IN-18 Nixie Tube CLock

At the heart of this clock is a 12V to 150V+ boost converter, through which four ~1970 IN-18 Nixie tubes are powered. At startup the voltage 'strikes' the filaments, after which the current is limited to only a few milliamps by a resistor. 32 of the filaments are driven by a HV open drain shift register HV5622, which is powered off the 12V input and interfaces to the microcontroller through a level-shifter.

​

The time is maintained with an I2C RTC DS3231 which retains its time with a backup CR2032 coin cell. The time, date, and format can be set through commands over UART. The HV is shut off for several hours in the middle of weekdays to prolong the lifetime of the tubes. HV is also shut off when the presence of a finger is detected by a capacitive 'guard ring' around the front of the clock.