	Distance	Pulse timing
IMU (MPU6050)	Tilt and rotation	I²C (a data bus)
Encoder	Wheel turns	Pulses

Examples

Example: turning a reading into a decision

int light = analogRead(A0);
if (light < 300) {
  digitalWrite(LED_BUILTIN, HIGH);   // dark: turn the light on
} else {
  digitalWrite(LED_BUILTIN, LOW);
}

Example: mapping to a useful range

map() rescales a number from one range to another:

int raw = analogRead(A0);                 // 0–1023
int angle = map(raw, 0, 1023, 0, 180);    // 0–180 degrees for a servo

Practice

Try it in the virtual Arduino

Press Start simulator, replace diagram.json with the wiring below, paste the sketch into sketch.ino, press ▶, then turn the knob (click and drag it) and watch the Serial Monitor.

diagram.json

{
  "version": 1,
  "author": "RoboLearn",
  "editor": "wokwi",
  "parts": [
    {
      "type": "wokwi-arduino-uno",
      "id": "uno",
      "top": 0,
      "left": 0,
      "attrs": {}
    },
    {
      "type": "wokwi-potentiometer",
      "id": "pot1",
      "top": -80,
      "left": 120,
      "attrs": {}
    }
  ],
  "connections": [
    [
      "pot1:GND",
      "uno:GND.1",
      "black",
      []
    ],
    [
      "pot1:VCC",
      "uno:5V",
      "red",
      []
    ],
    [
      "pot1:SIG",
      "uno:A0",
      "green",
      []
    ]
  ],
  "dependencies": {}
}

sketch.ino

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

void loop() {
  int raw = analogRead(A0);              // 0 to 1023
  float volts = raw * 5.0 / 1023.0;      // convert to volts
  Serial.print("raw: ");
  Serial.print(raw);
  Serial.print("   volts: ");
  Serial.println(volts);
  delay(200);
}

Reading a knob on A0Open full screen ↗

Start simulator

Runs on Wokwi, free, no account needed.

HalfwayEasy

Turn the knob until the reading is about 512. What voltage is that? About 2.5 V, half of 5 V.

PercentMedium

Print the knob position as a percentage (0–100%). int pct = map(analogRead(A0), 0, 1023, 0, 100); then Serial.println(pct);

A light switch from a knobHard

Turn the built-in LED on only when the knob is past three quarters.

Show an answer

void setup() { pinMode(LED_BUILTIN, OUTPUT); }
void loop() {
  digitalWrite(LED_BUILTIN, analogRead(A0) > 767 ? HIGH : LOW);
}

Quick check

1. A digital signal can be…

Any voltage from 0 to 5 VOnly HIGH or LOWOnly 2.5 VA number from 0 to 1023

2. What does analogRead return on an UNO?

true or false0 to 2550 to 10230 to 5

3. About what reading does 5 V on A0 give?

52555121023

Check my answers