While there are a number of IN-9 designs on the Internet, this one is exceptionally simple due to the judicious use of an 8-pin microcontroller.
So many stupid glowy vacuum tubes, so little time
The magic eye project is nearing completion. My little op amp preamp circuit works like a charm for driving the tube from line level. I have a couple of parts on order, but once everything arrives it should all come together very quickly.
So I ordered a batch of IN-9 neon bargraph indicators so I can iterate the design with yet another variety of obsolete communist vacuum tubes.
Experiment #3: digital output control
My first thought about building a widget around the IN-9 was to use an opamp chip to process an input signal and a timer chip to regulate power. That was what I did for the magic eye VU meter project. Magic eye tubes have an interesting property in that they have a logarithmic response curve, so if you're thinking of making something like a VU meter, that simplifies the requirements for the input circuit. As I looked into such a design for the IN-9, I realized that I not only would I need to add the log scaling (not a big deal), but that I'd also have to add temperature compensation. The expanding requirements got me to wonder if it wouldn't be simpler, and possibly even cheaper to use a small microcontroller instead. Digital logic never needs temperature adjustment.
I pulled out my MSP430 kit and put together a basic circuit, and after a few hours of dicking around with that, I realized that while there are many neat things about the MSP430 family, they are best suited for low-power, low-voltage projects, and are poorly suited to this particular high-voltage project. I managed to kludge something together using voltage regulators and output transistors, but as the circuit got uglier I realized that this was an unsatisfying design direction. So I scrapped it and started over.
My next choice, which turned out to completely exceed my expectations, was to use an attiny85 (I could easily have done this with an attiny25, but the '85 is what I have on hand). Being a committed minimalist, I am delighted by the utter simplicity of this circuit:
Experiment #4: ADC abuse
Add one resistor, one capacitor, one RCA jack, and 200 bytes of firmware to previous blinkenlight demonstration and voilà! The thing I really like about these tubes is that they are so obviously a plasma. You look at them and your mind just registers "electric flame."
Next up: properly connecting the audio input to the ADC
Build #5 with case
ADC input now biased with simple voltage divider and blocking cap. Case doesn't quite close properly, but I have some new, shorter, more closely spec'ed capacitors coming that should fix that. Or, you know, I could put it on a circuit board.