E-Ink wireless display

Battery powered E-Ink WiFi (MQTT) Display with ESP32 WROVER. 145 days without charging*.

MQTT topics (API)

Topics prefix: esp32/#ID#/. Eg: esp32/123456/info/startup/rssi.

#ID# = unique device identifier (last 3 bytes of MAC in hex)

Published data are published with QOS0 without retained flag (non-retained).

Input topics

• image: data bytes of PNG image to draw on display. Size has to match display's resolution (1200×825). Must be published as retained. TIP: Use imagemin with PNG quant to get the smallest size to use less power.
• ping: received data is published on topic pong. Dev purpose.

Output topics

• info/timedOut: true if image was not downloaded in timeout (10s)
• info/finish/freeHeap: free heap before sleep of ESP32 in bytes
• info/finish/totalHeap: total heap before sleep of ESP32 in bytes
• info/finish/totalTime: (total) elapsed time before going to sleep
• info/startup/rssi: RSSI of connected WiFi
• info/startup/freeHeap: free heap on startup of ESP32 in bytes
• info/startup/totalHeap: total heap on startup of ESP32 in bytes
• info/startup/time: elapsed time before MQTT is connected in microseconds
• info/startup/batteryRaw: raw ADC readings (without calibration) of battery voltage. Useful for ADC calibration
• info/startup/batteryVoltage: calibrated battery voltage in volts
• info/startup/batteryCapacity: battery capacity
• pong: answer to ping

Device workflow

flowchart
  A[Startup] --> B{Battery capacity < 20%}
  B --> |Low battery| I[Sleep 60s]
  B --> |Battery OK| D{Wifi connect}
  D --> |Connected| E[Power up e-ink]
  D --> |Not connected| L[Start Access Point\nesp32/12345678]
  L --> N[Start settings http server]
  N --> M[Wait for settings]
  M --> A
  E --> F[Start settings http server]
  F --> G[Connect MQTT]
  G --> K[Send startup MQTT info]
  K --> H{Subscribe to image \nMQTT topic and wait for data}
  H --> |Timed out 10s| C[Send final MQTT info]
  H --> |Data downloaded| J[Send image to e-ink and draw]
  J --> C
  C --> I
  I --> A  

Loading

Settings server

When unable to connect to WiFi settings server is started and waited for user to configure the display (WiFi, MQTT URL and credentials, ...)

VCom is voltage for e-ink display that adjusts contrast.

ADC calib A and B are coefficients for ADC calibration for battery voltage (capacity) measurement. Formula is calibrated_voltage [mV] = adc_sample * A + B.

_{*10s code execution (100mA) and 600s sleep time (200µA) on 8000mAh battery with 20% discharge safety}

Hardware

https://oshwlab.com/lubos.matejcik/esp32-wrover-battery-module_copy_copy

Drawn in easyeda.com.

Enclosure

Consists of three parts:

1. Wall mounted holder
2. Display holder/spacer
3. Cover

Display and boards are glued to display holder/space and cover is glued to display holder.

Whole enclosure is held by thin magnets inserted and glued into parts. This allows easy disassembly for recharing.

Display holder/spacer with cover and electronics

Wall mounted holder