Nixie Clock v2

December 25, 2012

For Christmas 2012, I built a Nixie clock for my girlfriend! I had previously built a four-tube Nixie clock for my parents in 2011. I decided to improve upon my previous design in various ways:

• This clock features six Nixie tubes versus four Nixie tubes, so the time can be displayed in hours:minutes:seconds format.
• The Nixie tubes are mounted on removable headers to easily swap out a burnt-out tube.
• The Nixie high-side "tube enable" circuitry makes use of high-voltage optoisolators. This is simpler and more robust than the dual transistor (MPSA42 & MPSA92) circuit used in the 2011 Nixie Clock.

Below are some pictures from the construction of the Nixie clock:

The design does not contain a microprocessor! Rather, the entire clock is controlled with custom digital logic written in VHDL. This design is implemented on my Mercury FPGA module, available at MicroNova Electronics. This module packs a Spartan-3A 50K FPGA onto a 64-pin DIP package with all of the necessary support circuitry and plenty of I/O!

The Mercury board is connected to the Soviet-equivalent of the 74141 driver IC (K155NA1), which converts BCD into one-hot digit cathode driver signals. It is also connected to a bank of TLP627 optoisolators, which control the anodes for each Nixie tube. A ring counter loops through and multiplexes the tubes such that all tubes are displayed within 16.67ms (1/60Hz). About 10% "dead time" was added in-between the display of each tube to mitigate a ghosting problem that occurred, where the previous digit would be displayed on the next tube! (I think this was a result of the slow switching speed of the Soviet K155NA1.

The clock is set via a USB interface on the back of the clock that connects to the Mercury FPGA board. Time is kept with a battery-backed DS3232 real-time clock connected via I2C interface to the Mercury FPGA. The DS3232 has an internal temperature-compensated crystal oscillator (TXCO), providing accuracy of ±2ppm, or about 1 minute per year.