Meet the PIR Sensor — Detecting Motion

By the end of this mission, you will be able to:

• Explain how a PIR sensor detects motion

• Describe what “PIR” stands for and what it actually senses

• Wire a PIR sensor to the ESP32 correctly

• Read and display motion detection in the Serial Monitor

Think about a motion-activated light that flicks on the second you walk into a room, or a security camera that only starts recording when someone walks by. In this lesson, you'll meet the sensor behind both of those: the PIR motion sensor.

Robo says: “This sensor doesn't measure distance, color, or sound — it senses something much more specific: the heat given off by moving bodies. Let's wire one up.”

PIR stands for Passive Infrared. It doesn't measure distance or exact position — it detects changes in infrared (heat) radiation, like the warmth from a moving body passing through its field of view. It's called “passive” because it doesn't send out any signal of its own (unlike a distance sensor); it simply watches for changes in the infrared radiation it receives.

The PIR sensor's OUT pin behaves just like a button: it outputs HIGH when it detects motion, and LOW when it doesn't. That means you can read it with the exact same digitalRead() function you've used before.

This lesson's circuit connects the PIR sensor like this:

• VCC → 5V (or 3.3V depending on module)

• GND → GND

• OUT → GPIO 4

Print “Motion Detected” in the Serial Monitor.

int pirPin = 4;
void setup() {
pinMode(pirPin, INPUT);
Serial.begin(115200);
}
void loop() {
int motion = digitalRead(pirPin);
if (motion == HIGH) {
Serial.println("Motion Detected!");
} else {
Serial.println("No Motion");
}
delay(500);
}

🔧 Try It Yourself

Try a few different movements in front of the sensor, then write down what the Serial Monitor printed:

Example: Staying still — printed “No Motion” the whole time.

• Staying completely still

• Waving your hand

• Walking across the room

🤔 Discuss With a Partner or Write Your Answer

Since a PIR sensor detects heat changes rather than exact position, what do you think would happen if a warm object (like a heater) turned on nearby, without anyone moving?

1. PIR stands for ________ ________.

2. A PIR sensor detects changes in ________ radiation, like body heat.

3. The word “passive” means the sensor doesn't ________ out any signal of its own.

4. A PIR sensor needs about ________ seconds to warm up before giving reliable readings.

Draw a line (or write the matching letter in the blank) to connect each word to its meaning.

• I can explain how a PIR sensor detects motion

• I can describe what “PIR” stands for and what it actually senses

• I can wire a PIR sensor to the ESP32 correctly

• I can read and display motion detection in the Serial Monitor

Question 1: What does PIR stand for?

🔍 Reveal answer (drag-select the blank space): B

Question 2: What does a PIR sensor actually detect?

🔍 Reveal answer (drag-select the blank space): B

Question 3: How long should you wait after powering on a PIR sensor before trusting its readings?

🔍 Reveal answer (drag-select the blank space): B

🏆 Level Up

Can you make your code count and print how many times motion was detected?

Name one real device in your home or school that likely uses a PIR sensor, and explain how you think it works.

Term
Definition
PIR Sensor
A sensor that detects motion by sensing changes in infrared (heat) radiation.
Passive
Not sending out any signal of its own; only observing.
Infrared
A type of radiation related to heat, invisible to the human eye.
Warm-up Time
The time a sensor needs after power-on before its readings are reliable.

🤖 Robo Says

Your circuit can now sense movement! Next up: Lesson 2 — you'll connect that motion detection straight to a light.