PROJECT 4 · SENSOR + TIMERS

PIR Motion Sensor + Buzzer

Detect movement with an HC‑SR501 and sound a buzzer — using a non‑blocking millis() timer.

⬇ Download the sketch Save Project_4_ESP32_PIR_Motion_Sensor.ino, then open it in the Arduino IDE (setup: Project 0b).

1 · How the PIR sensor works

HC-SR501 PIR sensor with pins and adjusters labeled — HC‑SR501: OUT goes HIGH on motion; two trimmers set range and hold time.

A PIR senses the infra‑red heat of living bodies and triggers on change — a warm, moving object.
OUT = HIGH on motion, LOW otherwise.
A person standing perfectly still may not be detected; a moving heat‑less object (robot, car) isn't either.

Warm‑up: after power‑on the PIR needs ~30–60 s to stabilize before it reads reliably.

2 · Non‑blocking timing: `delay()` vs `millis()`

delay(3000) freezes the whole program for 3 s. Instead we record when motion last happened and check elapsed time — keeping the loop responsive:

lastMotionTime = millis();                    // remember "now"
if (millis() - lastMotionTime >= holdTime) {  // has enough time passed?
  ...
}

This pattern scales to programs doing many things at once — a habit worth building early.

3 · Parts

Qty	Part	Notes
1	ESP32 · breadboard · jumper wires	—
1	HC‑SR501 PIR	Powered from 5 V
1	Active buzzer	Beeps on its own when powered

4 · Wiring

PIR OUT to GPIO 27, buzzer to GPIO 26, PIR on VIN/5V — PIR OUT → GPIO 27 · Buzzer → GPIO 26 · PIR powered from VIN (5 V).

From	To
PIR VCC	VIN (5 V)
PIR GND	GND
PIR OUT	GPIO 27
Buzzer +	GPIO 26
Buzzer −	GND

Why VIN, not 3V3? The HC‑SR501 needs 5 V (on VIN when USB‑powered). Its OUT signal is still ESP32‑safe.

5 · Code & expected serial output

The buzzer turns on when motion starts and off after ~3 s with no motion. Logging is event‑based — one line on, one line off. Open Serial Monitor at 115200:

==============================================
 Project 4: ESP32 PIR Motion Sensor + Buzzer
==============================================
PIR OUT -> GPIO 27
Buzzer  -> GPIO 26
Buzzer holds ON for 3000 ms after motion stops.
NOTE: the PIR needs ~30-60 s to warm up after power-on.
----------------------------------------------
[EVENT] MOTION detected  -> buzzer ON
[EVENT] No motion (hold elapsed) -> buzzer OFF

Improved from the original sketch: the kit's version re‑printed the same message every loop and had a confusing "stop alarm" line. This one logs only the two real events.

6 · Demonstration

PIR and buzzer wired to the ESP32 with serial output — Wave your hand → buzzer sounds & "MOTION detected" prints. Stay still → it stops after ~3 s.

7 · Troubleshooting

Symptom	Likely cause	Fix
Always triggers	Warming up / sensitivity too high	Wait 60 s; lower the sensitivity trimmer
Never triggers	PIR on 3.3 V, or OUT mis‑wired	Power VCC from VIN (5 V); OUT → GPIO 27
Buzzer silent	Reversed / wrong pin	+ → GPIO 26, − → GND
Buzzer never stops	Constant motion / retrigger jumper	Hold still; check the jumper
Monitor blank	Wrong baud	Set it to 115200