The TTY machine (a.k.a., Teletype/textphone/Minicom) is a communications device similar to a teleprinter that is used to send text messages over the public switched telephone network.

The TTY was developed in the 1960s to assist deaf and hard-of-hearing users in communicating over the telephone system. It consists of a keyboard, display, and acoustic coupler for a phone handset (some also included a small continuous roll printer or interface for external printers). The TTY converts typed characters to audio signals which can be sent over the phone system to TTY machine on the receiving end, where those audio signals would be converted back to text for display.

The TTY was supplanted in the 1990s by modern services such as instant messaging on computers and texting on phones (as well as video calls and video relay services for sign language use) but the technology is interesting to study, and in fact still works on some phone exchanges.

In this guide we'll take a look at how the TTY uses audio tones to communicate and build our own transmitter with a CLUE capable of sending messages to a TTY machine, both in standalone mode and in Bluetooth LE mode with messages being sent from iOS or Android.


Adafruit CLUE - nRF52840 Express with Bluetooth LE
Do you feel like you just don't have a CLUE? Well, we can help with that - get a CLUE here at Adafruit by picking up this sensor-packed development board. We wanted to build some...
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Adafruit STEMMA Speaker - Plug and Play Audio Amplifier
Hey, have you heard the good news? With Adafruit STEMMA boards you can easily and safely plug sensors and devices together, like this Adafruit STEMMA Speaker - Plug and Play...
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USB cable - USB A to Micro-B
This here is your standard A to micro-B USB cable, for USB 1.1 or 2.0. Perfect for connecting a PC to your Metro, Feather, Raspberry Pi or other dev-board or...
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Huge thanks to Tod Kurt and Jan Goolsbey for their insights into implementing a frequency shift keyed communications protocol and to Jeff Epler and Carter Nelson for helping make the code efficient and effective.

This guide was first published on May 05, 2020. It was last updated on May 05, 2020.

This page (Overview) was last updated on May 16, 2021.

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