PIR motion sensor

The HC-SR501: it raises its output for a few seconds when something warm moves in front of it.

Parts in the example:

PIR sensor
Relay
LED

The PIR sensor block as it appears on the canvas.

What it is

A passive infrared sensor watches for a change in the heat pattern in front of it — a person walking by — and holds its output HIGH for a while, set by a small trimmer on the module (a few seconds by default). No motion, LOW. It is the sensor of automatic lights and cheap alarms.

In the simulator

The block has a Motion button: click it and the output goes HIGH for the number of seconds chosen in Settings (Time in HIGH, 1 to 60 s, 3 by default), with a countdown shown on the block. Clicking again restarts the countdown, like a person still moving.

Pins

Pin	What it is
S	signal — HIGH while motion is detected — to a GPIO
3V3	power — to a 3V3 symbol
GND	ground — to a GND symbol

Wiring

The circuit below is the example Motion alarm — PIR + relay from the lab — open it with Project → Open example… and it comes ready to run.

PIR sensor pin	Goes to
S	D27 on the board (GPIO 27)
3V3	a 3V3 symbol
GND	a GND symbol

Settings

Double-click the block's title bar to open its card, then the Settings tab. Changes apply to the running simulation right away.

Setting	Accepts	Default	Notes
Time in HIGH	1 to 60 s	3 s	On the real sensor this time is a trimpot on the board.

Code

sketch.cpp

#include <Arduino.h>

const int PIR = 27;
const int RELE = 14;
const int LED = 2;

void setup() {
  Serial.begin(115200);
  pinMode(PIR, INPUT);
  pinMode(RELE, OUTPUT);
  pinMode(LED, OUTPUT);
}

void loop() {
  if (digitalRead(PIR) == HIGH) {
    digitalWrite(RELE, HIGH);          // switch the load through the relay
    digitalWrite(LED, !digitalRead(LED));  // fast blink = alarm active
    Serial.println("motion detected!");
  } else {
    digitalWrite(RELE, LOW);
    digitalWrite(LED, LOW);
  }
  delay(150);
}

pinMode(PIR, INPUT) and digitalRead(PIR) == HIGH. While it is HIGH the sketch energizes the relay and toggles the LED on every pass; otherwise both go off. The 150 ms delay sets the blink rate of the warning.

Run it

Press Build and run. The first build of a project takes a while; after that, only what changed is rebuilt.

The simulation running: the canvas reacts and the serial monitor shows what the code prints.

Click Motion: the relay's channel lights, the LED blinks fast and the serial monitor prints “motion detected!” until the countdown ends.

How the simulation models it

The block drives the level: HIGH from the click until the countdown ends, then LOW. Real modules also have a retrigger mode and a warm-up minute after power-on; neither is modeled.
The countdown survives redraws and the block being moved; deleting the block stops it.

Try this

Turn it into an automatic light: lamp on the relay (see the relay guide), off 10 seconds after the last detection.
Count detections per minute and print the rate.