ChirpKey-8

8-Pin ATtiny412-SSNR acoustic MFSK & capacitive touch 2-layer KiCad 8 hardware token

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ChirpKey-8: 8-Pin ATtiny412 Acoustic FSK & Capacitive Touch Hardware Token

ChirpKey-8 is a compact 32 × 24 mm two-layer custom PCB and 36 × 28 × 7.6 mm snap-fit pocket enclosure built around the 8-pin Microchip ATtiny412-SSNR (SOIC-8). It combines a PCB-etched capacitive touch pad (PA6), a tactile mode switch (PA3), differential push-pull piezo acoustic chirp drive (PA1 / PA2), status LED feedback (PA7), and single-wire UPDI programming (PA0) on a single 3.3V coin-cell or header-powered board.

Why I Designed ChirpKey-8

Most microcontroller boards lean on high-pin-count USB MCUs even when the task only needs a compact physical trigger and a short, audible/ultrasonic data frame that any laptop or phone microphone can decode without drivers or Bluetooth pairing. By constraining the entire design to the 8-pin ATtiny412-SSNR (SOIC-8), every single pin has a dedicated hardware role (VDD, GND, PA0/UPDI, PA1/PIEZO_P, PA2/PIEZO_N, PA3/MODE_BTN, PA6/TOUCH, and PA7/LED), driving the 12 mm external passive piezo transducer differentially at 6.6 Vpp from a 3.3 V rail while keeping the total parts + 3-board OSH Park PCB cost under $18.

Project Screenshots & Visual Overview

1. Two-Layer Routed PCB (32 × 24 mm, F.Cu + B.Cu)

ChirpKey-8 Two-Layer Routed PCB Layout

2. KiCad 8 Schematic (chirpkey8.kicad_sch)

ChirpKey-8 KiCad 8 Schematic

3. Independent KiCanvas Render of Repo Sources

ChirpKey-8 KiCanvas Verification

4. Interactive Pin Budget & Net Inspector

ChirpKey-8 Pin Budget and Component Inspector

5. Bill of Materials & Budget Breakdown

ChirpKey-8 Bill of Materials and Cost Breakdown

How to Assemble & Flash

Step-by-Step Assembly Guide

  1. Inspect bare PCBs: Check the 32 × 24 mm 2-layer board (chirpkey8.kicad_pcb) for clean SOIC-8 pads (U1) and the exposed copper touch electrode (TP1).
  2. Solder SMD passives & MCU (F.Cu): Reflow or hand-solder U1 (ATtiny412-SSNR, aligning Pin 1 dot to the top-left silkscreen marker), decoupling capacitor C1 (100 nF 0603), series resistors R1 (330 Ω), R2 (1 MΩ touch bleed), R3 (10 kΩ pull-up), R4 (220 Ω piezo current limiter), and status LED D1 (0603).
  3. Solder through-hole / tactile parts: Solder tactile pushbutton SW1 (PTS645SM43SMTR92LFS), external passive piezo transducer BZ1 (PKM13EPYH4000-A0), and the 3-pin 2.54 mm UPDI + 3V3 + GND header J1.
  4. Mount in 3D-printed enclosure: Snap the populated 32 × 24 mm PCB onto the four corner standoffs inside cad/chirpkey8_enclosure.stl (36 × 28 × 7.6 mm shell with UPDI header access window).

How to Flash Firmware (firmware/chirpkey8.ino)

  1. Install megaTinyCore in Arduino IDE (https://github.com/SpenceKonde/megaTinyCore) and select ATtiny412 / 402 at 20 MHz internal oscillator.
  2. Connect a 3.3V SerialUPDI programmer (CH340 / FT232R + 4.7 kΩ resistor between TX and RX) to header J1 (Pin 1 = UPDI / PA0, Pin 2 = 3V3, Pin 3 = GND).
  3. Flash firmware/chirpkey8.ino. Pressing SW1 or touching TP1 emits an 8-symbol MFSK acoustic chirp frame with CRC-4 checksum across PA1/PA2.

Bill of Materials (bom.csv)

Ref Part / Value Package Qty MPN Supplier Unit (USD) Ext (USD)
U1 ATtiny412-SSNR 8-pin AVR MCU SOIC-8 1 ATTINY412-SSNR DigiKey / Microchip $0.68 $0.68
BZ1 External Passive Piezo Transducer 4 kHz 12mm THT 1 PKM13EPYH4000-A0 DigiKey / Murata $1.12 $1.12
SW1 Tactile Switch 6x6mm SMD 4-Pad 1 PTS645SM43SMTR92LFS DigiKey / C&K $0.28 $0.28
D1 Green Status LED 0603 0603 1 LTST-C190KGKT DigiKey / Lite-On $0.18 $0.18
C1 100nF X7R Decoupling Capacitor 0603 1 CL10B104KB8NNNC DigiKey / Samsung $0.10 $0.10
R1 330 Ohm 1% LED Resistor 0603 1 RC0603FR-07330RL DigiKey / YAGEO $0.10 $0.10
R2 1 MOhm 1% Touch Discharge Resistor 0603 1 RC0603FR-071ML DigiKey / YAGEO $0.10 $0.10
R3 10 kOhm 1% Button Pull-Up Resistor 0603 1 RC0603FR-0710KL DigiKey / YAGEO $0.10 $0.10
R4 220 Ohm 1% Piezo Current Limiter 0603 1 RC0603FR-07220RL DigiKey / YAGEO $0.10 $0.10
J1 1x03 2.54mm UPDI + Power Header THT 1x03 1 PREC003SAAN-RC DigiKey / Sullins $0.28 $0.28
BT1 CR2032 Coin Cell Holder SMD/THT 1 BH-800 DigiKey / MPD $0.95 $0.95
B1 CR2032 3V Lithium Coin Cell 20mm 1 CR2032 DigiKey / Panasonic $1.50 $1.50
PCB1 32x24mm 2-Layer Purple PCB (3-pack) 32x24mm 1 OSHPARK-2L-1.19SQIN OSH Park $5.95 $5.95
  • Parts + 3-Board PCB Subtotal: $11.44
  • Estimated Shipping + Tax Allowance: $5.96 ($17.40 estimated grand total)

Known Limitations & Transparency Disclosure

  • AI-Assisted Engineering Disclosure: This repository was created with AI assistance (Made with AI disclosed) and deterministic KiCad 8 / STEP / STL generation tools (tools/generate.mjs).
  • Pre-Fabrication Verification Gate: Before ordering boards, run KiCad 8 native ERC/DRC on chirpkey8.kicad_pro, confirm ATtiny412-SSNR (SOIC-8) and PKM13EPYH4000-A0 footprints against current datasheets, and test firmware on a current-limited 3.3V bench supply.

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