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POLY555 Synth

poly555_and_okay_2-1

It’s an analog, 20 note polyphonic, square wave synthesizer based on the 555 timer chip. The enclosure and keys are 3D-printed, its models were programmed in OpenSCAD, and its PCB was designed in Kicad.

A little history

The monophonic (one note at a time) OKAY synths I made a couple years ago also used the 555 timer as their oscillator, as a sort of homage to the ubiquitous chip’s long history within the electronics community. When I showed one to folks in the scene, “the best 555 synth ever” was an easy, understandable pitch.

But the first question I always got was “How can it play more than one note?”. To that I’d pop the hood and explain that since it has only one 555 to make one oscillator to make one note, to have more notes and still use the 555 I’d need… more 555s!

And while there are more economical ways to get analog polyphony (at the least, the 556 immediately comes to mind), I kept with discrete 555s because I thought it’d be fun to take this train of thought to its logical, if inconvenient!, extreme.

(I consider the POLY555 to be a spiritual successor to the OKAYs, which I haven’t abandoned yet but absolutely need to give some TLC…)

Features

volume wheel Exposed circuitry
Obviously, right?! The 20 555 timer circuits are lovingly displayed in all their glory beneath a piece of plexiglass to protect them from dust. Each circuit also has an LED that lights up when it’s supplied power. BTW, I chose 20 and picked their notes so they’d line up nicely in a grid!
volume wheel Thumbwheel volume control
Just like an old Walkman, volume is controlled by a small wheel on the side. Instead of using a thumbwheel potentiometer, the wheel is 3D-printed and mounts to a pot identical to the timers’ tuners.
power switch Power switch on bottom
The power switch is similarly hidden away in a recessed cavity on the bottom of the enclosure with engraved binary labels.
pencil stand Pencil stand
Jam a writing utensil in there to prop it up like a picture frame. Admire it! It’s cool!

How does it work

Schematic

POLY555 main schematic

From left to right:

  1. A sliding power switch connects the 9v battery’s power to an on/off indicator LED, a 7805 voltage regulator, and a 386 amplifier.
  2. Regulated 5v is distributed across 20 tactile switch buttons, each of which is connected to a corresponding timer circuit, seen below.
  3. Those timers’ outputs bus to a standard linear trimmer pot tricked into having a logarithm sweep, before going to the 386 amp’s input.
  4. The 386 drives our speaker!

Timer circuit

All 20 555 circuits share the same circuit but have unique component IDs.

POLY555 555 schematic

Again, from left to right:

  1. An LED puts on a little light show
  2. C1 is a bypass cap for stability
  3. This 555 circuit is called an astable multivibrator (oscillator) and is modified to have a 50/50 duty cycle… in other words, a square wave!
  4. The oscillator’s frequency is determined by the C2 cap and the resistors R3 and RV1. RV1 is the 1k trimmer potentiometer for tuning.
  5. The chip’s output passes through a “mixing” 10k resistor before leaving. This prevents the 555s from “sinking” each other, which would sound more like modulation (like, a lower frequency superseding a higher one) than mixing together harmoniously.

3D-printed parts

Outside of the electronics and plexiglass, the rest of the POLY555 is 3D-printed.

Animation of POLY555's 3D-printed part assembly

  1. Enclosure bottom
  2. Mounting rail for the keys to rest on
  3. Keys
  4. Volume wheel
  5. Enclosure top

Design for Manufacturing

Per usual with 3D-printed stuff, there are various tricks to get it to print nicely:

  • Floating screw holes don’t have supports or the sacrificial bridge technique I previously wrote about. Instead, the cavity is first laid out as short, increasingly tighter rectangles, and the final cylinder cavity sits on that. Cool trick: it’s an OpenSCAD module.
  • Wherever possible, 45 degree angles are used to obviate supports: the key endstop, on/off switch exposure, volume wheel brace.
  • And where right angles won’t work and supports can’t be avoided, they’re baked into the models themselves so folks at home don’t have to have their slicer do it: plexiglass window pane supports, enclosure hitches
  • Exported STLs are oriented on the bed how they ought to be printed, with help from a shell script.
  • The engraved “OSKITONE” and “POLY555” enclosure texts are 3D-printed with the text side down. A chamfer helps to alleviate some of the “elephant foot” that happens on the a print’s first few layers.

Lessons learned from the OKAY

Better tuning

The POLY555 is both easier to tune and stays in tune longer than the OKAYs. Here’re the OKAY 2’s tuners on the key’s flip side:

OKAY 2's tuners

  1. That PCB is a resistor ladder. The further left on the keyboard the user plays, the more the resistance from the ladder, and the lower the note. So if one pot strayed out of tune, all the notes below it would too.
  2. The pots have 20 turns, which is great for high precision but painful on the wrist
  3. Should any of the buttons start to fail and introduce their own resistance, that gets added, causing an accidental vibrato.
  4. The 555s are sensitive to voltage changes. As the battery runs low, so do the frequencies.

All of that is fixed with the POLY555, where the tuning pots are independant and need less turning, the power buttons are separate from the tuning paths, and a regulator ensures constant voltage.

Hopefully easier to put together

Believe it or not, I think the POLY555 is easier (or at least more straightforward) to solder than the OKAY. Here’s the inside of the OKAY 2 again for reference:

OKAY 2 internals

  • Multiple PCBs connected with ribbon cable
  • “Floating” components that don’t mount to the PCB and also require wiring
  • Some component footprints aren’t actually the right size and require bending leads… yikes!
  • Optional or multi-purpose pads that were intended to engender hacking but were mostly ignored

The POLY555 suffers from none of that, and the end result is that its assembly should hopefully feel a lot closer to the Paint-By-Number kind of thing that, for better or wose, folks expect from an electronics kit.

Narrowing the gap between expected and actual complexity

The OKAYs were so adorable that they were cursed with looking much simpler than they actually were, which was good because I got sales but bad because (I’m almost certain) fewer folks got them assembled successfully in the end (which, IMHO, is the true heuristic for a product’s success. I’ve no interest in being a “shelfware” designer).

So in addition to hopefully making the POLY555 easier to assemble, I’m also intentionally making it look difficult by showcasing its circuitry. By celebrating the complexity instead of abstracting it away, I’m hopeful the kind of folks who are able to will self-select into purchasing and get to the finish line.

We’ll see!

Assembly guide

The OKAY kits shipped with their assembly guides printed out and folded into zines, and some even had makeshift-letterpressed covers. There’s no doubt a lot of old school charm to that, but it was hard to ensure folks got the best instructions with black ink on dead trees.

https://oskitone.github.io/poly555/

https://blog.tommy.sh/posts/poly555-synth/

 

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Bam

Musician since about 45 years(various flutes, synths) Composer since 1986 admin of the forum + blog Music-Society

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