KVM Switchinator

The KVM switchinator takes up only about 2.5" x 2.5" of desk space

The two KVMs under the desk sit neatly out of the way. Notice the visible LED indicators

A capturing tube for the IR signal makes sure that signals for the left KVM don't make it to the right KVM

Light pipes make it so you can see the KVM's LEDs, even though they face downward

📁 Source code: View on GitHub →

I’m no stranger to producing complex solutions to simple problems - and the KVM Switchinator is no different! I run a somewhat complex desktop setup whereby my two monitors are controlled by a KVM, each, so I can switch between four potential inputs per screen. This makes it so I can have my home and work machine as well as two others (one set up with ports on the outside of the desk so I can wire up a box quickly and temporarily) to the same monitors. This is awesome for when I’m bootstrapping a new machine so I can keep the right hand monitor on the bootstrap screen while tooling around with the keyboard, mouse, and left monitor on my main machine.

You may say, “That sounds pretty sweet. What was the problem?” - and you’re right. It is sweet, but the problem was simple. Having two 4-way KVMs sitting on the desk costs a lot of real estate. I really don’t like having a crowded desk - only stuff that I actually use should be on the desk.

While it took ages to find a KVM that fits all my needs, I finally settled on a TESmart HKS0401B2U after experimentation. This one features the important bits:

• Hotkey switching (double-tap scroll lock and then the number to switch to)
• IR switching (use the remote to select the target)
• USB passthrough (plug in any USB 2 device and it switches with the KVM)
• HDMI in and out w/ 4k@60Hz (I actually only use 2k@60Hz)
• EDID emulation (for when a PC comes online or the screen switches, the machine still “sees” the monitor)

While it sometimes has issues, and I’ve had to even replace one (more on that in a moment), it’s been a really solid solution. Unfortunately, because of how they are set up, they are quite wide, and take up a lot of space on my desk.

I first started by 3D printing up a simple vertical stacking mount so they’d only take up the width of a single unit at double the height. This worked well, but I still didn’t want that much desk real estate taken up, so I decided to relocate the devices under the desk.

This is when some “incompatibilities” came out:

• Only one keyboard is connected, so the hotkey switching controlled only the left monitor. This meant the physical button (or IR remote) to switch was needed
• Both devices use the same IR codes, so pointing the remote at them would switch both devices

I decided the solution was to use a small desk-mounted controller that would trigger each KVM independently via IR, and some 3D printed housings to keep it all together. The unit on the desk would have a small LED screen showing which input each KVM is set to with eight buttons to switch the left or right KVM to the desired input.

Learning to read IR codes

Through the course of creating the system, I needed to take a short detour to identify the IR codes used to switch which KVM input is active. Many years ago, I played around a lot with LIRC and knew that if you had an IR reader, it is easy to have it dump the raw IR signals that are sent to it. Unfortunately, I didn’t have a Linux IR reader handy anymore (got rid of it when I moved away from MythTV) so I decided to just toss an IR receiver on the board. This made for a decent plan because I could learn IR codes from the same Arduino library I want to use to emit IR codes. All I needed was a bit of code to do the learning, and I was set! So now, even while the Switchinator is running, all you have to do is point to the IR receiver and press a button - the human readable version and the source code version will be printed to serial.

Fitting it all onto the board

The ESP32 dev board that I selected (well, that I had sitting around in inventory) has 14 total pins (D13, D14, RX2, TX2, D18, D19, D21, D22, D23, D25, D26, D27, D32, D33) that can be used for whatever you want. This in itself posed a problem because I needed 13 pins to start with:

• 8 pins to detect button presses
• 2 pins for IR emitting
• 1 pin for IR receiving
• 2 pins for I2C to the OLED screen

For the first design, where I wanted to have separate boards, this was a nonstarter. I wasn’t about to go out trying to find commodity 14 pin wiring and connectors, so the solution was to use the famous HKS0401B2U parallel load shifter chip. This allows one to wire up 8 switches to the chip and read the status of all 8 switches as a single byte of data all at once (well, kinda - at once as far as we humans will know). This took the required 8 pins down to only two - and meant that I wouldn’t have to deal with connecting 14 total pins, much less how to route 8 inputs to the board. Nice!

Circuit Boards

Combinedinator - ESP32.fzz

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This combined board is a bit deeper, but locates all components onto a single PCB

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Switchinator.fzz

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Failed attempt - board just for buttons with an RF45 port for ethernet wiring

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Controllinator.fzz

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Failed attempt - board that held the IR gear and the ESP32 with an RF45 port for ethernet wiring

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The initial design split all the “brains” and the “buttons” by having the ESP32 mounted under the desk and the buttons with screen on a small board that sits on the desk. Connecting them was a simple RJ45 connector so I could just use readily available and cheap ethernet cable. For whatever reason, this simply didn’t work. I think it’s because the OLED screen wasn’t quite happy with all the “stuff” between it and the microcontroller - or maybe I just screwed up the wiring (very possible, and even likely). So after having the first set of boards made up and failed, I decided to combine the entire inator into a single custom board that included everything needed.