The system is structured around a 4-state Finite State Machine (FSM) running on the ATmega328P. All peripherals are driven using direct register manipulation.

Hardware Modules:

• ATmega328P (Arduino Nano) — central processing unit, 16 MHz, 5V
• PIR Sensor (HC-SR501) — motion detection for automatic arming
• Servo Motor SG90 — actuates the physical door lock mechanism (PC0)
• Passive Buzzer — variable-frequency acoustic feedback via GPIO toggle (PD7)
• 4x Password Buttons — 4-digit code input (PD3, PD4, PB0, PB1)
• Start/Reset Button — alarm trigger and system reset (PD2)
• Arming Button — PIR sensor arm/disarm toggle (PB4)
• Password Change Button — enters secure password-change mode (PC1)
• Green LED — correct password / system disarmed indicator (PD5)
• Red LED — active alarm indicator (PD6)
• PIR Status LED — visual indicator when PIR is armed (PB2)
• 16×2 LCD (HD44780 + PCF8574 I2C adapter) — real-time status messages (A4/A5)

Bill of Materials:

Electrical Notes:

• All push buttons are wired with the internal pull-up resistors enabled (INPUT_PULLUP equivalent in bare metal: set DDRx bit to 0 and PORTx bit to 1). Buttons are active-low.
• LEDs are connected through 220Ω current-limiting resistors to GND.

Algorithms and Structures:

• Password Validation FSM:

The system uses a simple finite state machine with 5 states:

1. `IDLE` — system waiting, buzzer silent
2. `ARMED` — alarm active, buzzer playing alarm melody via PWM
3. `INPUT` — user is entering the password sequence
4. `CORRECT` — correct password: green LED on, buzzer off, LCD shows “ACCESS GRANTED”, auto-return to IDLE after timeout
5. `WRONG` — wrong password: red LED on, buzzer plays intensified tone, LCD shows “ACCESS DENIED”, auto-return to ARMED

Software design

• Editor: VS Code with the C and “AVR Utils” extensions for syntax highlighting and register lookup
• Debugging: LED-based state signaling and logic analyzer on PWM/GPIO lines
• Target: ATmega328P X-Mini

Libraries and 3rd-party sources:

• `avr/io.h` — direct register-level access to all ATmega328P peripherals (DDRx, PORTx, PINx, TCCRx, OCRx, etc.)
• `avr/interrupt.h` — enabling global interrupts (sei/cli) and defining ISR handlers
• `util/delay.h` — used exclusively in the LCD initialization sequence and LED hold timers; all alarm timing is handled via hardware timers

Block Design

Laboratories Used:

*The implementation of this project is based on the concepts studied in the following laboratories:

1. Laboratory 0 GPIO - Used for basic interfacing. I configured the pins for the LEDs as outputs and the pins for the push buttons as inputs with internal pull-up resistors enabled.
2. Laboratory 1 UART - Integrated for debugging purposes. The system transmits the current state and the entered sequence to a Serial Monitor, allowing for real-time monitoring of the logic.
3. Laboratory 2 Interrupts - Used for the reset button and potentially for handling button presses to ensure immediate response from the microcontroller without constant polling in the main loop.
4. Laboratory 3 Timers - Essential for the acoustic feedback.
5. Laboratory 6 I2C - Essential for the UX.

Schematics

Pins Layout