I found this great (fully assembled) clock for sale on eBay this morning; it is an all ttl based nixie clock. It is a six digit display based on IN-12a tubes. The time base is provided by a mains reference and supports both 50hz and 60hz systems. Additionally, it supports 12h or 24h mode with (optional) leading zero suppression. And - it looks nice!
Lastly, I just told that these will be available as kits in the very near future.
THEORY OF OPERATION:
Time keeping is derived from the AC mains line frequency (50/60Hz selectable) which is divided down using a divide by 10 and a divide by 5 or 6 counters to obtain a 1Hz signal to count seconds. From there the seconds and minutes each contain divide by 10 and divide by 6 counters and lastly the hours uses a presettable divide by 10 and 3 or 4 resettable counter for 12 or 24 Hr selectable operation.
The counters are comprised of flip-flops (F-F's) configured in a series or "ripple" configuration generating a binary count displayable by turning on the "BINARY DISPLAY" LED's. The binary count is then converted to decimal using a handful of basic NAND and NOT logic gates and displayed by turning on the NIXIE tubes.
Power is supplied by a standard 9VAC/1A wall transformer (wall-wart), full wave rectified, filtered and linear regulated to 5VDC used for powering all the IC's. The high voltage required by the NIXIE tubes is derived from a dual primary 10v transformer hooked up "backwards" with its primary winding in series to produce approx. 200-250 volts, rectified and filtered.
Display dimming is performed using a total of six F-F's (two F-F's per counter) to make three 2-bit counters offering four levels of brightness. The two bits are combined resistively creating a stepped analog signal fed to a common collector voltage source for the two LED displays, the last counter is fed to a pulse width modulator (PWM) to drive the NIXIE tubes. Utilizing PWM topology permits the NIXIE tubes to dim while maintaining even lighting of their segments.
1. NIXIE DECIMAL DISPLAY - Utilizes Russian made IN12A neon NIXIE tubes to display time in conventional decimal format.
2. BINARY DISPLAY - Contains six vertical rows of soft glowing purple LED's used to display the time in binary format or "base 2" which is explained HERE. This section also contains the logic IC's to count seconds, minutes and hours.
3. 1Hz INPUT PRESCALER - This is a selectable divide by 50 or divide by 60 counter used to scale the line input frequency down to 1Hz (one cycle per second) to drive the main clock circuit. The output of this two stage counter is displayed as a moving "dot" on these soft glowing purple LED's.
4. NIXIE - This button sets the brightness level of the NIXIE tubes.
5. BINARY - This button sets the brightness level of the BINARY DISPLAY LED's.
6. 1Hz GEN - This button sets the brightness level of the 1Hz INPUT PRESCALER LED's.
7. "0" Hr DISP - This jumper is used to display or blank the "0" in the 10's Hours digit. Example: Jumper can be set to display "09:23:45" or "9:23:45".
8. 12/24 Hr MODE - This jumper sets the clock to display in 12 Hour or 24 Hour modes.
9. 50/60Hz FREQ - This jumper is used to select the incoming line frequency. Example: For operation in USA set to 60Hz, for Europe set to 50Hz.
10. AC POWER INPUT - Plug AC power plug in here.
11. HOUR - This button is used to set the Hours time.
12. MINUTE - This button is used to set the Minutes time.
13. HOLD - This button "freezes" the Seconds time for precise time setting.
Images and text used with permission from Koolklox.