Nixie Tube Calculator
From Alpha One Labs
I will put design notes here, and encourage everyone to kibitz. The calculator will be built into a box approx 9x12". It will be "landscape" mode, so you'll hold it with the wider end opening toward you.
Contents |
[edit] What is a nixie tube?
A nixie tube is a gas discharge tube, with 10 cathodes, in the shape of the numbers 0-9, and an anode. There will be 10 'digits' in this calculator, plus 10 neon bulbs for the decimal points, and two more for the minus signs, so 112 elements to control in all! These will be driven as an 11x10 matrix, where one bank of switches selects the tube, and the other bank of switches determines the segment. The minus signs will be controlled individually.
[edit] Why two minus signs?
Scientific notation. The third tube from the right will be blank for a positive exponent, and overlaid with a - sign for a negative exponent. So 2.345E+06 will be represented as "2.345 06" and 2.345E-06 will be represented as "2.345-06". The 'standard' minus sign for which side of zero we're on will be off to the left of the leftmost tube.
[edit] Subsections
The main board will be an Arduino (most likely) or a Olimex LPC-2103 development board. The 2103 has 32k flash and 8k of ram and runs up to 60MHz, plenty for a calculator! It will run off a 9v battery, with a 3v lithium to keep the real time clock.
The nixie power supply will be a store-bought item. I could probably do it myself, but what you don't know can hurt you at 180V!
The nixie and driver section will consist of 23 optoisolators, which will form a matrix with the tubes and the neon bulbs. These will either be driven by multiplexers or 74595 shift registers, depending on what's available and cheaper.
The keyboard will sit on top of it all and provide an attractive cover. I will look around for some old typewriter keycaps, as I have a Steampunk motif in mind for the project. This will probably consist of about 32 tactile switches on a pc board.
[edit] Software
The main calculator loop is already written, and will end up on Google Code most likely, and linked from this page. I am trying to think of ways to maximize the battery life, esp as the display will consume about 60mA when on. I think the processor consumes between 100µA and 2mA depending on clock speed and what's enabled. The trick will be to turn everything off when it's not in use.
To that end, the calculator will run at the slowest clock speed that won't seem too slow, and allows the tubes to multiplex without flicker. This will probably be in the 8-16MHz range. During the brief times when the calculator is actually calculating, the clock speed will momentarily be set to 60MHz. This is easily done through the processor's PLL registers. All the analog circuitry will be turned off, further saving power.
The processor will go into a lower power sleep mode after 1 second of idle time, and turn off the display after 30 seconds idle. The static ram will be maintained, so pressing a key will wake it up again with nothing lost.
Closing the cover will go into lowest power sleep mode, where only the real time clock will be maintained off the lithium cell. Not sure if the ram contents will be kept, this is a design decision.
[edit] Issues
How to display the - sign over the third digit without blocking it?
How cheap are multiplexer chips? I know there's the 74141, and I ordered two, though the website said currently not available, so we'll see if I get them. Otherwise, I'll string 3 74595s together to the SPI port, which is plenty fast.
[edit] Notes
Nixie Tube Multimeter! http://www.flickr.com/photos/wwward0/3967737874/in/photostream/
Counting to (almost) 100 http://www.youtube.com/watch?v=ndfzolUxcsI
