What is this?

This is project of DIY USB 25-keys MIDI sensor keyboard using stm32f103c8t6 (bluepill).

Video with test: https://www.youtube.com/watch?v=1aOL8qs3DmQ

How this works? 🤔

All piano keys is a capacitive touch sensors. The microcontroller continuously measure needed time for change pin state from 0 to 1 and and back. That's have little lag, because any gpio pin have parasitic capacitance. And when finger touches pin there is a current leak happens and time to charge/discharge changes.

The algorithm is simple:

1. Change pin to input with internal pull-up
2. Measure time until pin have 1 state
3. Change pin to output=1 for complete charge parasitic capacitor
4. Change pin to input with internal pull-down
5. Measure time until pin have 0 state
6. Change pin to output=0 for complete discharge parasitic capacitor

Any deviations in these measured times is meaning for key press.

Shematic

Just connect piece of foil to microcontroller. And that's all, lol.

Current pinout which use in the code:

Pin	Note
PB13	C3
PB14	C3#
PB15	D3
PB12	D3#
PA8	E3
PB8	F3
PB9	F3#
PB7	G3
PB3	G3#
PA1	A3
PB1	A3#
PA0	B3
PB4	C4
PB0	C4#
PA7	D4
PA2	D4#
PA4	E4
PB5	F4
PB11	F4#
PA5	G4
PA15	G4#
PA3	A4
PA6	A4#
PB6	B4
PB10	C5

But you can connect keys in any random order and just generate new App::m_pins using DEBUG_SORT_KEYS=true mode.

That's exactly what I did 🤦‍♂️

Hardware design

I use keys cut from tin and these mounted to part of fruit box 🤦‍♂️

All keys lacquered for isolation. That important if you don't want to fried your MCU.

(See full size: docs/device-photo.jpg)

To simplify this I wrote piano-layout generator in php: gen-piano.php

Usage:

# gen-piano.php <number of keys>
php gen-piano.php 25

For example, layouts for 25-keys MIDI for this project:

• Black keys layout for cutting: docs/piano-black-keys-for-cut.png
• White keys layout for cutting: docs/piano-white-keys-for-cut.png
• Drill layout: docs/piano-drill-map.png

All of these images have 300 ppi resolution. Just print and cut! :)

Configuration

Okay you built this device. And as expected, nothing works. Oh no, this is scam???? 😱

No, that's okay. Different constructions have different parameters and my settings maybe not suitable for you.

Mainly, you need tune these parameters for your device:

Parameter	Default	Description
SENSOR_WHITE_KEY_THRESHOLD	1000	Threshold for white key, at which deviation (in %) will mean key is pressed. This value in percentages multiplied to 100. 100000=100%
SENSOR_BLACK_KEY_THRESHOLD	1000	Threshold for black key, at which deviation (in %) will mean key is pressed. This value in percentages multiplied to 100. 100000=100%

Other parameters:

Parameter	Default	Description
NOTE_OFFSET	48	MIDI note for first key: 36 - C2, 48 - C3 See for more info: https://studiocode.dev/resources/midi-middle-c/
KEY_VELOCITY	64	Velocity of key press
DEBOUNCE_CYCLES	1000000	Time for debounce. Value in the cpu cycles. This value used to reduce jittering keys state.
CALIBRATION_CYCLES	100	Cycles for measure average capacitance for initial state.
SAMPLES_CNT	8	Cycles for averaging measured capacitance
DEBUG_SORT_KEYS	false	Debug mode for generatin new App::m_pins in right order
DEBUG_PRESS_KEYS	false	Debug mode for tune thresholds

You can find all of these parameters in src/App.h

Build firmware

You need arm-none-eabi compiler for arm: https://developer.arm.com/downloads/-/gnu-rm

git clone https://github.com/Azq2/stm32-sensor-midi-keyboard
cd stm32-sensor-midi-keyboard
./build_libopencm3.sh
make

After this you have app.bin and app.elf. Just flash one of this in any appropriate way.

In case of BlackMagic Probe you can simply flash using this command:

make install

How to use?

Just connect to USB and it works. I love linux for this.

I really don't know what actions needed for windows/osx :)

Debug

I use USART1 for debug output. Connect any USART-to-USB adapter to these pins:

MCU	USART-to-USB
PA9 (TX)	RX
PA10 (RX)	TX