Did you ever notice how, in the crazy world of tech, there's always that one quirky little project trying to solve a problem so niche that its only competitors might be a left-handed screwdriver and a self-hiding alarm clock?
I use two different computers in my daily workflow and share a single keyboard/mouse pair between them. Trying several USB switching boxes found on Amazon made me realize they all suffer from similar issues - it takes a while to switch, the process is quite clumsy when trying to find the button and frankly, it just doesn't get any better with time.
All I wanted was a way to use a keyboard shortcut to quickly switch outputs, paired with the ability to do the same by magically moving the mouse pointer between monitors. This project enables you to do both, even if your computers run different operating systems!
The device acts as an intermediary between your keyboard/mouse and the computer, establishing and maintaining connections with both computers at once. Then it chooses where to forward your mouse and keystrokes to, depending on your selection. Keyboard follows the mouse and vice versa, so just dragging the mouse to the other desktop will switch both.
To get the mouse cursor to magically jump across, the mouse hid report descriptor was changed to use absolute coordinates and then the mouse reports (that still come in relative movements) accumulate internally, keeping the accurate tally on the position.
When you try to leave the monitor area in the direction of the other monitor, it keeps the Y coordinate and swaps the maximum X for a minimum X, then flips the outputs. This ensures that the cursor seamlessly appears at the same height on the other monitor, enhancing the perception of a smooth transition.
<palign="center"> Dragging the mouse from Mac to Linux automatically switches outputs.
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The actual switch happens at the very moment when one arrow stops moving and the other one starts.
## Keyboard
Acting as a USB Host and querying your keyboard periodically, it looks for a preconfigured hotkey in the hid report (usually Caps Lock for me). When found, it will forward all subsequent characters to the other output.
To have a visual indication which output you are using at any given moment, you can repurpose keyboard LEDs and have them provide the necessary feedback.
It also remembers the LED state for each computer, so you can pick up exactly how you left it.
Alternatively, you can use the [pre-built images](binaries/). Take the Pico board that goes to slot A on the PCB and hold the on-board button while connecting the cable.
It should appear as a USB drive on your system. Copy the corresponding board_A.uf2 file there and repeat the same with B.
## Upgrading firmware
Option 1 - Open the case, hold the button while connecting each Pico and copy the right uf2 to it.
This will make the corresponding Pico board enter the bootloader upgrade mode and act as USB flash drive. Now you can drag-and-drop the .uf2 file to it (you might need to plug in your mouse directly).
Ever tried to move that YT video slider to a specific position but your mouse moves too jumpy and suddenly you are moving your hand super-carefully like you're 5 and playing "Operation" all over again?
**Holding right ALT** while moving the mouse will slow it down considerably, enabling you to get the finer precision work done and still have your mouse moving quickly otherwise.
[The circuit](schematics/DeskHop.pdf) is based on two Raspberry Pi Pico boards, chosen because they are cheap (4.10 € / pc), can be hand soldered and most suppliers have them in stock.
The Picos are connected using UART and separated by an Analog Devices ADuM1201 dual-channel digital isolator (~3€).
While they normally don't have support for dual USB, thanks to an [amazing project](https://github.com/sekigon-gonnoc/Pico-PIO-USB) where USB is implemented using the programmable IO wizardry found in RP2040, there is support for it to act both as an USB host and device.
## PCB
To keep things as simple as possible for DIY builds, the traces were kept on one side and the number of parts kept to a theoretical minimum.
USB D+/D- differential lines should be identical in length, but they are slightly asymmetrical on purpose to counter the length difference on the corresponding GPIO traces PICO PCB itself, so the overall lengths should match.
Zd (differential impedance) is aimed as 90 ohm (managed to get ~107, close enough :)).
The thickness is designed to be 1.6 mm for snap-fit to work as expected.
Planned changes:
- Add 15 ohm resistors for D+ / D- lines
- Add decoupling capacitance near VBUS line on USB-A connector (~100 uF)
- Add an ESD protection device near the USB connector
Since I'm not very good with 3d, the case is [simple and basic](case/) but does the job. It should be easy to print, uses ~33g of filament and takes a couple of hours.
Horizontal PCB movements are countered by pegs sliding through holes and vertical movements by snap-fit lugs on the side - no screws required.
Micro USB connectors on both boards are offset from the side of the case, so slightly larger holes should allow for cables to reach in.
The lid is of a snap-fit design, with a screwdriver slot for opening. The markings on top are recessed and can be finished with e.g. crayons to give better contrast (or simply left as-is).
If you have some experience with electronics, you don't need this. However, some of you might not, and in that case this video might help guide you through the process. Please note, after soldering you should clean the flux from the PCB to prevent corrosion.
The standard process to do that is using isopropyl alcohol and an old toothbrush. But guess what? I'm not putting my old toothbrush online, so you'll just have to improvise that part :)
This step is not required, but it can be handy if your screens are not perfectly aligned or differ in size. The objective is to have the mouse pointer come out at exactly the same height.
Just park your mouse on the LARGER screen at the height of the smaller/lower screen (illustrated) and press ```Right Shift + F12 + Y```. Your LED (and caps lock) should flash in confirmation.
Repeat for the bottom border (if it's above the larger screen's border). This will get saved to flash and it should keep this calibration value from now on.
#### Other configuration
Mouse speed can now be configured per output screen and per axis. If you have multiple displays under Linux, your X speed is probably too fast, so you need to configure it in user_config.h and rebuild. In the future, this will be configurable without having to do that.
#### Functional verification
When you connect a new USB peripheral, the board will flash the led twice, and instruct the other board to do the same. This way you can test if USB and outgoing communication works for each board.
Do this test by first plugging the keyboard on one side and then on the other. If everything is OK, leds will flash quickly back and forth in both cases.
1. I just have two Picos, can I do without a PCB and isolator?
*Sure. Having an isolator is recommended but it should work without it.*
2. What happens if I have two different resolutions on my monitors?
*The mouse movement is done in abstract coordinate space and your computer figures out how that corresponds with the physical screen, so it should just work.*
3. Where can I buy it?
*I'm not selling anything, this is just a personal, non-commercial hobby project.*
[update] There is a manufacturing/assembly company in China that might offer PCBs in qty of 1, assembled boards and boards + case, so stay tuned.
I **don't want to take any commission** on this - the only goal is to provide an alternative for people who don't feel confident enough to assemble the boards themselves.
- Windows 10 broke HID absolute coordinates behavior in KB5003637, so you can't use more than 1 screen on Windows (mouse will stay on the main screen).
- Not tested with a wide variety of devices, I don't know how it will work with your hardware. There is a reasonable chance things might not work out-of-the-box.
So, what's the deal with all the enthusiasm? I can't believe it - please allow me to thank you all! I've never expected this kind of interest in a simple personal project, so the initial features are pretty basic (just like my cooking skills) and mostly cover my own usecase. Stay tuned for firmware updates that should bring wider device compatibility, more features and less bugs. As this is a hobby project, I appreciate your understanding for being time-constrained and promise to do the best I can.
I kindly request that anyone attempting to build this project understands and acknowledges that I am not liable for any injuries, damages, or other consequences. Your safety is important, and I encourage you to approach this project carefully, taking necessary precautions and assuming personal responsibility for your well-being throughout the process. Please don't get electrocuted, burned, stressed or angry. Have fun and enjoy!
