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Second iteration of a Rapid Prompting Method (RPM) letterboard with capacitive touch sensors & Bluetooth connectivity

In a nutshell (full story in the prototype link): The Rapid Prompting Method (RPM) is a technique developed as a vehicle for non-verbal autistic children (and adults) to communicate, that, in effect, unlocks them from a silent isolation and can prove that they are intellectually intact. Itcan be life-changing for the autistic child and for the people who love them.

RPM uses a letterboard for the student to spell and communicate during the interaction with the facilitator. There are a lot of sensory variables involved. Practice, patience and consistency are only a small portion of what’s required.

This letterboard that acts as a Bluetooth keyboard, sending its output to an iOS device that provides typing feedback through its text-to-speech functions.

Design Considerations
The Board

A first requirement is to recognize the selected letter by a very soft touch, so capacitive touch sensing was chosen. This also allows for custom printed faceplates. Also, it has to be very light for comfortable use in one hour sessions (this version weighs in at 200 grams, similar to a small iPhone. Compare to 600 to 900 grams of a Bluetooth keyboard). Finally, it has to survive constant manipulation by the facilitator.

The Electronics

I chose an ESP32 as MCU because of its speed, BLE and Wi-Fi capabilities (plus a large community for support and libraries). To handle the capacitive touch functions of the letterboard, I chose the MPR121 breakout boards from Adafruit: they are relatively well documented, well developed, and, high quality (as is their HUZZAH ESP32 Feather board). Communication between the three MPR121’s (for a total of 36 sensors) and the ESP32 is via I2C. I added a piezo buzzer to provide status feedback in conjunction with the HUZZAH32’s built-in red LED. Finally, a 350mAh LiPO battery feeds the ESP32 via a simple On/Off mechanical switch.”

Link to article