The pedal

The app's Live view, taken off the phone and put under your foot.

The pedal connects by itself, over Bluetooth, to the Spark 2 or the Spark NEO. The phone is only needed to put banks together and send them over; after that it stays in your pocket.

• Footswitches 1–4: the four presets of the half on screen (A1–A4 or B1–B4).
• Footswitch 5: shows the other half without changing the sound; held for 3 seconds it opens the looper.
• Bank buttons: previous and next bank. «Amp Preset» is the amp's own slots; you send the others from the app.
• LEDs above the footswitches: they show what's playing, red for half A and green for half B.
• Tempo LED and click: they beat the looper's tempo and count in before recording.
• MIDI mode: the same pedal controls a program on a computer or an iPad.

The names of the parts

Throughout this guide each part has one name only:

Everything inside the prototype. Buy more resistors and connectors than you need: they cost less than the shipping.

Every wire and where it ends up. The next chapters build it one piece at a time.

The XIAO, pin by pin

XIAO, expander, the two rails and the connectors the top panel's cables plug into.

1. The rails. On a perfboard make two strips of bare solder from one side to the other: 3V3 rail at the top, GND rail at the bottom. A bare wire laid over the holes and soldered every two or three is fine. The two rails must never touch: that would short the supply.
2. The XIAO's headers, top left, with the USB socket towards the edge: that's where the extension that brings the socket out of the case plugs in. The headers let you pull it out to replace it.
3. The expander to the right of the XIAO, and the capacitor soldered between its supply pin (V+ or VDD) and ground, as close as possible. A ceramic capacitor has no polarity.
4. Expander supply and address. V+ and RST to the 3V3 rail; GND, A0, A1 and A2 to the GND rail. If RST is left floating the chip stays in reset and doesn't answer. If the A pins aren't grounded the address isn't 0x20 and the firmware can't find the expander.
5. XIAO to the rails and the bus. 3V3 to the 3V3 rail, GND to the GND rail. D4 to the expander's SDA and to the display connector; D5 to SCL, likewise. SDA and SCL swapped: nothing breaks, but display and expander stay silent.
6. The JST-XH connectors. Each one has its own ground wire, taken from the GND rail next to it:
   • display, 4 ways: VDD (3V3 rail), GND, SDA (D4), SCL (D5);
   • footswitches, 5 ways: G, FS1, FS2, FS3, FS4;
   • footswitch 5 and bank buttons, 4 ways: G, FS5, bank L, bank R;
   • LEDs 1 and 2, 5 ways: G and four lines PB4…PB7;
   • LEDs 3 and 4, 5 ways: G and four lines PB0…PB3;
   • tempo LED and click: two wires from D1 and D3 plus ground (a 3-way works too).
   Which line goes to which button and LED is in the table in the next chapter.
7. Clean off the flux with isopropyl alcohol and a small brush, before powering up. In the prototype, flux residue under the expander made it heat up: it looked like a dead chip.

The display

1. Look at the back of the display. There's a row of pads with labels (some in Chinese); the one marked D2 must be closed, i.e. its two halves joined by a blob of solder or a 0 Ω resistor. Open, the display doesn't answer on the bus and stays black. On the prototype's unit it was already closed at the factory.
2. Wire it to its connector with four wires: VDD, GND, SCL, SDA, following the display's labels, not the wire colours.

The antenna

The XIAO's stick-on antenna snaps onto the tiny u.FL connector, pressed straight down until it clicks. Only once: that connector survives few insertions. Stick the antenna inside the case, away from metal (footswitches, cell). Wood doesn't block the signal.

One reads and the other drives: the expander's port A listens to the buttons, port B lights the LEDs.

Which line goes where

This is the map the firmware expects. If you wire it differently nothing bad happens: the guided mapping in «The test» tells you yours, and you write it into the firmware.

PA2 stays free.

Footswitches and bank buttons

1. Each footswitch has (at least) two terminals: one to its line, the other to ground. Polarity doesn't matter.
2. Ground is daisy-chained: one wire from the connector's G to the first footswitch, from there to the second, and so on. Same for the bank buttons.
3. Each line wire goes to its way on the connector, as in the table. Mark on the connector which way is which.

The LED strip

1. Find each LED's legs with the multimeter on diode test: black probe on the longest leg (the common cathode), red on one of the others. The LED glows faintly in its colour. Usually the order is red, cathode, green, blue, but check it. Blue isn't used: cut it short. An LED fitted backwards doesn't light, but at this voltage it doesn't break either: just turn it round.
2. Mount the four LEDs on a strip of perfboard, spaced like the footswitches (70 mm).
3. On the red leg a 330 Ω resistor, on the green leg a 560 Ω one; all the cathodes together to the connector's ground. Each resistor under its own heat-shrink.
4. Keep the cable from the strip to the board away from the footswitches' metal: those wires have no protection.
5. Hot glue to hold the wires goes on after you've seen all four LEDs light up.

Tempo LED and click

1. Tempo LED: another RGB LED, of which only the red is used. D1 → 330 Ω resistor → red leg; cathode to ground. It goes in a 5 mm hole in the top panel.
2. Click: the piezo disc has two wires, and polarity doesn't matter. D3 → 330 Ω resistor → one wire of the disc; the other wire to ground.
3. The disc is louder when held only by its edge, over a gap: glued flat to the wood it's barely audible. The click only sounds during the looper's count-in, never over the amp: it doesn't need to be loud.

Cell, charger, switch and terminal: the part that does damage if you get the polarity wrong.

1. Cell holder → charger BATT, with a JST-PH plug. Polarity is given by the board's silkscreen, not by the wire colours (pre-crimped leads are sometimes reversed): with the USB-C at the top, + on the left and − on the right.
2. LOAD + goes to one terminal of the switch; from the other terminal a wire to the terminal block's +.
3. LOAD − goes straight to the terminal block's −.
4. Test. Cell in, switch on, multimeter on voltage across the terminal block's two screws: between 3.6 and 4.2 V, with positive where it should be. Then switch off and take the cell out.
5. Terminal block → XIAO. The + to the BAT+ pad on the underside of the XIAO, the − to BAT− (the labels are on the board).
6. The divider: one 100 kΩ resistor from the terminal block's + to D0, another from its − to D0. In the prototype they sit in two short leads, under heat-shrink. It's after the switch: with the pedal off it draws nothing. Without it the pedal still works, but the battery icon doesn't tell the truth.
7. The charger's «PWR» LED stays on even with the pedal off and drains the cell in about six weeks (2.1 mA measured). In the prototype it was removed, prised off with the tip of a screwdriver. The other LEDs are useful.

Wood, because it's transparent to Bluetooth's 2.4 GHz.

• The switch on the side, never on the top panel: it snaps at the first stomp.
• The charger behind the side, with its USB-C socket facing a hole, and firmly fixed (in the prototype on a plywood bracket, with four screws): every tug on the cable reaches its solder joints.
• The window: never cyanoacrylate glue, which fogs the acrylic. A diode laser doesn't cut acrylic, it melts it: score it and snap it.
• The display sometimes whines (you hear it in the sound, near the amp): the cause is the coil marked 4R7 on its back. A drop of hot glue on the coil calms it down.
• The XIAO's socket is brought out with a plain USB-C extension, 20 cm, male into the XIAO and female towards the side: that way firmware and USB MIDI work with the case closed. The panel-mount lead tried first carried power but not data.

You load it once; after that the banks come from the app, no cable.

1. Install Arduino IDE 2 from arduino.cc.
2. The board package: in Tools → Board → Boards Manager search for esp32 (by Espressif Systems) and install it. The pedal is tested with version 3.3.
3. The display library: in Tools → Manage Libraries search for U8g2 (by olikraus) and install it.
4. Download the project from github.com/mazzrelaz/SparklingTones (Code → Download ZIP) and unzip it outside the Documents folder.
5. The test first: open pedale/prova-espansore/prova-espansore.ino. In the Tools menu: Board XIAO_ESP32S3, USB Mode Hardware CDC and JTAG (the default), and the Port that appears when you connect the XIAO to the computer. Then the Upload arrow. It's for checking the wiring: that's the next chapter.
6. The real pedal: open pedale/prova-ble/prova-ble.ino (it brings along the other files in the folder) and change two settings: USB Mode USB-OTG (TinyUSB) and USB CDC On Boot Enabled. For English screens, change the line #define INGLESE 0 near the top of the file to #define INGLESE 1 (out of the box they're in Italian). Same port, Upload. On restart the display shows the logo and the version for four seconds. With any other «USB Mode» the pedal won't compile, on purpose: without TinyUSB there's no MIDI mode.

The times after

With the pedal's firmware the USB socket becomes the MIDI controller, and the Arduino IDE can no longer upload on its own. Before each upload you put the XIAO back into listening mode, without pressing any button: you open its port at 1200 baud and close it again.

1. Connect the pedal and look up its port number in Tools → Port (for example COM3).
2. In PowerShell, with that number:

   $p = New-Object System.IO.Ports.SerialPort 'COM3',1200; $p.Open(); $p.Close()

   A «device does not exist» error is normal: the port disappears because the XIAO restarts.
3. A new port appears (on the prototype COM13): choose it in Tools → Port and press Upload.

Before closing the case: every button and every LED on the right line.

With prova-espansore loaded (its screens are in Italian):

1. Hold any two buttons for a second and a half: the guided mapping starts.
2. The buttons: the display asks you to press them one at a time, in order — footswitches 1 to 5 from the left, then the left and right bank buttons.
3. The LEDs: the display lights one line at a time and asks what lit up. Answer with the footswitch under the LED that lit (with the fifth if nothing lit), then with the colour: left bank button red, right green.
4. At the end the display shows the two tables and stays put: compare them with the ones in chapter 5. If they match, the wiring is right. A line you answered «nothing» to is a loose wire or an LED turned round. A button that doesn't respond is skipped after 30 seconds.
5. The manual test: right after, the four LEDs light in the bank colour, the footswitch you press leaves only its own lit, the fifth changes colour.

Then upload prova-ble, and the pedal is ready.

Switch on the Spark, switch on the pedal. It does the rest.

The drawings and photos show the pedal's Italian screens, with translations where it matters. With INGLESE 1 (chapter 8) the pedal shows them in English.

1. Switch on the Spark, and make sure no phone is connected to it (neither the official app nor this one): the Spark only accepts one connection.
2. Switch on the pedal with the switch on the side. Four seconds of logo, then it looks for the amp.
3. The first time, choose the amp: left bank button Spark 2, right NEO. The pedal remembers.
4. When the square in the top right fills in, play.

What each control does

Banks

• «Amp Preset» is the presets stored in the Spark (A1–B4 on the Spark 2, CH1–CH4 on the NEO). The pedal reads them on every connection and plays them by selecting the slot: the change is instant.
• The other banks you put together in the app and send to the pedal (next chapter). It holds eight of them, even with the power off. One of their presets reaches the Spark in half a second, a second at most.
• The pedal never writes to the amp's slots: bank presets play «on the fly». While one is playing, the Spark's light blinks: that's normal.
• On the NEO there's only one half: four places, CH1–CH4.

Battery

The icon at the top has four bars. Below 3.5 V it blinks and the display says batteria scarica (battery low). A charge lasts about 17 hours of pedal on (measured). With a USB cable plugged in the icon shows full: that's the cable, not the cell.

The Live view's banks, copied into the pedal.

1. On the pedal, hold both bank buttons together for a second and a half. At the top appears ponte 1:59 (bridge), counting down: you have two minutes. While the bridge is closed the pedal can't be found: that way nobody passing by makes it drop the amp.
2. In the app, Live view → ⋯ menu → Pedal → Find the pedal, and choose SparkPedale.
3. Choose the bank and the place in the pedal (1–8), then Send. A bank goes over in a couple of seconds.
4. In What's in the pedal you can move and remove banks; nothing happens until you press Update the pedal.
5. Disconnect, or close the panel: the pedal takes the Spark back by itself.

The Spark 2's looper, run with your feet. The NEO has no looper.

• The LEDs: red above FS1 while recording or overdubbing, green above FS3 while playing, green above FS4 with the loop stopped and ready.
• The tempo LED blinks on every beat, at the preset's bpm (or the one tapped on FS5).
• The count-in: if the looper click is on in the Spark, FS1 with an empty loop counts four beats before recording — one red LED per beat, from the left, and the piezo click — and recording starts exactly on «one».
• The ring on the right of the display has one slice per bar, and fills one beat at a time. In the middle: ● recording, ◉ overdubbing, ▶ playing, ■ stopped, ○ empty.
• Bars and tempo can only be changed with the loop empty: with a loop recorded the Spark hangs, and the pedal refuses.

The same pedal for a program on a computer (AmpliTube, for example) or on an iPad (BIAS FX).

FS1 + FS4 held for a second and a half: the pedal switches to MIDI, lets go of the Spark, and an M appears in the top right. Same combination to go back. The pedal remembers which mode it was in.

FS5 switches between the two pages.

• From a computer, over USB: the pedal shows up as SparkPedale MIDI. You need a cable that carries data all the way to the XIAO. On Windows it's the only way: Windows doesn't pass Bluetooth MIDI on to programs.
• From an iPad, over Bluetooth: BIAS FX has no menu to connect it; connect it from GarageBand (gear → Advanced → Bluetooth MIDI Devices) or from midimittr, and it stays connected for the whole iPad.

Things that really happened to the prototype, and what fixed them.

The pedal is a personal project, built and tested on a single unit. The code and the story behind every choice are on GitHub (in Italian). ← The app guide