GSM-Based Intrusion Detection System with PIR Motion Sensor and SMS Alert

Homes, hostel rooms, small shops and storerooms are frequently left unattended, and a simple break-in during those hours can go unnoticed for a whole day. Conventional alarm systems are either wired, expensive, or tied to an app that needs constant internet — overkill for a student room or a small premises. This project demonstrates a compact, low-cost alternative built from standard modules: a PIR motion sensor watches the protected area, an Arduino Uno processes the trigger, and a SIM800L GSM module sends an SMS alert directly to the owner's mobile phone the moment motion is detected. Because the alert travels over the GSM cellular network, the system works anywhere with mobile coverage — no Wi-Fi, no broadband, no cloud account. A buzzer sounds locally at the same time for on-site deterrence, and an arming delay gives the owner time to leave before detection starts. Every part of the build — sensor interfacing, serial communication with the GSM module, AT commands — is a standard embedded-systems exercise, making the whole system genuinely explainable in a viva.

1. The system powers up in a disarmed state; pressing the arm button (or on power-up, as configured) starts the arming delay countdown shown on the status LED.
2. Once the delay elapses, the Arduino begins polling the HC-SR501 PIR sensor's digital output pin.
3. When motion enters the sensor's cone, the PIR output goes HIGH; the Arduino detects the rising edge and registers an intrusion event.
4. The firmware starts the local buzzer alarm immediately and wakes the SIM800L over its UART serial connection.
5. The Arduino sends the AT command sequence (AT+CMGS) through the serial port, and the GSM module transmits the alert SMS to each configured phone number.
6. After a cooldown window, the buzzer stops and the system re-arms itself automatically, ready for the next event.

Tech stack:

• Arduino Uno (ATmega328P)
• SIM800L GSM module (quad-band, SMS via AT commands)
• HC-SR501 PIR motion sensor
• Arduino IDE (C++ firmware)
• 5 V active buzzer module
• Breadboard + jumper-wire wiring
• Regulated power supply (SIM800L needs a stable ~4 V rail)

• [PIR motion detection] The HC-SR501 PIR sensor monitors the protected zone and drives its output pin HIGH when it detects a moving warm body, giving the Arduino a clean digital trigger with adjustable range and retrigger delay.
• [Instant SMS alert over GSM] The SIM800L module sends a text message to the owner's phone via standard AT commands the moment motion is detected — no Wi-Fi, internet or cloud service required.
• [Local buzzer alarm] A 5 V buzzer sounds at the site alongside the SMS for immediate on-site warning; the alarm is silenced and re-armed automatically after a configurable cooldown.
• [Arming delay] A configurable exit delay (e.g. 30 seconds) runs when the system is armed, so the owner can leave the room without triggering the alarm themselves.
• [Multiple alert recipients] Up to three phone numbers can be stored in the firmware, so the alert SMS reaches every configured recipient.
• [Retrigger debounce] The firmware ignores further triggers for a cooldown window after each alert, preventing a burst of duplicate SMS messages from continuous movement.
• [Status indication] An onboard LED and serial-monitor messages show armed, triggered and GSM-network-registration states, so setup and fault-finding are straightforward.

• Working prototype (Arduino Uno, PIR sensor, SIM800L GSM module, buzzer, breadboard wiring)
• Complete firmware source code (Arduino C++) with comments
• Circuit and wiring diagram with pin-connection table
• Component list with exact module names
• Setup, calibration and SIM800L testing guide
• Project abstract PDF (10 sections)
• Project report PDF (background, design, methodology, results, limitations)
• PPT presentation for final review
• Viva Q&A preparation document (PIR working, GSM AT commands, serial communication, power design)