▶ Open the 3D model in your browser  ·  User manual (German)  ·  Schematic  ·  Firmware

A small alarm system on an Arduino Uno R3, built as a school project for learning field 7 (LF07). You arm and disarm it with a PIN on a 4x4 keypad or with an RFID card. While it is armed, the motion sensor triggers a 30-second siren. The OLED only shows icons: a heart when the system is disarmed, a skull when it is armed, and a flashing skull during an alarm. You add cards right on the device, no PC needed.

Features

• Two ways to arm and disarm: PIN + # on the keypad, or a stored RFID card. Both do exactly the same thing.
• Exit delay: After arming you have 10 s to leave the room. The buzzer beeps once per second in the meantime.
• Alarm: It starts on motion (only while ARMED) or after 3 wrong tries (wrong PIN or unknown card). The siren runs for 30 s or until the correct PIN or a known card is used.
• Feedback: A bright double tone means OK, a rough buzz means an error. The PIN only shows up as *, and a wrong entry briefly shows a ✖.
• Admin menu on the device: A + admin PIN + # opens it. There you add cards (just hold them to the reader), or browse and delete the stored ones. Cards live in the internal EEPROM and survive a new upload.
• RFID reset: B restarts the RC522 in case a clone module hangs.

Quick controls

Key	Action
0–9	enter the PIN (shown as *)
#	confirm the entry
*	clear the entry · in the admin menu: back / exit
A	while DISARMED: enter the admin PIN → admin menu
B	while DISARMED: re-initialise the RFID reader
Hold a card to the reader	same as the correct PIN (if the card is stored)

The full end-user manual (in German) is in MDs/anleitung.md, and as a PDF in Anleitung/.

Hardware

Part	Uno connection	Note
Arduino Uno R3	–	ATmega328P, 2 KB SRAM
4x4 keypad	rows R1–R4 → D9–D6, columns C1–C4 → D5–D2	the sketch scans the matrix itself
1.3" OLED 128x64	SDA → A4, SCL → A5	SH1106 controller, I²C address 0x3C
RFID-RC522	SS → A0, RST → A1, MOSI → D11, MISO → D12, SCK → D13	VCC to 3.3 V, 5 V destroys the module
Motion sensor (PIR)	OUT → A2	active HIGH
Passive buzzer	signal → D10	driven directly from the pin, no transistor
9 V battery	barrel jack	power supply

All other VCC pins go to 5 V, all GND pins to a common ground. This uses up every pin on the Uno; only D0/D1 (serial) are left free. The full parts list is in Teile.xlsx.

The original Cirkit Designer schematic is in Schaltplan/ (PNG and SVG).

Getting started

1. Install the Arduino IDE and get these libraries from the Library Manager:
   • Adafruit SH110X (also install its dependencies Adafruit GFX and Adafruit BusIO)
   • MFRC522 (GithubCommunity)
   • Keypad (Mark Stanley / Alexander Brevig), only needed for hardware_test
   • Wire, SPI and EEPROM already come with the IDE.
2. Set the PIN and admin PIN at the top of alarm_system.ino (PIN_CODE, ADMIN_PIN).
3. Select the Arduino Uno board and the right port, then upload.
4. Set the serial monitor to 9600 baud. On start-up it shows the RC522 version (0x91/0x92 = OK), the number of stored cards and the free SRAM.
5. On the very first start the EEPROM is empty, so the sketch writes the factory list DEFAULT_UIDS into it. After that you manage cards only through the admin menu.

If something doesn't work, the test sketches check each part on its own:

Sketch	Purpose
Code/alarm_system/	Main firmware: state machine, PIN, RFID, admin menu, sounds
Code/hardware_test/	bring-up test: OLED, keypad, buzzer, motion sensor
Code/display_test/	OLED only (icons + free SRAM)
Code/rfid_test/	RC522 only (version, UID of every card)

3D model in the browser

▶ rexi255.github.io/Arduino-Security-System

An interactive 3D model of the build (three.js), laid out after the schematic. It runs the firmware logic from alarm_system.ino, so the states, timings, sounds and OLED graphics are the same:

• Press keys on the 3D keypad or in the side panel. Your keyboard works too (Enter = #, Esc = *).
• Click the cards or the key fob: they float over to the RC522 and the SPI lines light up.
• Click the motion sensor to trigger the alarm while ARMED.
• A signal pulse travels along each wire. A key press lights up exactly its row and column wire, and the L LED on D13 flickers during SPI traffic.
• The buzzer plays through WebAudio, and there is a serial monitor and an EEPROM card list.

The model uses demo PINs (1234 and admin 0000), not the real ones from the sketch. To use it offline, just open docs/viewer/index.html in your browser.

Pitfalls we solved

• The OLED is an SH1106, not an SSD1306. With the SSD1306 library it stays blank or only shows stripes. The right class is Adafruit_SH1106G.
• SRAM is tight. The OLED buffer alone takes 1024 of the 2048 bytes. With the Keypad library only ~205 bytes were left, which caused resets and wrong key presses. So the sketch scans the matrix itself, and all strings are kept in flash with F().
• SPI.begin() drives D10 HIGH, because D10 is the AVR's SS pin. That is exactly where the buzzer sits, so the sketch sets the buzzer pin LOW only after SPI.begin().
• The motion sensor moved to A2, because D11 is the fixed SPI MOSI pin. A2 works just like any digital pin with digitalRead().
• Polling the RC522 blocks for ~25 ms when no card is present. So the sketch only polls the reader every 200 ms, otherwise the keypad gets sluggish and the siren stutters.

Project structure

Folder/file	Contents
Code/	Arduino sketches, one subfolder per sketch
docs/	animated README graphics (SVG) and the 3D viewer (docs/viewer/)
MDs/	docs (German): hardware spec, user manual, development log
Anleitung/	user manual as a PDF
Schaltplan/	schematic (PNG/SVG)
Teile.xlsx	parts list
CLAUDE.md	project context and hardware facts for development with Claude Code

The firmware was written together with Claude Code. Every prompt and answer is logged (in German) in MDs/entwicklungsverlauf.md.

License

MIT. The 3D viewer uses three.js (MIT, bundled as docs/viewer/three.min.js).