DIY Alarm — Core

The core is the Arduino Mega 2560 firmware for a DIY wired/wireless intruder

• fire + water alarm. It talks to a Raspberry Pi bridge over USB serial (500 kbaud, CRC-framed), which in turn talks to Home Assistant and a Zigbee keypad / RFID reader.

Features

• 16 wired DEOL zones + 16 remote (serial-fed) zones, with optional RS485 expanders.
• Alarm state machine: exit delay → armed away/stay → entry delay → alarm.
• 24-hour zones: fire, water, panic, tamper.
• Outputs: sirens, buzzer, water valve, relays, indicators, chime.
• Credential-based arming/disarming (PIN + RFID), with a keypad programming mode.
• Fully non-blocking (millis()-based timing, no delay()).

Topology

Zigbee keypad / RFID ──► Raspberry Pi bridge ──USB serial──► Mega (this repo)
                                                            │
                              RS485 ── zone expanders (0 by default)
                              RS232 ── power-monitor Nano (AC / battery / 12V)

Key concepts

Topic	Doc
Zones (types, states, DEOL, bypass)	docs/zones.md
Outputs (types, attributes, sirens/LEDs/buzzer)	docs/outputs.md
Alarm state machine & severities	docs/alarm-logic.md
Serial protocol (all commands + events)	docs/serial-protocol.md
Programming mode (PIN / RFID)	docs/programming.md
EEPROM layout & persistence	docs/eeprom.md
System overview & architecture	docs/architecture.md

Repository layout

core/
  src/               firmware (Arduino Mega)
    main.cpp         setup + loop
    alarm.cpp        alarm state machine glue
    zones.cpp        zone scanning / debounce
    outputs.cpp      outputs, sirens, LEDs, buzzer
    serial_cmd.cpp   command dispatch
    pin.cpp          PIN/RFID store + programming mode
    expander.cpp     RS485 zone-expander bus
    power_monitor.cpp  supply Nano (Serial2)
    inputs.cpp       panel inputs (reset button, AC/batt/12V)
    telemetry.cpp    status / zone reporting
    eeprom_map.h     central EEPROM region map
  lib/
    alarm_core/      pure, unit-tested logic (alarm, zones, timing, PIN, SHA-256)
    serial_parse/    frame parsing + CRC8
  test/              PlatformIO Unity tests (native)
  docs/              documentation
  platformio.ini     build config

Building

PlatformIO:

# build the firmware
pio run -e megaatmega2560

# run the unit tests (native)
pio test -e native

The default environment is megaatmega2560 (ATmega2560, Arduino framework). A second native environment runs the pure-logic unit tests on the host.

Configuration

Compile-time configuration lives in:

• lib/alarm_core/config.h — zone/output counts, timings, thresholds, pins, enums.
• src/zone_config.cpp — per-zone type + attributes, zone pin map.
• src/output_config.cpp — per-output type + attributes, output pin map.
• src/eeprom_map.h — EEPROM region layout.

Credentials (PINs/RFID tags) are stored in EEPROM and managed via programming mode; see docs/programming.md.

AI Usage

The overall design, features, and concepts of this project are the maintainer's. An AI coding agent (Cline) was used as a collaborator during development — the concepts were discussed with the AI, which helped with the implementation. The documentation (this README and the docs/ set) was written by the AI.

The maintainer actively reviewed and challenged the AI's responses where appropriate, and made the final design decisions.