FretNought

FretNought places addressable LED strips beneath the strings of an acoustic guitar to display chord shapes directly on the fretboard. A wrist-worn smart pick reads strum motion through an IMU, while a microphone on the fret device captures the resulting audio. Both events stream over Wi-Fi to a Node-RED backend, which fuses them inside a 500 ms window before advancing to the next chord.

There are two modes. Practice lets the user step through chords manually, advancing on a confirmed strum. Performance walks through a progression at user-set tempo and capo for full-song playback. A web dashboard handles chord input, mode selection, and live state.

Three independent components talk over Wi-Fi and MQTT. Two SiWx917 MCUs handle the embedded side, one in the fret device and one in the pick. Both publish to an MQTT broker hosted with Node-RED on an Azure VM, which runs the fusion logic and serves the dashboard.

The fret device PCB was designed in Altium around the SiWx917 module, including power distribution, decoupling, and radio circuitry. It integrates a microphone for audio detection and an SPI-based LED strip. The pick device is a smaller wrist-worn board built around its own SiWx917 module, with an onboard IMU for detecting strum motion. After the move to dev boards, the IMU was broken out from the custom pick PCB and connected to the dev board over I²C through an on board QWIIC connector.

Bring-up surfaced a persistent Timeout 102 over SWD on the custom board that resisted every fix tried. To stay on schedule, firmware was migrated to SiWx917 dev boards while the standalone PCB remained as a hardware deliverable.

Three peripheral drivers were written from scratch on the SiWx917. The I²S driver streams mic audio into DMA buffers for frequency analysis. The I²C driver handles register-level reads from the IMU. The SPI driver encodes the LED strip protocol's pulse timing directly into the MOSI bit pattern, avoiding bit-banged GPIO.

The Node-RED backend correlates IMU strum events with mic audio events inside a 500 ms window, filtering out incidental motion and ambient noise before advancing the chord. The dashboard exposes chord, mode, tempo, and capo controls and reflects current state back to the fret device. OTA images are hosted on the same Azure VM.