By Jon Stanley
http://www.electronixandmore.com/project/numitronclk/index.html
Here is a great little single digit Numitron clock based on an RCA DR2100. Jon graciously provides both the schematic and the code for the PIC microprocessor on his site. The clock itself has only the single tube and 4 LEDs to display the time. The leds show what digit the display is currently showing (HHMM).
All images and video are copyright Jon Stanley.