HackSpace – September 2019

LENS

MIDI input

MIDI (which stands for ‘Musical Instrument
Digital Interface’) is a technology used extensively
in studios and live gig situations to connect
synthesizers, drum machines, sequencers, and
more. For our final feature, we’re going to add a MIDI
input to the glockenspiel so that it can be played
digitally by another instrument.
MIDI outputs are relatively simple to wire up, since
you’re just sending an electrical signal down a wire,
but MIDI inputs are a little more complex. Since you
can’t really trust signals coming from external
devices, a MIDI input uses an optocoupler which
converts the incoming signal into pulses of LED light,
then immediately back into an electrical signal again.
Add the extra components to your existing circuit.
In the Arduino software, install the ‘MIDI Library’ by
Forty Seven Effects and upload the sketch. You may
actually need to disconnect the jumper wire from
pin 0 in order to do the upload, because the Arduino
gets confused between the computer’s USB signal

and the keyboard’s MIDI signal – you can replace the

jumper after the sketch has been uploaded.

GETTING THERE SLOWLY

If everything is working properly, pressing certain keys

on the keyboard (middle C, and the octave of keys

above it) will now cause the servos to move. The code

is written so that holding down a key will move the

arm into position, and releasing the key will cause the

beater to strike. While this is a little counter-intuitive, it

actually makes the instrument easier to play accurately

• if the Arduino attempted to move to a note and hit it
  as soon as a key was pressed, there would be a
  noticeable delay before each note while the arm
  moved into position. With this method, as long as the
  player can get used to playing ‘in reverse’, the latency
  is much better, and it’s possible to play along in time to
  a piece of music.

Left

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You might notice that some numerical values in these

sketches are of the type ‘unsigned long’, rather than

just ‘int’ (integer). The Arduino usually deals with time in

milliseconds, but the number of milliseconds in a day is too

large to be stored as an ‘int’ value, so instead we use ‘long’

variables which can contain much larger integers.

The ‘unsigned’ part means that the value can only be

positive, and allows for even larger numbers. If you find

any strange behaviour while writing or modifying code for

your glockenspiel, think about whether any of the variables

need to be ‘long’ or ‘unsigned long’ instead of ‘int’.

Since this project was already based around an Arduino Uno, a plug-and-play Ethernet

shield made sense, but there are loads of different ways to add internet connectivity to an

electronics project. You could swap your regular Arduino Uno for an Uno WiFi with wireless

capability, although it’s about twice the price of a regular Uno.

A Raspberry Pi would also be a good choice for this project, since many models come with

WiFi. You could either connect a RPi to the Arduino via USB, or run the whole build solely

from a Raspberry Pi. There’s a huge number of options for small controller boards at the

moment, and this project should work on most that can control servos (which is the majority).

Signing off

Switching brains

Quick Tip

If you made changes

to the note positions or

delay times in the main

sketch, remember to

copy these over to the

other sketches.