Tiny single-digit numitron tube clock

There is no "dumb" display easier to connect to an Arduino (or any microcontroller, for that matter) than a numitron tube, even compared with the 7-segment LED display. This is because:

• unlike LEDs, it does not need current-regulating resistors;
• unlike LEDs, the segments are not polarized; just connect any end of a segment to Vcc and the other to ground and it lights up; basically, every segment is the filament of a light bulb, like those used in flashlights in the old days; like light bulbs, they even get warm after a while (hence their inefficiency);
• voltage range is pretty wide, between 2V and 5V;

The most popular numitron tube is IV-9, which can be purchased these days on ebay for $5.
Size-wise, IV-9 fits perfectly on a Arduino ProMini board, and so it can be soldered directly into the bottom holes of that board. The clock I made displays the HHMM-formatted time as a sequence of 4 digits. Since there is no RTC on board, the processor keeps track of passing time. The rechargeable LiPo battery plays the role of the backup power, in case the USB cable is unplugged. The current consumption is about 20mA per segment, making the clock draw between 50mA and 160mA. With the on board 240mAh LiPo battery, the power would last for a maximum of 2 hours. The display can be turned off (left button) to save power, thus making it suitable as a wearable device (watch). In this case, the clock draws only about 10mA.
The right button is used for setting up the time.

The numitron clock doesn't get any smaller than this.

The cost of building this tiny numitron clock is about $12:
- Arduino ProMini, with ATmega328 (about $3 on ebay);
- IV-9 numitron tube ($5 on ebay)
- LiPo charger module ($2 on ebay)
- 240mAh LiPo battery ($2 on ebay)
- 2 tactile push buttons ($0.5 on ebay)

The only problem I had was with the ProMini bought on ebay. Sometimes they come with no bootloader burned on the microcontroller. Once anything is soldered on the board (e.g. headers), burning the bootloader through the ICSP becomes almost impossible (because of space constraints).
Therefore, before including it in a project, always test the bare ProMini by uploading a sketch.