I started this project when asked by a friend of mine, a guitar player, to try to make a better tuner than those commercially available. Their usual problems are bad digital signal processing and small and hardly leggible displays. To solve the latter problem, I decided to use IN-13 nixie tube as a tuning indicator (large and easily readable at greater distance and poor lighting), and IV-11 VFD indicator for displaying tone letters, and a few more INS-1 indicators.

I programmed ATTiny45 (in assembly code) to run a second order digital filter loop (with adjustable peak frequency, and automatic gain control). Two such Tiny45 filters in series work quite fine to extract a basic sine harmonic from a signal coming from an acoustic (the tuner also has an electret mic and preamplifier for acoustic guitar) or an electric guitar. Voltage sources for nixies and VFD are buck and boost converters controlled by two ATTiny13 MCUs. All the components (including Russian nixies and VFDs) are ubiquitous and easily obtainable. I have a circuit schematic drawn on paper and I started to draw a PCB layout.

On the attached photos you can see the circuit working on a project board, and unfinished PCB in .pdf file (along with PCB for my random numbers generator). It was tested by a guitar player and it's working nice. I will redraw the circuit scheme in Protel and e-mail it to you if you are interested.