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

Blog » 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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