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Experimenting with Numitron filament tubes
A while ago I built a battery-powered single digit Numitron clock. There was nothing challenging about it. Each segment of a Numitron takes less than 20mA, like a LED, and requires between 3.5V and 4.5V to light up. So the segments can be driven directly from the processor or other TTLs (shift registers etc). Since Numitrons behave so similar to (7-segments) LEDs, why not use one of my old LED matrix shields to multiplex 4 of them, I thought. And instead of the current limiting resistors, I would use Schottky diode s, to bring them even closer functionally to LEDs. I would even be able to use the same interrupt-based code for multiplexing. I started the practical experiment with just one Numitron. The 7 segments are driven by a 595 shift register and the common electrode gets grounded through a ULN2803 gate. The diodes insure the current flows in one direction only and they also guarantee a maximum voltage of 4.5V on each segment. In practice, multiplexing does not work...
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