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Typie-Walkie with E-Paper Screen

Everyone knows the talkie-walkie, here is a variant with an e-paper screen.

Thanks to its capacitive touch, the e-paper screen turns into a human-machine interface. The user type the message with the keyboard and then press Send. The message is sent and displayed on the remote screen.

The touch e-paper screens were sampled by Pervasive Displays.
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Hardware

The central part of the project is the monochrome 3.70" e-paper 416x240 screen with touch. It is connected to the MCUs through the EXT3-1 and EXT3-Touch interface boards, stacked together.

​We need two of them, one for each device.

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For simplicity, I took two ESP32 boards, the ESP32-Pico-Kit-v4 and the ESP32-DevKitM-2.

Alas, the pinouts are different and some pins are missing. The documentation, although dramatically better than for the ESP8266, is still unclear.

​For example, the ESP32-DevKitM-2 was crashing and rebooting endlessly because the default SPI interface was raising an exception.
Picture
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The problem was solved by specifying the pins of the SPI interface explicitely.

​As a tradeoff, the JTAG signals are no longer available.
Code Editor

    

Software

The application is the same for the two devices. The management of WiFi and MQTT relies on off-the-shelves libraries on top of the Wiring/Arduino SDK.

For the screen, the Pervasive Displays Library Suite manages all the elements of the interface:
  • The input keyboard is used to type in the messages (TX);
  • The Send button sends the messages (TX); and
  • The output text-box displays the messages received (RX).
 
The keyboard contains three sets of keys:  capital letters, lowercase and symbols.

​Additionally, the § key changes the layout, and the « key acts as backspace.
The MQTT broker is provided by Mosquitto. It runs on a PC or a single-board computer like a Raspberry Pi.

However, recent versions of Mosquitto require the prior configuration of the mosquitto.conf file to allow the access from outside the PC where it is running on.

Alternatively, a cloud-based broker like mqtt.eclipse.org, test.mosquitto.org or broker.hivemq.com could be used to manage the MQTT messages.
mosquitto.conf

    

Conclusion

The e-paper touch-screen is surprisingly responsive when using the keyboard to type a message. 

WiFi is not exactly a low-power solution and doesn't match the ultra-low power capabilities of the e-paper technology. A variant uses the low-power LoRa radio instead.

However, this project explores an interesting low-power alternative to standard TFT screens, and shows that e-paper screens can act as input devices as well.

Links

  • ​​Pervasive Displays website    
  • EPD Extension Kit Gen 3 (EXT3/EXT3-1) product page
  • Touch Expansion Board for EXT3 (EXT3-Touch) product page
  • 3.7" e-paper touch-screen product page
  • Pervasive Displays Library Suite
  • Espressif website
  • Espressif ESP32-Pico-Kit-V4
  • Espressif ESP32-Pico-DevKitM-2
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Posted: 09 Aug 2022
​Updated: 17 Jan 2023
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