Tired of having to get out of your filthy hole every day to see if mail has arrived? Want to finally catch that bastard that brings physical spam to your physical mail box in order to subject him to various force fields? Phyff is for you.

Just install the sensor in your mailbox, connect the receiver to the serial port of your Unix box, and filter the logfiles with something like swatch to trigger an audible beep.

I designed and built this thing as my first serious RF transmission project. Working on weekends and holidays and waiting for parts from Digikey, this took me about 2 months from mid-November 2005 to January 2006.

Technical specifications

The PCB of the detector/transmitter. The TLP434 module can be seen at the bottom left. This is a double-sided PCB. A big mess due to a bad soldering iron that burned some tracks. Double-side soldering can be difficult.

Transmitter

Mail detector: infrared barrier
Barrier range: 1m
MCU : ATmega8 at 8MHz
Power : 7-20V, 13mA
RF module : Laipac TLP434 433.92MHz ASK
Antenna : 1/4 wavelength dipole
Baud rate : 4608 baud
Bit rate : 4608 bit/s
Line coding : baseband
Total packet length : 1148 bits plus sync and training.
Convolutional code : rate 1/4, constraint length 7
Block code : Hamming (7,4,3)
CRC : 16-bit
Encryption : Symmetrical XTEA , 128-bit key, CBC with 64-bit authentification
Sensor ID : 16-bit
Encrypted payload : 18 bytes
Temperature sensor : LM335Z, 0.48 degree Kelvin resolution

On the left, the IR transmitter LED with its reflector, and on the right, the IR receiver photodiode.

Receiver (hardware)

RF module : Laipac RLP434, SAW version
MCU : AT90S4433 (older version of ATmega8) at 8MHz
Power : 7-20V, 30mA
Antenna : 1/4 wavelength dipole
RS232 tranceiver : MAX232
Baud rate : 57600

Screenshot of the receiver software running on my PC. The sensor is placed in my mailbox 4 stairs below.

Receiver (software)

Functions : synchronization, ECC decoding (Viterbi and table lookup), decryption, parsing, logging.
Language : Objective Caml
CPU requirement : Cyrix 166 or better (i.e., 100 bogomips is enough)

Pictures

3D preview of the board, generated using a 3D script for Eagle. My home-built UV exposure box. Notice the quality workmanship. Hey, it's not worse than the thermal insulation wrapping on the Apollo "lunar" module. The board. Patched to correct various PCB mistakes. This case will hold the board, the batteries and the antenna, and will be inserted in the mailbox. The board, encased and with batteries and wires. Not very pretty, I admit. The mailboxes of my previous appartment, in the 11th district of Paris. I temporarily left the antenna outside for testing. My mailbox. The board is glued to the top of the mailbox and thus can't be seen. The "Uncle Ben's" cardboard box simulates delivered mail. You can see the infrared barrier below it.