Nixie Tube Thermometer
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Inspired by the many Nixie clocks currently found on the internet, and the obvious lack of possible uses for Nixie tubes, I decidied to make a thermometer. First thing to deal with when dealing with nixie tubes is voltage. Nixie tubes operate at ~180VDC and draw a few mA per tube. Not an impossible power to achieve, but not an especially easy one either. There are various ways of getting to this voltage, I was a little lazy and ordered a 12VDC to 180VDC converter from allspectrum.com. Since I was using a microcontroller and only needed to power 3 tubes, I really should have just made my own power source. However, I didn't know too much about dealing with these circuits at the time so I used this one.
Here's a look at the circuit:
I used the 74141 BCD converter/driver Russian equivalent chip which I got on ebay. I see a lot of people that use transistors instead, but I just don't see the benefit. I got 50 chips for about $30 on ebay, so less than a dollar each, which allows you to use only 4 pins on a microcontroller to control a tube. Also, a lot less soldering and much cleaner board in the end. High voltage transistors aren't expensive, but you need 10 (11 if you're using the decimal) for each tube. So I used the BCD driver, which was made for Nixie tubes, having high voltage transistors built in to each output line.
As you can see, nearly the entire circuit is comprised of integrated circuits. This makes for some easy organization when it goes to the circuit board for soldering. In addition, I used only 3 of the pins on the 3rd nixie tube for the hundreds position in the display. Since I was only using three, I decided to save a 74141 IC and just use a few high voltage transistors (MPSA42). I figure the thermometer will very unlikely be in any situation where a temperature between 0 and 399 will ever need to be displayed, and if it is, the electronics will probably not work anyways.
The other thing to point out here is IC4, the AD595 thermocouple amplifier. Again, here I could have easily done this without purchasing somewhat of a costly IC. A few resistors and a little more creative programming allows for pretty much the same result. However, at the time I hadn't really thought that through all the way and this seemed like an easy and quick solution. The AD595 also has many other features which in the future could be used to do some fun stuff with. Check out the spec page for details. The AD595 also provides a simple formula for dealing with non-linearities in the thermocouple.