Synthiboard

Synthiboard Release status: stable
Description	Synthesizer development board, powered by an Arduino Pro Mini
Last Version	0.1 (2017-08-06)
Platform	Arduino Pro Mini
License	CC BY-SA 4.0
Download	GitHub
Shop	Tindie

An Arduino board that can run a number of different synth firmwares (maybe with minor modifications). Bunch of potentiometers, bunch of buttons. And some space to experiment.

The source for hardware and available firmware is up on GitHub now: makervan/synthiboard.

Hardware

We’re using an Arduino Pro Mini as the core of our synth. Input devices are up to 6 buttons and up to 5 potentiometers. Output is done via PWM and a low-pass filter. Unfortunately, there’s not enough oomph on that output to make it work with headphones, you need powered speakers.

The required minimal input configuration depends on the type of firmware you are using (see details below).

There’s a perfboard section of the board that allows you to make your own addons to the synth. Feel free to share your hacks.

Parts

Part	Partnumber	Source	Notes
PCB	Synthiboard v0.1	Tindie
Arduino	Pro Mini, 5V, 16Mhz	DigiKey AliExpress
Micro USB Connector	Amphenol ICC (FCI) 10118194-0001LF	DigiKey	Used for power only
DC barrel jack	e.g. CUI Devices PJ-002A	DigiKey
3.5mm stereo jack	CUI Devices SJ1-3525N, Switchcraft Inc. 35RAPC4BHN2	DigiKey	min. 1 as output, there's space for two more
Buttons	Tactile Switch SPST-NO	DigiKey	There are many different colors available, as well as different lengths of the button. You'll need minimum one for the reset button, and 0-6 for the synth itself.
Potentiometer	Bourns Inc. PTV09A-4225F-B103	DigiKey	There's space for up to 5 pots on the board
Resistor R1	1k Throughhole	DigiKey
Electrolyte capacitor C1	10µF radial, 2.0mm leg spacing	DigiKey
Capacitor C2	10nF, 2.5/2.54mm leg spacing	DigiKey
Pin headers	2.54mm single row	DigiKey	3 pins are necessary for the second channel on the audio out (sync/audio) Get some long rows, and then cut them to size as necessary
Pin sockets	2.54mm, single row, 12pin or 2pin	DigiKey (12pin) DigiKey (2pin)	For socketing the Arduino. Don't forget the two 2-pin headers on the inside of the Arduino board.
FTDI Programming adapter		DigiKey	To program the Arduino from your computer. If you already have one, you don't need another one.

Firmwares

There are currently three different firmwares running on the board. They are all in the GitHub repository.

Auduino

The Auduino is a Lo-Fi granular synthesizer, initially developed by Peter Knight. It runs on the Synthiboard pretty much out of the box, but there’s the chance to do some hacks, which we’ll describe in the workshop (and on here sometime soon).

• Firmware for the Synthiboard

Input Components

• 5 potentiometers
• No buttons.

Drummachine

The Drummachine is a 4 sample 16 slot drum sequencer, based on code written by Sebastian Tomczak, which has been adapted to work with the Synthiboard. The original uses an R2R DAC, the Synthiboard version PWM for audio output instead.

There’s a few hacks for this one as well, which we’ll talk about in the workshop.

• Adjusted Firmware for the Synthiboard

Input Components

• 1 potentiometer
• 4 buttons ( + reset)

Glitch Storm

The Glitch Storm is a chiptune synth by Spherical Sound Society. It’s available as a kit on tindie, but runs on the Synthiboard as well (with some minor adjustments). We’re missing the LEDs still, that’s for a future correction.

• Adjusted Firmware for the Synthiboard

Input Components

• 3 potentiometers
• 2 buttons ( + reset)

Development

R2R DAC vs. PWM

There are a few ways to generate sound from an Arduino: PWM, a simple R2R DAC, or a dedicated DAC IC.

• https://www.reddit.com/r/AskElectronics/comments/15azgw/what_are_the_pros_and_cons_of_using_an_r2r_vs_a/
• https://electronics.stackexchange.com/questions/188790/signal-generator-pwm-or-r2r
• https://circuitplay.blogspot.de/2013/11/active-filtering-of-arduinos-pwm-output.html
• https://books.google.de/books?id=kK1PCwAAQBAJ&lpg=PA456&dq=sallen-key%20filter%20arduino&hl=de&pg=PA456#v=onepage&q=sallen-key%20filter%20arduino&f=false

Additional Firmware

Drumkid

• Uses 6 buttons (✔), 4 pots (✔), 5 LEDs (✘, could be put on the prefboard section), PWM output (✔)
• HackADay.io Project
• GitHub Repo
• Audio Out PWM uses a different timer and different output pin, so maybe for another HW revision
• maybe we can also add sync out

Helios One

• http://bloghoskins.blogspot.com/2020/07/helios-one-arduino-synth-part-31-minor.html

Ellitone Multi-Synth

The Ellitone Multi-Synth is a budget-friendly digital synthesizer that can produce a wide variety of sounds thanks to its 6 selectable synthesis/operation modes. Each of these 6 synthesis/operation modes utilize the instruments 4 knobs, 4 keys, and center patch bay in different ways.

• Demo video
• Explanation of different modes
• Firmware source on Google Drive
  • Code release post on reddit
  • GitHub mirror

Uses Mozzi library, which uses the “wrong” audio output pin. So it won’t work out of the box, but I’ll give it a shot, and I’ve been thinking about a new hardware revision to address this.

BleepDrum

• DrumSynth by BleepLabs
• Source Code
• Other BleepLabs Synths, e.g. Nebulophone

Mozzi Examples

The Mozzi library has some examples that might be worthwhile to take a look at:

• https://github.com/todbot/mozzi_experiments/blob/main/eighties_arp_uno/eighties_arp_uno.ino

Also there are loads of other synths using Mozzi:

• Arduino Atari Punk Synth

PCB redesign

• Straight ATmega328 instead of ProMini board
  • A little more expensive but looks better
• remove the USB port
  • more trouble than it is worth
• Battery power (3* AA)
  • Also add reverse polarity protection to the DC input jack
• Add option for different PWM pin, to support more synth sketches
• MIDI in, e.g. via 3.5mm jack (TRS): A simplified guide to TRS MIDI
  • Use existing 3.5mm jacks, add pinout for optocoupler chip?
• Version using the RPi2040

References

Other Projects

• Rachel’s Nightmare
• Electronic Instrument Toolkit
• Wee Noise Maker (GitHub Repo, Blog Post)