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Word clock


It's described in the blog post by me.


  • show the time in rainbow color schema
  • show a color schema based on a CO2 PPM value that needs to be handed in via MQTT topic (see API for reference)
  • light up all LEDs to have a beautiful panel of letters instead the time
  • adjust the brightness via API and via an existing AWTRIX that has a LUX sensor and MQTT API enabled (it then asks the AWTRIX API once a minute for the current LUX value and adjusts it's brightness accordingly)
  • connect to Wifi to fetch the current time and to connect to an MQTT to provide an API


  • add the NodeMCU board to Arduino
    • open "Settings" and add as "Additional boards manager URL"
    • open "Board Manager" via "Tools" > "Boards"
    • search for "NodeMCU" and install esp8266 package
    • select board "Tools" > "Boards" > "NodeMCU 1.0"
    • optional: raise the transfer speed to not wait too long during upload (921.600 worked fine for me)
  • install following dependencies via the Arduino IDE:
  • copy config.h.dist to config.h and adjust the config options
  • adjust the MQTT_MAX_PACKET_SIZE option in the PubSubClient library to 250 to allow proper AWTRIX lux sensor support

From PubSubClient: "The maximum message size, including header, is 128 bytes by default. This is configurable via MQTT_MAX_PACKET_SIZE in PubSubClient.h"

error: 'boolean' has a previous declaration as 'typedef bool boolean'
 typedef bool boolean;

Known limitations

  • DST is not yet respected (plan is to announce this via MQTT)
  • the AWTRIX part is not yet optional. One can remove the code segments that start with // TODO only include if AWTRIX is enabled


There is an MQTT based API available. The topics can be specified in the config.h.

Color schema

  • MQTT_SUBSCRIBE_COLOR_PPM_TOPIC: send an integer to this topic and the clock will adjust it's color schema based on the value. Thresholds are 800, 1000 and 1400. It will use a purple, a blue, a green-yellow and an orange-red color schema for the ranges between those thresholds. If something is send there it will stay in this color schema until the next reboot or MQTT_SUBSCRIBE_COLOR_RAINBOW_TOPIC is triggered.
  • MQTT_SUBSCRIBE_COLOR_RAINBOW_TOPIC: send anything there and the clock will move back to the classic rainbow color schema.

Clock mode vs full mode

  • MQTT_SUBSCRIBE_VIEW_FULL_TOPIC: send anything there and the clock will enable all LEDs making it a colorfull panel instead of a clock
  • MQTT_SUBSCRIBE_VIEW_TIME_TOPIC: send anything there and the clock will show the time again


  • MQTT_SUBSCRIBE_BRIGHTNESS_TOPIC: send a value between 0 and 255 there and the brightness will be adjusted to that value

Time offset

  • MQTT_SUBSCRIBE_OFFSET_TOPIC: send the offset in seconds to this topic and the clock will change it's display time to UTC + offset. This can be used to work around the non-existing DST option.

Related projects


Arduino project for a word clock







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