Frutamidi

FrutaMIDI — single-sided PCB (version 1.1)

A 100 × 70 mm board with copper on the bottom side only, designed for chemical etching at home (toner transfer or photosensitive board). It takes a Raspberry Pi Pico and an Adafruit MPR121 module (1982), both socketed, plus 14 screw terminals, a button, a potentiometer, a 3.3 kΩ series resistor on each electrode, two I²C pull-up resistors and two 220 Ω I²C series resistors.

Status: the schematic passes ERC and the PCB passes DRC, both with no violations, and the PCB matches the schematic. The board has not yet been fabricated or physically tested. If you make one, open an issue with the result.

Contents

• Files
• Bill of materials
• Design rules for home etching
• 1. Print
• 2. Transfer and etch
• 3. Drill
• 4. Solder
• 5. Module configuration: JP1 and two wires
• 6. Test before powering
• 7. Firmware and first power-up
• Common problems
• Electrical safety
• Modifying the board

Files

Bill of materials

Components

If you cut 40-way female header to length, you lose one pin at each cut: 2 strips of 1×40 are enough: one for 20 + 13, the other for 20 + 7.

Fabrication materials

Design rules for home etching

1. Print

1. Open 1-copper-for-transfer.pdf and print at actual size / 100 %, without "fit to page".
2. Measure the printed outline: it must be 100 × 70 mm. If it is not, the printer scale is wrong.
3. Check that the text "FrutaMIDI v1.1" appears reversed (mirrored) on the paper. That is intended: once transferred to the copper it reads correctly.
4. For toner: laser printer, maximum density, transfer paper. For photosensitive board: transparency film, preferably two copies stacked so it is fully opaque.

The small dots in the centre of the pads are left without copper after etching and act as a guide for the drill.

2. Transfer and etch

Preparing the board: cut it to 100 × 70 mm and rub the copper with steel wool until it shines. Clean it with alcohol and do not touch it with your fingers again.

Toner

1. Place the paper with the ink facing the copper and hold it with tape.
2. Iron on maximum, no steam, 3–5 minutes with pressure, going well over the corners.
3. Let it cool, soak in warm water for 5–10 minutes and remove the paper by rubbing with your finger.
4. Touch up broken tracks or pads with holes using the permanent marker.

Positive photosensitive

1. Place the transparency with the ink facing the resist (it gives a sharper result).
2. Expose to UV light as the manufacturer specifies (usually 1–3 minutes), develop, rinse in water and check that the tracks are sharp.

Etching

• Ferric chloride at 40–50 °C (water bath), agitating: 10–20 minutes. Take the board out when all the copper between the tracks has gone.
• Rinse with water, remove the toner/resist with acetone, and check against the light that there are no bridges between tracks, especially in the electrode fan-out, around the resistor row, at JP1 and between the Pico pads.

Safety: wear gloves and goggles, in a ventilated place. Ferric chloride stains everything permanently and is corrosive. Do not pour the used solution down the drain: it contains dissolved copper, so take it to a recycling/waste collection centre. If you use hydrochloric acid with hydrogen peroxide, always mix it in small quantities and outdoors, because it releases toxic gases.

3. Drill

Drill from the copper side, using the dots as a guide. There are 131 holes in total: 129 round holes and the 2 potentiometer slots.

Potentiometer tab slots. Drill the two potentiometer tab holes with a 1.8 mm bit, then widen each one along the long side of its pad (left-right, the 4 mm direction) to about 2.1 mm with a round needle file or by rocking the 1.8 mm bit sideways. Test-fit the pot before soldering; the tabs must drop in without force.

If you like, apply flux or protective lacquer to the copper after drilling. You can also tin the tracks with the iron so they do not oxidise.

4. Solder

Solder from the lowest component to the tallest, so the board sits flat on the table. Use the assembly map.

1. Resistors R1–R12 (3k3). They stand vertically in a row under the module's electrode header, one per electrode. Do not mix them up with R13/R14.
2. Resistors R13 and R14 (4k7), the I²C pull-ups, vertical next to J1. Then R15 and R16 (220 Ω), also vertical: R15 just below J1, R16 between the Pico and the SCALE button.
3. Button SW1 ("SCALE").
4. Pico female headers (2 × 1×20). Trick: plug both headers onto the Pico, set the assembly on the board and solder. They end up parallel and at the right spacing. Then remove the Pico.
5. Module female headers (1×13 and 1×7). Same trick: first solder the male header to the module, plug on the female headers, set it on the board and solder.
6. Potentiometer RV1 ("OCTAVE"), including the two mounting tabs.
7. Terminals J2. Lock the 7 blocks together before placing them on the board, with the wire entries facing away from the board.
8. JP1 and the 2 configuration wires: see the next section.

5. Module configuration: JP1 and two wires

The Adafruit 1982 exists in two versions, with the 7 power/I²C pins in a different order. That is why the board does not connect them directly. Above the 7 module pins there are 7 holes S1–S7 (J3), each one directly above a module pin. You configure the board for your module in two steps.

Step 1: power (JP1). JP1 is a 3-pad solder bridge. The centre pad is 3V3. Bridge it to one side with solder or a resistor lead off-cut:

• ORIG (pad 1 → S1): original version, 3V3 goes to VIN.
• QT (pad 3 → S7): STEMMA QT version, 3V3 goes to VCC.

The silkscreen legend to the right of the module repeats this: "JP1 3V3 selector: / ORIG = no white connectors / QT = 2 white connectors".

Step 2: I²C (two wires). J1 has 2 holes, SCL on the left and SDA on the right. Connect:

• J1 SDA → the S hole above the module's SDA pin;
• J1 SCL → the S hole above the module's SCL pin.

To confirm, read the letters printed on the module next to the 7-pin row, and check this table:

Use insulated solid wire and run it on the component side. Solder the two wires underneath, on the copper. The other S holes stay empty. The module's GND needs no wire: it comes in through the electrode row and is the same in both versions.

A wrong JP1 position can damage the sensor. ORIG on a STEMMA QT module puts 3V3 on the module's ADDR pin; QT on an original module puts it on IRQ. The MPR121 chip is then unpowered, so 3V3 on that pin is above its absolute maximum rating (VDD + 0.3 V), and it can back-feed the module through its 10 kΩ pull-ups. There is no direct short to GND, but the module is not supplied correctly, the sensor does not respond and the Pico LED blinks fast. Check JP1 against the table above before inserting the module (section 6). Never bridge all three pads.

6. Test before powering

With the multimeter, without the Pico or the module plugged in:

• Continuity between the JP1 centre pad (3V3) and a GND terminal: it must not beep.
• Resistance between each terminal 0…11 and the corresponding pin of the module's 1×13 header: about 3.3 kΩ (the series resistor). It does not beep in continuity mode.
• Continuity between neighbouring terminals (0–1, 1–2, …): it does not beep.
• Continuity between the two GND terminals at the ends: it beeps.
• Resistance between the module's SDA pin and pin 31 of the Pico socket (and SCL and pin 32): about 220 Ω (R16 / R15), with the configuration wires already soldered. It may not beep in continuity mode.
• Before inserting the module: continuity between the JP1 centre pad and the S hole for your version (S1 for ORIG, S7 for QT): it beeps. The other side (S7 or S1) must not beep.

7. Firmware and first power-up

The board uses different pins from the breadboard, so it needs the PCB version of the firmware:

1. With the Pico off the board, hold BOOTSEL, plug in the USB and drag the .uf2 onto the RPI-RP2 drive (on the Pico 2: RP2350).
2. Unplug the USB. Plug in the Pico with the USB facing the top edge (the "1" on the silkscreen is next to pin 1).
3. Plug in the module with the 13-pin row facing the terminals: the module's lettering ends up upside down, and that is intended.
4. Plug in the USB. The Pico LED should give 2 slow blinks (sensor found) and then light up whenever you touch a terminal.
5. Open the online sampler, touch the screw of terminal 0 with your finger and the kick drum should play.

If you compile it yourself, uncomment #define FRUTAMIDI_PCB at the start of FrutaMIDI.ino. This definition moves I²C to GP26/GP27 (Wire1), the button to GP22 and the potentiometer to GP28, and enables the potentiometer.

Common problems

Sensitivity problems (fruit, cables, ground) are the same as on the breadboard: see the design notes in the main README.

Electrical safety

Each electrode has a 3.3 kΩ series resistor (R1–R12) between the screw terminal and the sensor. Its protection against IEC 61000-4-2 contact discharge at ±4 kV is an estimate that rests on assumptions (the sensor pin's surge limit is inferred, not specified), not a pass. The resistor is not rated for the pulse voltage across it. Only an ESD-gun test, ideally with a pulse-rated resistor, can confirm it. The board has not been evaluated for higher discharge levels or for touching while it is connected to mains-powered equipment. See the electronics analysis. Never connect the electrodes to anything plugged into mains power.

Modifying the board

The schematic and the PCB are generated by the scripts in scripts/, not drawn by hand. To change something, edit gen_schematic.py / gen_pcb.py and run:

sh hardware/scripts/build_all.sh

The script regenerates everything, runs ERC and DRC (it fails if there are violations), checks that the PCB matches the schematic and exports the fabrication/ folder again. The KiCad files also open and edit normally in KiCad 10, but if you edit them by hand, stop regenerating them with the script.

The module footprint was measured from Adafruit's Eagle files (CC BY-SA). Both versions have the same geometry: only the order of the 7-pin row changes.