• Initial Design Considerations

    Ryan Davis01/11/2017 at 16:37 0 comments

    The Numitrons I'll be using for this project are the Sovtek IV-9s, as they seem to be standard across most Numitron projects like this. I'll wire up 6 of them total, so I can display HH:MM:SS, one Numitron for each digit.

    As far as control, I decided to use an ARM microcontroller, because I'm trying to move away from Arduino's level of abstraction. The mBed online IDE was recommended to me as being easy to use, and compatible with many different boards; all I needed to do was choose one. After looking through various mBed boards, the F303K8 board seems to be exactly what I need: It provides a chip with an internal RTC, a compact form factor, and low power usage, all at a $10 price point.

    The chip can't control Numitrons directly due to current requirements, so I needed some way to connect them. I was going to use 6 standard 74HC595 shift registers, but then I realized the Numitrons require 20mA of current per filament, which is way more than the HC595s can supply. Luckily, TI makes a power logic version called the TPIC6B595 that can supply up to 150mA per output, so I decided to use 6 of those (There are 8 filaments per Numitron, and conveniently 8 outputs on each shift register). I can also daisy-chain the shift registers, so controlling the 6 Numitrons only requires 3 pins.

    The first step towards building this clock is getting the chip to interface with the display. I've already wired up the prototype, and I'll make another log once I get pictures and videos showing my test code.