Every decision already
made, and every place
we need a pair of hands

S-A · Pulse — 555 resistance oscillator

The plant’s tissue sits in the timing path of a 555 running astable. Its changing resistance — typically 50 kΩ to 2 MΩ between electrodes — changes the pulse rate. The microcontroller timestamps every rising edge on an interrupt pin and works with pulse-interval deltas, not absolute values, which is what cancels most of the drift.

• NE555, a 4.7 nF film cap, two 10 kΩ resistors, a protection diode and electrode leads — about €1.50.
• A 1–10 kHz pulse train. The jitter is the data.
• It measures bulk resistance — water, ions, temperature — not true biopotentials, and leaves no waveform to log.

Verdict: every residency unit and every kit. This is the musical instrument.

S-B · Wave — biopotential front-end

An AD8232 single-lead ECG module (~€6) repurposed for plant surface potentials: a differential measurement between two tissue electrodes against a reference rod in the soil, the instrumentation-amp gain bringing µV–mV potentials into ADC range. Sample it with an ADS1115 at 128–250 SPS — the ESP32’s internal ADC is too noisy for this. Add an RC low-pass near 40 Hz and notch the mains in firmware: 50 Hz in Austria, 60 Hz in Mexico.

Fussier — gel quality matters, motion shows up as artifact, and it needs a fifteen-minute settle — but you can see an action potential when a leaf is cut, and the waveform is loggable for the open dataset.

S-C · Both on one board

The 555 plays music with no latency while the AD8232 logs science-grade waveforms. Two GPIOs and one extra module. This is the v1 architecture, and it is where the instrument and the sensor stop being different products.

Electrodes

• Ag/AgCl ECG or TENS gel pads — leaves and stems. Best signal, hygienic per plant, available in any pharmacy. The default.
• Gold or stainless clips with conductive gel — performance, where plants are swapped fast. Reusable; carry a gel tube.
• A stainless rod, 3–5 cm — the soil reference for S-B. Any hardware store.

Leads are shielded audio cable into a 3.5 mm TRS jack on the enclosure: tip and ring are the electrodes, sleeve is the reference.

All three run the same firmware behind build flags. Fifteen M-A boards, two M-B, and M-C waits for the wildfire work.

V-1 · BLE MIDI to a phone or laptop synth

The ESP32 advertises as a standard MIDI-over-BLE device and pairs straight with iOS, Android and macOS synth apps and DAWs. Latency lands around 10–20 ms. Zero extra hardware, and it is how the residency units ship.

V-2 · Standalone voice — I2S DAC and speaker

A MAX98357A amp (~€4) and a 3 W speaker, driven by a firmware wavetable synth: two oscillators, sine and triangle, one envelope, around three hundred lines. The unit sings by itself in the garden. A SAM2695 module does General MIDI in hardware for ~€15 if the crew would rather not write DSP.

V-3 · CV and gate — for the rig

An MCP4725 12-bit DAC gives 0–5 V pitch CV on a 3.5 mm mono jack, with a gate line from a GPIO through a transistor. It plugs into Eurorack, semi-modular and volca-class gear. One €3 module turns Savia into stage equipment. The H11L1 optocoupler circuit adds DIN-5/TRS serial MIDI for €2. Both go into the three performance units.

V-4 · Network voice

Features travel over LoRa and the far node re-synthesizes locally. The plant’s rhythm travels as a score, not a recording — which is also the engineering truth: LoRa cannot stream audio.

Input is timestamped rising edges from the 555, and on the science path a sampled waveform.

• Keep a rolling buffer of the last N = 32 intervals.
• Compute the absolute difference between consecutive intervals, with rolling mean and variance.
• Threshold: emit a note when that change exceeds k·σ, k default 1.0. The plant’s changes play; a resting plant is nearly silent. That is both musically and philosophically correct.
• Pitch: map the magnitude logarithmically onto a scale table — C-minor pentatonic over three octaves by default, with dorian, hirajoshi, whole-tone and chromatic-off selectable.
• Velocity: the normalized change, so soft events are soft notes.
• Gate: the current mean interval, clamped between 80 ms and 2 s.
• Controllers: rolling variance to CC1, the five-minute baseline drift to CC74, spike density per minute to CC11. These make the plant’s state audible as timbre even between notes.
• Root and key are externally settable over a BLE characteristic or serial — the hook that lets a performer set the key the plant plays in.

The science path adds a spike detector — derivative threshold with a 2 s refractory — that accents real action potentials, and logs (t, mV) CSV at 100 SPS with a session metadata file: species, location, weather, event tags.

Ensemble mode gives each unit a MIDI channel from 1 to 8 and a name — several plants into one DAW is the garden quartet. Configuration is a WiFi captive portal at savia.local: scale, root, channel, thresholds, output toggles, over-the-air update. There is no app to build.

LoRaWAN needs gateways and a network server. Reticulum is serverless and encrypted end to end, and it matches how the rest of this project is built.

The regulation writes the protocol

EU 868 MHz in Austria at roughly 1 % duty cycle means about 36 seconds of airtime per hour per node. Mexican and US builds use 915 MHz. So the unit of the protocol is a feature packet, never raw signal: a CBOR object of 48 bytes or less carrying an id, a timestamp, the mean interval, the variance, spikes per minute, baseline drift, battery, temperature and humidity — every 60 s in a garden, every 600 s for a sentinel, plus an event-triggered packet on spike bursts.

The receiving node feeds the packet into its own mapping engine, and the far plant plays here. Stock antennas give 1–5 km line of sight; valley walls will shape the real number, so we walk it and publish the map. Every node carries a BME280 — session data without weather context is scientifically worthless.

Hard rules: a power switch on the enclosure face; charging while playing must work; the battery lives in a printed cradle, never loose; a fuse or PTC on the cell.

FreeCAD or Onshape, so the files are open and shareable. Export DXF and SVG for the laser, STL and 3MF for the printer, and STEP masters.

E-1 · Residency edition

3–4 mm birch ply, a finger-jointed six-panel box, roughly 110 × 70 × 35 mm. The face carries the power switch, the electrode jack, the CV jack, DIN/TRS MIDI, USB-C, two status-LED light pipes, and an engraved unit number. Inside: a printed standoff tray and a battery cradle. Panel thickness and kerf are parameters, not constants — the laser’s kerf is measured before any panel is cut.

E-2 · Kit edition

A two-part PETG snap fit, same port layout, printing in under three hours with no supports. Made to be remixed.

E-3 · Garden stake

A printed body with an O-ring lid, a ground spike, an antenna gland, and a solar lid variant. It is a later build, but the mounting gets modelled now.

Accessories, all printable: an electrode clip with strain relief, a soft TPU leaf clip, and a wall or tripod mount with a ¼-20 insert for performances.