PIR Motion-Activated Burglar Alarm with Arduino

PIR (Passive Infrared) sensors detect infrared radiation emitted by warm bodies — humans and animals. When motion is detected, they output a HIGH signal that you can read with any microcontroller. This alarm adds a 10-second arming delay, a 30-second alarm siren, an LED status indicator, and a key-switch style arm/disarm button.

What You’ll Need

• Arduino Uno or Nano
• HC-SR501 PIR sensor module
• Active piezo buzzer (5 V)
• Green LED (status) + red LED (alarm) + two 220 Ω resistors
• Push button for arm/disarm + 10 kΩ pull-down resistor
• 9 V power supply or battery
• Project box for mounting

Step-by-Step Instructions

1. Set up the PIR: Connect PIR VCC to 5 V, GND to GND, OUT to Arduino pin 2. The HC-SR501 has two onboard potentiometers: one for sensitivity (range), one for hold time — set both to minimum for testing.
2. Wire outputs: Green LED (armed status) through 220 Ω to pin 4. Red LED (alarm) through 220 Ω to pin 5. Buzzer positive to pin 6, negative to GND.
3. Wire the button: Button between pin 7 and 5 V, 10 kΩ from pin 7 to GND.
4. State machine logic: Implement four states: DISARMED (green LED off), ARMING (green LED blinks for 10 s), ARMED (green LED steady), ALARM (red LED + buzzer for 30 s). Button transitions DISARMED → ARMING → ARMED; alarm auto-resets after 30 s back to ARMED.
5. PIR warm-up: PIR needs 30–60 seconds to stabilise after power-on — add a startup delay and flash the green LED during this period.
6. Test and calibrate: Adjust sensitivity potentiometer until the detection range suits your room. Adjust hold-time pot so each trigger lasts 3–5 seconds.

Safety Notes

• This is a demonstration project, not a substitute for a certified security system.
• Piezo buzzers at close range are uncomfortably loud — keep volume in mind when testing indoors.

Tips & Troubleshooting

• False triggers often come from sunlight shifting, air conditioning vents, or pets — reposition the sensor to avoid these.
• Add a GSM module (SIM800L) to send an SMS when the alarm triggers for a real remote-notification system.
• Store arm/disarm events in EEPROM to create an event log.