An expressive midi controller based on the stm32 ecosystem. It uses the MPE midi protocol to pass through signals that act as gestures/effects including velocity/note to any music DAW. (like ableton)
- Uses the powerful STM32H7
- Uses USB-C OTG to connect to a device
- Has per key PWM leds for indication
- Low profile/one sided pcb design
- Small key footprint for compactness
- Two octaves + extra key before an octave
See it in action here!
This controller will be able to make music more accessible by being more affordable to the typical person. Most MPE midi controllers on the market cost between $500 USD to $2000. I also made this project to touch the stm32 ecosystem and learn how to integrate and use it. This midi keyboard could also make my workflow nicer for making music in Ableton.
To reduce the cost of production, I decided to design a compliant mechanism (flexure) that would be only one part. It would reduce the labor and cost by a lot. This is because most keyboards/pianos have between 10 to 100 which I have reduced to a minimum of 2. A hall effect sensor is used to measure the position and rotation of a magnet embedded in each key.
There were many problems while prototyping and laying out things. I had bare knowledge of how flexures worked and had to create as many prototypes as needed until the right movement was made. I also didn't know that the size of the board would be a big constraint that was completely ignored.
I highly recommend to look over these issues before building another stm32pe device!!! A lot of tolerances for the 3d prints are based on my 3d-printer. Make sure to print segments before making the entire thing to test for fit between the pcb, magnet, and everything.
Problems during fabrication
- TMAG5273 has a fixed i2c address which only differs by buying different models (B1, C1, D1) or by running extra interrupt traces
- M screw spacers broke off when screwing into plastic
- Too high of a tolerance between the board and 3d printed parts, had to cut off bottom two aligners on all keys
- Connection (led mux) from main-board to other board was made through really tiny & painful to solder wires
Here's a list of problems after fabrication + firmware
- The Black key flexure design suffers too much with plastic deformation permanently, as well as temporarily
- ^ This issue also causes a permanent distance change with less distance to measure and use
- TMAG5273 library has no calibration feature and the measured angle is offset when brought closer
- Keys are a bit too slippery and need a dimple to grip the finger
- Flexures on both key types are a little on the firm side, takes a little time to get used to
- Last white key (b) is impractical as header wires are blocking its roll to pitch change up
- The power on led is a bit too bright, not changeable
- The base/case of the stm32h7 is kinda slippery
- PLA flexures can get deformed under heat in a car which can leave a permanent loss of height on keys or mess more with the already high tolerance to allow press fitting
These issues do not make the stm32pe midi keyboard worthless or horrible to use, but are just little tiny problems that could be polished in the future.
- tinyusb lib on MIT license
- i2c-mux stm32 lib on BSD-2-Clause license
- stm32 tmag5273 lib on No license (added a i2c address rewrite based on the datasheet as well as a crucial fix)
Thank you to all the open source libraries above to make this project possible. I don't think I would have the expertise to implement all my ics this well.
The firmware for the stm32pe midi controller is written in c/cpp on stm32cubeide and flashed with stm32cubeprogrammer. Amazing platform. Outdated UI (lol). To be as transparent as possible, some of the firmware was written/assisted with AI. About 20% of it was written mostly with AI, while the rest + logic that ties everything together was me. mpe.c, 80% of helper.c, 60% of tlc5940.c, and 5% of main.c with some fixes to TMAG5273.c was assisted with AI. I do understand what the firmware does. Depending on your perspective on AI, it shouldn't really matter as I know what I'm doing.
All of the code is stored in firmware/stm32pe_midi/Core/Src or /Core/Inc. Src/main.c contains some user preferences that can be set through variables.
//midi
int start_octave = 4; //midi code shift
int mt_send_black = 40; //midi on threshold
int mt_send_white = 30; //midi on threshold
//mpe
int16_t pitch_mpe_st = 682; //mpe semitone pitch range (682 = 4st, 341 = 2st)
float angle_mpe_padding = 15; //mpe pitch
float angle_mpe_realistic_max = 25; //mpe pitch
float aftertouch_mpe_white_padding = 9; //mpe aftertouch
float aftertouch_mpe_black_padding = 15; //mpe aftertouch
//octave
uint32_t octave_change_mode_time_threshold = 200; //diff change between mode/octive
uint32_t octave_led_flash = 500; //ms between led flash during mode
//led
int startup_cutoff_wait = 100; //led cutoff + cutoff for initial sensor readings saving
//mode
uint32_t octave_mode_hold_down = 1500; //octave mode keys milliseconds to hold down & change
uint32_t mpe_mode_hold_down = 3000; //~ same above but for mpe mode keys
//debug
bool disable_comport = false; //silences comport to possibly make processing faster?
This is an approximate bill of materials that is a little flexible. It would be around $333.27, ±5 dollars in case of shipping or extra taxes/fees. Thank you Hack Club - Highway for funding this.
| Part | Cost | Count | Supplier | Note | Link |
|---|---|---|---|---|---|
| PLA Filament (white) | $16.99 | 1 | Amazon | I have a 3d printer and would like to print most of the parts. | https://www.amazon.com/dp/B07PGZNM34 |
| PLA Filament (black) | $16.99 | 1 | Amazon | ~ (for the black keys) | https://www.amazon.com/dp/B07PGY2JP1 |
| UV Plastic Glue | $14.15 | 1 | Amazon | This is for the underside of the keys to stop scraping. | https://www.amazon.com/dp/B00QU5M4VW |
| Header 10 Pins | $0.78 | 2 | DigiKey | (does not include shipping/taxes) | 68000-110HLF |
| Header 5 Pins | $0.63 | 1 | DigiKey | ~ | 68000-105HLF |
| PCB Fabrication + Assembly | $151.09 | 2 | JLCPCB | *no coupons applied yet | |
| PCB Shipping | $33.52 | n/a | ~ | ||
| Customs duties & taxes | $83.10 | n/a | ~ | ||
| Sales Tax | $15.24 | n/a | ~ | ||
| TOTAL | $333.27 |
I forgot to add two more parts to the bom. They have been covered out of pocket which is fine
| Part | Cost | Count | Supplier | Note | Link |
|---|---|---|---|---|---|
| 5mm Cube Magnets | $16.99 | 45 | Amazon | I could not find any listings with lower quantities | https://www.amazon.com/dp/B0CW9GVTX2 |
| M3 5mm Screws | ¥2.14 | 50 | Taobao | Pretty cheap! (30 cents usd) | https://detail.tmall.com/item.htm?id=722759297726 |
| Female 10pin Headers | $5.62 | 1 | Mouser | This is for the main board. The part itself costs $0.63 without taxes/shipping. | https://www.mouser.com/ProductDetail/710-61301011821 |
| JLCPCB Two Design Fee | $38.99 | n/a | JLCPCB | 😭 i did not think about this while designing, i tried negotiating and doing everything | |
| TMAG5273-B1/C1/D1 | $23.28 | 18 | Mouser | i stg the datasheet said the sensor's i2c addresses were reprogrammable.. but they didn't say i needed to hookup interrupts to them too... lesson learned :p | |
| Total out of pocket | $85.18 | ||||
| REAL TOTAL | $418.45 |