Slidarr: A Resistance-Sensing MIDI Musical Instrument

The Idea: A Guitar That Thinks Like a Piano

What if a single wire could replace an entire keyboard? The Slidarr began as a thought experiment: map piano tones across a conductive string, let the player’s finger position determine the note, and use resistance measurement as the underlying sensing mechanism. No keys, no frets — just a wire, a finger, and physics.

The concept draws from both stringed and keyboard instruments. Like a guitar, the player touches a string. Like a piano, the position maps to a discrete tone. The twist: sliding continuously between positions creates a live pitchbend, giving the Slidarr an expressive quality that neither instrument natively offers.

Architecture

Resistance-Based Position Sensing

The wire acts as a voltage divider . When a copper-tipped finger touches the string at any point, the circuit measures the resistance from that contact point to a reference end. This resistance value maps directly to a position along the wire, which in turn maps to a note on the chromatic scale .

The system must be calibrated before use — a calibration pass sweeps the full wire to establish minimum and maximum resistance bounds, then maps that range onto one octave starting at C4 (261 Hz).

Embedded Signal Processing on TM4C

The microcontroller (TM4C / compatible with Arduino ) runs the core sensing loop: sampling the ADC , converting resistance to note index, and generating MIDI messages. Three distinct interaction modes are handled in firmware:

• Note on/off — touch and release triggers standard MIDI note events.
• **Pitchbend ** — continuous sliding sends incremental pitchbend messages. When the bend reaches its maximum, the firmware steps to the adjacent note and resets the bend, creating a seamless glide.
• Octave scroll — holding the scroll button while sliding shifts the entire frequency window left or right across the keyboard, allowing access to all octaves on a single wire.

MATLAB Integration

MATLAB was used for signal analysis and validation during development — visualizing the resistance-to-note mapping, confirming calibration accuracy, and prototyping the pitchbend logic before embedding it in embedded C .

Outcome

• Designed and built a functional single-string MIDI instrument prototype, from hardware wiring through embedded firmware to synthesizer integration.
• Implemented real-time MIDI generation including note on/off, continuous pitchbend, and octave-shift scrolling entirely in embedded C.
• Collaborated across a three-person team, dividing hardware design, firmware development, and signal processing responsibilities.
• Demonstrated the proof of concept live, with the Slidarr driving a software synthesizer in real time.

The project was intentionally scoped as a proof of concept — the goal was to validate the interaction model and demonstrate the hardware feasibility, not to produce a final instrument. The architecture is designed to scale: multiple strings, polyphony, and richer sensing are all natural extensions.

Source code: github.com/Deffendor/slidar
Full report: Construction of the Slidarr (PDF)