IN-9 neon bargraph experiment #3: digital output control

Posted on 31 Dec 2011 04:28

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:

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Blog tags: attiny25 attiny45 attiny85 bargraph in-9 meter neon nixie tube vu

One could think of the modern democratic social contract thusly:

Posted on 13 Dec 2011 20:49

Even those who do not understand prisoner's dilemmas deserve to be protected from them, and everybody benefits when we do.

The reactionary mind rebels against this logic and believes that it will come out ahead in a unconstrained contest. In the overwhelming majority of cases, this belief is mistaken. That is because such a belief is most often based entirely upon emotional intuition and is thus impervious to reason. This is an essential problem of contemporary political economy.

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First working build of magic eye level meter

Posted on 11 Dec 2011 00:13

Now using sooper-kool EM800 tubes instead of frumpy old 6E2 tubes.


Circuit to the left of center is 555-based boost converter. Circuit to the right is LM358-based preamp.

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Blog tags: 555 6e2 em800 em84 em87 eye lm358 magic meter tube vu

You must squeeze, Rabban, *squeeeeeze*...

Posted on 07 Dec 2011 00:55

I coaxed another 20% speedup out of the spectrum analyzer for a 3fps gain. The radio module SPI interface is rated for 2MHz, but it turns out you can overclock it a bit, which I achieved by tweaking the OSCCAL register. I see that cringe, but hey, this is very far from a production design! The radio stabilization period is now the unambiguous limiting factor, which means it's unlikely to get any faster :(

Okay, I thought of two more possible ways to speed it up: 1) Switch to a different radio module that might be faster e.g. the TI CC2500. 2) Parallelize the sweep by adding a second radio module. That would complicate the algorithm, but I already know exactly how to do it.

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So many stupid glowy vacuum tubes, so little time

Posted on 02 Dec 2011 10:04


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.

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