There is a better idea, Implementing HID KM with ESP32C3 BLE.

[qq8322302]

Hello,hrvach:
I am an electronics enthusiast from China and came across your project by chance. It is a fantastic project that can solve many operational problems with multiple hosts.
I have an idea now, to use ESP32C3 BLE to simulate keyboard, mouse, and communication with multiple devices, and to add lithium batteries to get rid of wiring harnesses.

My thoughts:
ESP32C3 serves as the main controller, and after turning on BLE, it broadcasts four names: KM1, KM2, KM3, and KM4. Four PCs are connected to each of these four BLE names.
The IO of ESP32C3 can use software to simulate USB-HOST mode for connecting keyboards and mice.
(ESP32C3's ble_hid_device_demo: esp-idf-v5.2.1/examples/bluetooth/bluedroid/ble/ble_hid_device_demo)
(ESP32C3 software simulation USB-HOST: https://github.com/sdima1357/esp32_usb_soft_host )
ESP32C3-FH4， In China, it only costs RMB 5 yuan, and materials such as batteries and casings are also very cheap. I can design and produce PCB hardware, but my software level is not very good. I wonder if you are interested in researching together?We look forward to your reply.

[chri2]

Sounds nice - I couldn't use it.

It will be a different device for end users without security needs.

My use case includes that I'm forbidden by rules at my work to use any keyboard that is connected wireless.

Making your project secure includes having a secure bluetooth stack which until now proved many times to be a wrong assumption.

Another point for me would be the needless use of batteries. That is not sustainable. There is no reason I can see that your idea couldn't work with one computer being connected through usb providing the power needed to drive your board (or just use an external usb power adaptor which in on most desks anyway.

I'd build or buy one for testing if it would work without a battery just for the fun of it and maybe to give it to my son to switch between his mobile and his desktop. On my desktop I couldn't use it for security reasons.

[qq8322302]

@chri2 If for absolute safety, then wireless and lithium batteries are indeed not a good idea. Both will have certain security risks.
If it's for convenience and a clean desktop, then wireless and lithium batteries will definitely be more convenient.
This involves different usage scenarios.

[tegger2000]

Maybe this project could be a base for your idea: https://github.com/Noskcaj19/hardware-kvm

[hrvach]

This is a cool idea - I've investigated ESP32 initially but it didn't have usb. My plan is to expand the nunmber of devices one can control, my initial thoughts were most people who would use this need 2, a small number might need 3 and very few people need 4 or more. I agree this would be a desirable feature for sure, I occasionally needed a third output, maybe something happens later this year :)

Some of the technical limitations mentioned were actually chosen on purpose, like omitting Bluetooth - I simply don't believe there is a vendor who could implement this in a way that stands the test of time (https://github.com/engn33r/awesome-bluetooth-security for reference). Bluetooth stacks are often written by coders faced with unrealistic timelines and then never ever patched after vulnerabilities are discovered. The protocol itself is a bit too complicated for a secure implementation.

If you're still interested in helping out, I'd be curious how cheap this board could be made, i.e. what the ultimate price point is for small(er) batches. I'll be working on a new PCB variant when time permits...

[qq8322302]

Firstly, I would like to share a simplified version of the deskhop project that was created using the Raspberry Pi PICO development board and RP2040 * 2. The PCB was redesigned and streamlined to match the metal casing. Flashing the firmware of the desktop hop project has a very good effect.

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I found a similar project for ESP32C3 BLE: USB-HID-BLER, URL: https://github.com/dnstzzx/usb-hid-bler
This project is to convert wired keyboard and mouse to BLE. I built a simple circuit and tested it. The mouse can be used normally, but the keyboard cannot be used.

I have already drawn the PCB according to my idea, and it will take about a week to produce it. I will test it first when the time comes.
There are the following software issues that need to be addressed:

1. I don't know how ESP32C3 can emit KM1, KM2, KM3, KM4, multiple BLE names.
2. The compatibility of USB-HID-BLER is not perfect and needs to be addressed.
3. Need to merge the functions of deskhop into it.

Regarding small batch costs:
PCB+materials+shell+production, about RMB 55 yuan. ( 8$ or 8€ )

I don't know if you're interested in helping write software? I can provide you with hardware.

[qq8322302]

Additionally, for users who prioritize safety and do not want to use BLE, we can use wired USB. I also have a better idea that can be extended to hardware solutions with 4 or even 8 hosts.
It can be expanded using RP2040 * 1+analog switch+CH9329 * N, and the price is also not expensive.
RP2040 is connected to a keyboard and mouse, and at the same time, it is connected to 4 analog switches through UART * 1. By selecting which CH9329 to communicate with through GPIO, it can easily communicate with 1 out of N PCs. The disadvantage is that it can only communicate with one CH9329 at a time and cannot communicate simultaneously.
If interested, I can also create this form of hardware, but I also need your software support.

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UPDATE

1. No need to simulate switches, multiple CH9329 can be connected through one UART, and different communication addresses can be set.
2. 0xFF is the broadcast address, which can broadcast messages to all CH9329 online.
3. This hardware solution can easily support N hosts and does not require significant software changes to the desktop.(It is necessary to set PICO-PI-USB to 2 USB-HOST and connect the keyboard and mouse separately. The code for PICO-PI-USB seems to support it, but I am not sure.)
4. Small batch price is about 6-8$