7

LENS

Six separate strings (actually lengths
of threaded metal bar) are fixed to the
guitar’s body, each one again acting as a
digital input. When a string is touched with
the metal plectrum (which is part of the
circuit), a voltage is detected and the note
is triggered.
While this playing mechanism takes some
time to get used to, it is perfectly possible
to play both chords and melodies accurately
with practice. However, generating musical
data is only half the battle – we also
need sound...

A TEENSY BRAIN
To produce sound (and manage the data
from the guitar’s many inputs), I needed a
microcontroller, and decided on a Teensy
3.6 with an audio add-on board. For the
uninitiated, the Teensy is roughly like an
Arduino but with more processing power,
more inputs, and a brilliant audio library
which allows you to build a fully functioning

synthesizer from within the Arduino IDE. I

did consider using a Raspberry Pi, but the

near-instant boot time of the Teensy was

the deciding factor.

I was able to write most of the code

(and test it fairly extensively) before the

project started to look anything like a guitar.

Although the Teensy is pretty fast, I still

had to do a lot of optimisation before I

could get the latency (the time between

plucking a note and hearing it) down to an

acceptable value. The sheer number of

digital and analogue inputs being read made

things difficult: there are 72 fret contacts,

12 ‘strings’, 28 potentiometers, five toggle

switches and an arcade push-button, not to

mention the LEDs.

One sacrifice I had to make during this

breadboarding stage was capacitive touch.

My original idea was for everything to

work by just touching the frets and strings,

allowing naturalistic left-hand playing

and finger-picking with the right hand.

Sadly, however hard I tried, I couldn’t get

satisfactory speed or sensitivity using this

method. It’s possible that different materials

could have helped, or that my code or wiring

was inefficient in some way, but there

came a point (while hacking the Teensy’s

core libraries!) that I decided I had spent

enough time on this one feature, and gave

up. I added a metal plectrum to the body

and modified the design of the neck so

that both would simply read conductivity

Teensy audio library

The guitar’s synthesizer code features

a huge number of oscillators, filters,

envelopes, and other effects, all

connected together with a complex

web of virtual audio cables. Luckily,

there is an intuitive drag-and-drop

interface for the Teensy audio library,

which allows you to create your audio

setup in flowchart form, then click

‘export’ to generate code which you

can copy and paste straight into the

Arduino IDE.

Left

Building electronics into the neck of the guitar

Below

The join between neck and body; ribbon

cables carry fret data to the main board