#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..

Notes

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:

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.

Construction

Credits and References

• Arduino Guitar Tuner by nikoala3/Nicole Grimwood
• Arduino Frequency Detection by amandaghassaei
• Guitar tunings - wikipedia
• Passive Low Pass Filter
• Low Pass Filter Calculator
• LM386 microphone amplifier
• TI Single-Supply, Electret Microphone Pre-Amplifier Reference Design
• Electret microphone: selecting a bias resistor
• A Digital Guitar Tuner by Mary Lourde R., Anjali Kuppayil Saji - a scholarly article that explores the critical parameters that need to be addressed while designing a guitar tuner
• Digital Guitar Tuner by Dmitry Berenson - this is perhaps the best description of a realistic circuit using and LM358 op amp and ATMEGA32 microcontroller
• ..as mentioned on my blog
• ..as posted on hackady.io