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Touch-to-Ground Tuning Kit

A field notebook for the "touch a penny, play a string" build. Punk-rock lab notes meet teachable troubleshooting.

Intent

  • Let educators walk a class through touch-to-ground sensing without frying patience or gear.
  • Offer swappable resistor/capacitor combos tuned for different room moisture levels.
  • Provide a spoken-aloud troubleshooting flow so learners hear the logic, not just the fix.

Quick Bill of Materials (per string/contact)

Component Humid Room Default Dry Lab Default Notes
Sense electrode Copper tape square (2–3 cm) Same Clean with isopropyl before each class.
Return electrode Alligator to large foil plate Same Tape to table leg for less trip hazard.
Bias resistor 2.2 MΩ 4.7 MΩ Larger value = more sensitivity, more noise.
Series resistor (guard) 220 Ω 470 Ω Limits inrush when students touch ground.
Shunt capacitor 10 nF film 4.7 nF film Forms RC filter with bias resistor.
Reference ground Instrument sleeve to earth ground Battery-powered synth case Keep it consistent across the room.

Recommended RC Combos

Target vibe Room humidity Bias R Cshunt RC time constant Notes
Crisp plucks 30–45% 2.2 MΩ 4.7 nF ≈10 ms Fast decay, great for articulations.
Legato bows 45–60% 3.3 MΩ 10 nF ≈33 ms Gives a lazy envelope that loves CC1.
Grainy scrapes 60%+ 4.7 MΩ 22 nF ≈103 ms Embrace the noise, ride the threshold.

Rule of thumb: double the resistor when the room gets drier; halve the capacitor when you can see condensation.

Troubleshooting Flow (Humid Room)

Start → Quiet output?
  ├─ No → Celebrate; log the combo.
  └─ Yes → Check bias resistor solder.
        ├─ Solid → Swap to lower-value bias (e.g., 1 MΩ).
        └─ Sketchy → Reflow / tighten alligator clip.
              └─ Still dead? → Dry the electrode with a fan, retry.

Troubleshooting Flow (Dry Lab)

Start → Wild jitter?
  ├─ No → You're golden; document the room temp + humidity.
  └─ Yes → Add/Increase shunt cap (up to 22 nF).
        ├─ Still jittery → Touch a grounded laptop chassis; if noise drops, improve earth ground.
        └─ Stable-ish → Raise bias resistor (up to 10 MΩ) and retune firmware thresholds.

Classroom Narration Prompts

  • “When I say pluck, you say NoteOn velocity burst—that's literally the MIDI packet we fire.”
  • “Bow gestures feed the CC1 envelope; ask students to map it to tremolo or filter cut-off and listen for expressive range.”
  • “Scrape is our friendly glitch: micro-notes at ~50 velocity. Notice how the RC combo above shapes how gritty it feels.”

Field Notes Template

Date Room Humidity (%) RC combo Threshold tweaks Surprises

Fill this after each workshop. Punk-rock discipline is still discipline.