STTS

STTS — Pocket Synth

A behbeh credit card sized synth for your wallet. The keys are a full octave ad you can go up and down 2 more. The noise from a piezo disc which is why theres a hole in the board.

Features (this feels like a sales pitch lol)

• 13 touch keys (C to high C), octave down, octave up, and mode
• Three play modes of plain, arpeggio, vibrato
• Octave shift of ±2
• Differential piezo drive for hopefully double the volume of a normal buzzer
• Runs on one CR2016 coin cell, auto sleeps after 30 s, hold MODE to wake

How it works

• MCU: ATtiny1617 (24 pin QFN), programmed over UPDI
• Touch sensing: each key is a copper pad under soldermask, connected to a MCU pin and to a shared TOUCH_SEND line on a 1 MΩ resistor. The firmware times how long each pad takes to charge a finger adds capacitance and makes it slower. Each pad is calibrated at power on.
• Speaker: a 15 mm piezo disc glued over a 10 mm hole, driven from two pins in opposite phase. Touch readings are timed between speaker transitions to avoid interference.
• Power: CR2016 → B5819W Schottky diode (blocks the programmer from back feeding the coin cell) → 10 µF + 100 nF decoupling.
• Layout: all components are on the back. Nothing sits behind the white keys, and no key's trace runs behind a different key, to keep keys from triggering each other.

Repo layout

| Folder | Contents |

| hardware/ | KiCad 9 project (schematic, PCB, custom footprints in libs/) | | firmware/STTS/ | Arduino sketch (megaTinyCore) | | production/ | Gerbers, BOM, and placement (CPL) files for JLCPCB | | images/ | Beautiful renders and screenshots |

Bill of materials

Assembled by JLCPCB (Economic PCBA, bottom side), 5 boards.

Full BOM: bom.csv

Assembled by JLCPCB (Economic PCBA, bottom side), 5 boards.

Part Qty per board LCSC ATtiny1617-MNR 1 C614176 1 MΩ 0603 resistor 16 C22935 10 µF 0805 capacitor 1 C15850 100 nF 0603 capacitor 1 C14663 B5819W Schottky diode 1 C8598 CR2016 SMD holder 1 C964740 15 mm piezo disc (hand-soldered) 1 — Item Cost (USD) JLCPCB order (5 PCBs, assembly, components, shipping) $52.12 Piezo discs $5.00 3D-printed case (own printers) $0.00 Total $57.12

JLCPCB quote (5 assembled boards, 0.8 mm): $52.12

Piezo discs (bought separately): $5

Total: $57.12

Building and flashing

1. Install the Arduino IDE and add megaTinyCore through the Boards Manager.
2. Settings: Chip ATtiny1617, Clock 5 MHz internal, millis() timer TCA0 or TCD0 (not TCB0, which drives the speaker), Programmer SerialUPDI.
3. Wire a 3.3 V USB serial adapter as a SerialUPDI programmer to the GND / UPDI / VCC pads on the back. Take the coin cell out before connecting VCC.
4. Run Burn Bootloader once to set the clock fuses, then Upload.

Status

Schematic yeye PCB layout (DRC clean) yeye Firmware v0.1 (compiles untested on hardware) yeye Front and back 3d printed covers yeye Boards ordered nono Bring up and touch tuning nono

Case

A two-piece 3D-printed case: the bottom piece holds the card, and the top piece slides on after it's in. There's also a bottom-only version that works as a back cover. The piezo has a clear window so it can still vibrate, and the touch keys stay exposed.

• STLs and STEP: case/
• Editable Onshape source: Onshape document

Notes

Designed by yours truly EthanKerman for Hack Club Forge