Maker Nino Ivanov has designed a telegraph machine with a distinction: it transmits its messages over a serial connection between two Microchip ATtiny85 microcontrollers, encoded in flashing lights as entries from a “3×3×3 Polybius dice” matrix.

“The Communications Hub & Reactive Instrument for Merely Sending Info, a backronym for CHRISSI, is a tool based mostly on the [Microchip] ATtiny85 microcontroller (Digistump kind board),” Ivanov explains of the gadget, “that includes a mere 5 I/O [Input/Output] pins, which permits serial communication (RX, TX and GND at 9600-8-N-1 baud), and show of the 26 letters of the alphabet + one house (27 symbols, all in all, or thrice thrice three).”

That “thrice thrice three” is essential to how the messages are encoded. Every CHRISSI unit has simply two LEDs as a show, each of which mild on the similar time. Its messages, then, are proven as a sequence of flashing lights — however reasonably than go for a tried-and-tested encoding scheme like Morse code, Ivanov opted for a “3×3×3 Polybius dice,” named for the Greek historian Polybius and usually present in historic use as a two-dimensional sq. grid for easy cryptography.

Ivanov’s interpretation of the Polybius sq. is prolonged into the third dimension, making a 27-cell matrix by which the 26 letters of the alphabet are positioned — with the twenty seventh cell holding an area character. “Upon receipt [each letter] is expressed by means of three co-ordinates, X, Y and Z,” Ivanov explains, “given as consecutive blinks and expressing the character’s place in a 3×3×3 grid.”

Whereas decoding a message requires data of the grid and a eager eye for counting the blinks, transmitting is less complicated because of a keyboard which has one change per letter. There, although, Ivanov encountered an issue: a scarcity of general-purpose enter/output (GPIO) pins on the ATtiny85. The answer: a resistor ladder which varies an analog voltage relying on which secret is pressed, permitting all 27 switches to be linked to only two analog inputs on the microcontroller. There’s even a hidden bonus function: messages might be saved within the ATtiny85’s EEPROM for automated transmission.

Schematics and supply code for the mission have been revealed to GitHub below the reciprocal GNU Affero Normal Public License 3.