1/7/2026 - The goals:

This tactile control panel is primarily designed to get HMI for the RGB Connect Project (see my other projects): Even though I can control the RGB LEDs on the mobile phone App, it's still better to have an actual hardware controller, that doesn't require a phone nor Wifi. So it'll be connected to the RGB Connect Main board with an 8P FPC ribbon cable (a few 80cm long, because it has to be accessible, easy to reach (not like the Controller)).

However, I want this module to be usable with many more projects, so I'm going to add more ways to access the IO, like an 6P FPC connector (so I could use it with the IOX-77 devboard) and simple 2.54mm header pins. These additional connectors won't be soldered by default, but at least the footprint will be present on the PCB, so it's future-proof.

I'll use 2 MPR121 ICs for the capacitive touch monitoring. I'll get 24 touch keys in total: some for tactile key switches, some other for a tactile RGB color wheel (maybe more of a squarish one, so that it fits the rectangular design...). For The LED status bar, I'll add a few WS1812 (maybe 8 or 10?)

I've found a cute vibration motor on LCSC (part number C2759984), so I'll drive it with PWM with a mosfet (AO3400, I've got plenty of those).

Then, the remote will be organized this way: a sandwich made of a bottom PCB (with all the components, and LEDs), a 3d Printed case, and on top another PCB with all the copper areas (tactiles key areas) and the coin vibration motor. The two PCBs will be connected via an FPC 24P 1mm connector that goes from the top side of the bottom PCB to the bottom side of the top PCB (tactile areas). Then I'll add somewhat of a diffusing layer on the right side of the control panel, then add on top a black semi transparent 1mm laser cut acrylic sheet (the LEDs will shine through it from below).

I've designed a quick basic Control panel: this would certainly be the rough overall shape:
(tactile keypad on the left, and the light bard on the right)

I guess the goals are now clearer, so let's try building it!

1/8/2026 - Schematic Part 1

I started the schematic, not by adding the main ICs, the MPR121, but the components around (don't ask me why...)

So we've got the coin vibration motor, with it's AO3400 Mosfet for PWM driving, an LED indicator, and FPC connectors: There are 3 footprints, but I'll only use one depending on the project (for the RGB connect, I'll use the 3.3V 8P FPC, but if I want to connect it to the IOX-77 devboard, I've got another 6P FPC connector). So there are multiple footprints available, and I'll choose which one to solder when I'll need it.

As I said, I want it to be versatile, so it has to work either with 3.3V, or 5V. For this purpose, I've added a 1A 3.3V LDO (TLV75733PDBVR), and an I²C level shifter. Same thing, if I don't need 5V compatibility, I don't need to solder these components and can bypass them; but at least, I could do it easily if needed. Here too, there is a dedicated ''5V'' 8P FPC connector footprint.

The LDO can be disabled through it's PWR.EN pin (pull low to disable the 3.3V rail, to achieve really small current consumption (a few µA)).

1/10/2026 - Shematic Part 2: Addressable LEDs and MPR121

I added 9 WS2812 (2.2mm by 2.2mm SMD ones): 8 will be on the right side below the acrylic, and another one under the control area (to display the current RGB color of the strip LED).

There are 2 available voltage for VDD LEDs, +5V or +3.3V (depending on how the PCB will be used), so there are 2x 0 ohm resistors (shunt) to choose the voltage source. Obviously, it means that we won't solder both at the same time, 'cuz you know, 3.3V and +5V connected together may not be the greatest idea ever 😅 ...

The 2 MPR121 communicate via I²C, so they have different addresses set with a pull up/down on the ADDR pin. all the electrodes coming out of the ICs will be send over the top PCB (with the copper electrodes on the top side) with a 1mm pitched, 5cm long FPC ribbon cable. We've got all the 24 electrodes along with a Ground attachement point for the Ground copper pour that I might add around the electrodes for shielding. I still hadn't added the designator for the electrodes on the FPC connector (ELE?) because I'll follow the hardware configuration of the MPR121 ICs and adjust the schematic accordingly.

I think everything is good so far:

Edit: I updated the schematic because I forgot to had decoupling caps all over the place. (ICs, FPC connectors... Here is the V2:

1/17/2026 - Changed a few things on the schematic...

I changed my mind on the way the MPR121 will be connected to the sensor Pads after thinking about it:
I initially wanted all the electronics to be on one PCB, and then a 24P FPC ribbon cable would connect the MPR121 pins to the electrodes on a PCB on top. The thing is, the FPC and routing on the top electrodes PCB would have added way too much parasitic capacitance, as the traces would be stacked on top of each other... (traces on the bottom PCB => FPC => traces on the top PCB for routing it to the electrodes...)
So I think that I'll add the MPR121 ICs directly on the bottom layer of the electrode Top PCB, and only the I2C lines will be connected via FPC to the main board.

So here is the updated version: