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#271 cordwood/tuner

Use the Boldport Cuttle & Cordwood as a guitar tuner, with a custom input preamp.

Here's a quick video showing it in action..


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The Cordwood has six LEDs. Six is one of my favourite numbers .. like the number of strings on a standard guitar.

So is it possible to use the Cordwood to detect (even tune) the six strings of a guitar?

I was thinking about doing a complete analogue solution with bandpass filters (probably active filters in order to get very tight passband frequencies). Maybe later, but to start I wanted to try an Arduino-based approach I'd seen in an Instructables article by Nicole Grimwood.

Input Preamp and Filter

I've modified the circuit to take an input from an electret so I can use it with acoustic instruments, and adjusted the OpAmp configuration for simple single-supply operation using a basic LM386 audio amp.

The electret microphone is biased with a 10kΩ resistor and coupled to the amplifier with 100nF capacitor.

The LM386 uses a standard 200x gain configuration with a 10µF gain control and 10µF bypass capacitors.

On the output:

  • 22kΩ voltage divider and 220µF capacitor biases the output to VCC/2
  • a low-pass filter at a corner frequency of 723Hz cuts out most of the high frequency harmonics and noise that just confused the tuning algorithms

I mounted this all on a small prototyping board to get a nice clean signal. It is laid out in such a way that it plugs into a breadboard. Here's the contruction I used:

mic_preamp_front mic_preamp_rear

And a typical output signal (open D string):


Arduino Sketch

The initial code is from the Instructables article by Nicole Grimwood.

I've update the sketch quite a bit. It now doesn't even try to be a tuner, but just a pretty stable and accurate indication of which root note is playing.





Credits and References