Perhaps a project already done but i try to propose it !

The clock is based on 4 TAF1317A Nixie tubes drived by a K155ID1 IC and MPSA42 transistors.

The particularity of this project is the use of a MCP23017 IC. This IC is an I2C 16 bit I/O expander, with it the ATMega328P controller has all its I/O pins free.

The clock also include a RTC DS3231 IC and a switchmode boost converter for nixies high voltage.

The attach schematic does not perfectly match with the prototype board and does not include the high voltage converter. The converter is based on a NE555 timer and a MOSFET used to switch current in an inductor (inspired from circuits found on the internet)

Brief description of the clock operation:

• The ATMega328P controller is used to drive the MCP23017 IO expander  and collect/set data on the DS3231 RTC with I2C bus,
• The MCP23017 drives the K155ID1 nixies cathode driver and injection of high voltage on nixies anodes,
• The nixies multiplexing works as follows:
• All matching cathodes of the nixies are connected together,
• During 5 ms the high voltage is injected on the first tube anode and desired cathode is activated,
• During the next 5 ms, high voltage is injected on second tube anode and desired cathode is activated,
• Etc with the two last tubes. So the four tubes are swept over a 20 ms period.

I have also attach the AVR studio project of the clock (i'm sory because i'm french all comments are written in french).

Ideas for clock improvement:

• Adding date and temperature display (available on the DS3231),
• Clock setting by bluetooth,
• make a clean PCB !

Sorry for my english which is probably not very good ;)

i'm able to be more precise on the clock operation if someone is interresyted by this project