Arduino Uno PIR Sensor Simulator: Wiring, Code and a Live Circuit
This is an HC-SR501 PIR motion sensor wired to an Arduino Uno, running live: the serial monitor says when motion starts and when it ends. In the editor, press the sensor to walk past it.
Three wires: VCC to 5V, GND, and OUT to pin 2. No library is needed.
- 5 V logic
- No library
- OUT on pin 2
- Powered from 5 V, 3.3 V output
Wiring the PIR Sensor to the Arduino Uno
Every connection in the circuit above, with the board’s own pin names.
| PIR Sensor pin | Arduino Uno pin | Why |
|---|---|---|
| VCC | 5V | The HC-SR501 takes 4.5 to 20 V and regulates it down to 3.3 V on board. |
| OUT | pin 2 | A 3.3 V high while it sees motion, then low. |
| GND | GND | Common ground. |
The output is a 3.3 V high, which the Arduino Uno reads as HIGH: its threshold is half of 5 V. Nothing else is needed.
Pins to leave alone on the Arduino Uno
Pins 0 and 1 are the USB serial port on a real Uno, so anything wired there fights the upload and the serial monitor. Pin 13 carries the onboard LED.
On a real bench
On a real bench the Uno stands beside the breadboard: the PIR Sensor goes into the breadboard and male-to-male jumpers run from the Uno’s female headers to its rows. Power the Uno from USB while you build, and unplug it before you move a wire.
The code
The sketch the circuit above runs. Open it in the editor, change a line and press Run: it compiles in your browser.
const int PIR_PIN = 2; // OUT, pin 2
int lastState = LOW;
void setup() {
Serial.begin(9600);
pinMode(PIR_PIN, INPUT);
Serial.println("Watching for motion");
}
void loop() {
int state = digitalRead(PIR_PIN);
if (state != lastState) {
lastState = state;
Serial.println(state == HIGH ? "Motion detected" : "Motion ended");
}
delay(50);
}
The PIR module does all the sensing itself; the board only reads one digital pin. OUT goes high when the infrared pattern in front of the lens changes, which is what a warm body moving does, and stays high for the hold time.
The sketch prints only changes, not every reading, by remembering the last state. That is the pattern for any sensor that holds a level: act on the edge, not the level.
Serial.begin(9600) opens the serial monitor. Serial goes over the USB cable, and the serial monitor in the editor reads it at whatever baud rate the sketch sets.
Common mistakes with the PIR Sensor on the Arduino Uno
Not waiting for it to settle
A real HC-SR501 needs up to a minute after power-up to learn the room, and gives false triggers until then. Ignore it for the first 30 to 60 seconds.
The hold time knob
The orange trimmer next to the sensitivity one sets how long OUT stays high, from about 3 seconds to several minutes. A sensor that seems stuck on is usually turned all the way up. The simulated part has a hold property for the same thing.
Using pins 0 and 1
Pins 0 and 1 carry the USB serial port on a real Uno. the PIR output wired there fights every upload and every Serial.print(). Start at pin 2.
Questions and answers
Can I simulate a PIR sensor with an Arduino Uno?
Yes. The circuit at the top of this page is an Arduino Uno with a HC-SR501 PIR sensor, running in your browser on a simulated ATmega328P. Press the sensor on the drawing for motion; its hold and block times behave like the module’s. Press Run it in the editor to change the wiring or the code; it is free and needs no account.
Is the 3.3 V output enough for a 5 V board?
Yes. The Arduino Uno reads anything above 2.5 V as HIGH, and 3.3 V is comfortably above that.
What do the jumper positions H and L do?
H is repeat trigger: OUT stays high as long as motion keeps happening. L is single trigger: OUT goes low after the hold time even if motion continues, then can fire again. The part’s retrigger property chooses between them.
Can I use an interrupt for the PIR?
Yes, OUT is a clean digital edge with no bounce, so attachInterrupt() on RISING works well. Polling every 50 ms as here is just as good for a person walking past.
Build your own PIR Sensor project
Open this circuit in the editor, change the wiring or the code, and keep your version in a free account.