STM32 Security Sentry

System Demo

Watch full demo video here: https://youtu.be/kZfAan9jCs8

Overview

This project is a stationary sentry system built on the STM32F401RE NUCLEO platform to monitor and protect four physical sectors. The system integrates PIR motion detection, Time-of-Flight (ToF) LiDAR ranging, and mechanical actuation within a clean Finite State Machine (FSM). Unlike simple looping Arduino-style designs, it uses a bare-metal HAL-based approach without an RTOS, relying on hardware interrupts (EXTI) for event-driven responsiveness and non-blocking timers for real-time scanning, control, and state transitions.

The system mimics a radar station:

1. Sleeps until motion is detected (PIR).
2. Wakes and performs a mechanical sweep of the area (Servo + LiDAR).
3. Maps intrusions into specific sectors.
4. Notifies the user and enters a "Protect Mode" loop with active scanning if the threat persists.

Hardware Architecture

Component	Role	Interface	Protocol
STM32F401RE NUCLEO	Primary Controller	-	-
VL53L0X LiDAR	Precision Ranging	I2C1	I2C
HC-SR501 PIR	Motion Trigger	PA9	EXTI (Interrupt)
SG90 Servo	Pan Mechanism	TIM2 CH1	PWM
16x2 LCD	User Interface	I2C1	I2C

Hardware Setup

Software Architecture: Finite State Machine

The core logic replaces standard blocking delays with a 3-Stage Finite State Machine. This allows the CPU to process sensor data and update the display simultaneously while managing mechanical movement physics.

1. IDLE (Low Power / Monitor)

• Behavior: The system is static. The servo is parked at 0°. The CPU waits for an external interrupt from the PIR sensor.
• Logic: motionDetected flag is set via HAL_GPIO_EXTI_Callback.
• Transition: Motion $\rightarrow$ SCAN.

2. SCAN (Active Mapping)

• Behavior: The servo sweeps 0° to 180° in 10° increments.
• Physics Handling: The system reads the LiDAR data from the stable current position before moving the servo to the next angle, preventing "blurry" sensor data caused by motor vibration.
• Data Processing: Distance data (< 500mm) is mapped into one of 4 angular sectors (45° slices).
• Transition: Sweep Complete (180°) $\rightarrow$ NOTIFY.

3. NOTIFY (Decision & Feedback)

• Behavior: The system analyzes the sector map.
• Intruder Logic:
  • If Objects Found: The LCD maps the threat location (e.g., [X] [ ] [ ] [X]). The system resets the servo and immediately re-enters SCAN (Protect Mode) to actively verify if the threat persists.
  • If Clear: The LCD displays "All Clear". A Smart Wait loop monitors the PIR sensor for 1 second. If motion occurs during the message, the delay is aborted immediately to re-arm the system.

Display Logic:

Objects Detected	All Clear

Wiring & Pinout

STM32 Pin	Component Pin	Description
PA0	Servo Signal	TIM2 Channel 1 (PWM Generation)
PB8	LCD/LiDAR SCL	I2C1 Clock
PB9	LCD/LiDAR SDA	I2C1 Data
PA9	PIR Signal	EXTI Line 9 (Rising Edge Interrupt)
5V	VCC	Power for Servo & LCD
3.3V	VCC	Power for LiDAR & PIR
GND	GND	Common Ground

Key Engineering Challenges Solved

1. Whip Pan Bug

Problem: Resetting the servo from 180° to 0° takes physical time (~400ms). The code runs in microseconds. This caused the system to start the next scan while the motor was still flying back to the start, resulting in "ghost" detections on the left side of the room. Solution: Implemented a non-blocking mechanical settle delay only during the reset phase, ensuring the motor is physically stable before the LiDAR begins ranging.

2. Blind Spot Security

Problem: Using HAL_Delay(1000) to display an "All Clear" message froze the CPU, leaving the system blind to intruders for 1 full second. Solution: Replaced blocking delays with a while loop that checks HAL_GetTick() but breaks immediately if the motionDetected interrupt flag is raised. This prevents the system from missing a detection

Build Instructions

1. Clone the Repo:

   git clone https://github.com/TechNotebookYT/stm32_sentry.git

2. Open in STM32CubeIDE:
   • File > Open Projects from File System.
3. Drivers:
   • Ensure VL53L0X drivers and i2c_lcd drivers are in Core/Src and Core/Inc.
4. Compile & Flash:
   • Build Project (Hammer Icon).
   • Run > Debug As > STM32 Cortex-M C/C++ Application.

Future Improvements

• UART Logging: Outputting precise distance maps to a Python script for a real-time radar GUI.
• Buzzer Integration: Adding an audible alarm during the NOTIFY state if proximity is < 20cm.
• Wireless: Integration with ESP8266/ESP32 via UART to send alerts to a mobile device.