His binary clock uses an Arduino Duemilanove (which fits perfectly in the tin) to keep time and control the indicator LEDs. The LEDs are arranged in two rows as you would expect, representing ...

“Use your Arduino Nano or Particle Photon board and build Binary Nixie Clock with 18 x IN-2 nixie tubes. By connecting DHT22 sensor it is also possible to build binary thermometer and hygrometer.

His binary clock uses an Arduino Duemilanove (which fits perfectly in the tin) to keep time and control the indicator LEDs. The LEDs are arranged in two rows as you would expect, representing ...

Nixie tube clock build has been classed as an intermediate project and should take approximately eight hours to complete using an Arduino MKR WiFi 1010 microprocessor together with IN-1 Nixie ...

From your 3rd-to-last picture (end view of arduino/board inside clock) you should have plenty of room for such a module. Something like a Chronodot or DS3231 module should work well.