Desk Control Panel

Design Requirements

A control panel for under my desk that allows me to control my KVM as well as other peripherals.

A MCU like an ESP32-C3 enables a fancier interface with a 0.96" OLED screen.

Peripherals

• 2 x HDMI switch (2IN-1OUT)
• USB hub switch
• Speaker channels (left and right)
• 3 x USB power

The HDMI switches and the USB hub switch are existing standalone devices so these will be controlled by buttons directly without the MCU. The USB power switches will be controlled directly by the ESP32.

HDMI Switch

The voltages of the switch pins are as follows:

With the stock switch:
          ┌─────┐
┌─────────└─────┘─────────┐
│       5.2V   GND        │
│ ┌───┐  │      │   ┌───┐ │
│ │ON │ 5.2V   0V   │OFF│ │
│ └───┘             └───┘ │
│       4.9V   3.2V       │
└─┌─────┐─────────┌─────┐─┘
  └─────┘         └─────┘

          ┌─────┐
┌─────────└─────┘─────────┐
│       5.2V   GND        │
│ ┌───┐  │          ┌───┐ │
│ │OFF│ 4.9V   3.2V │ON │ │
│ └───┘  │      │   └───┘ │
│       4.9V   3.2V       │
└─┌─────┐─────────┌─────┐─┘
  └─────┘         └─────┘

With the stock switch removed:
          ┌─────┐
┌─────────└─────┘─────────┐
│       5.2V   GND        │
│ ┌───┐             ┌───┐ │
│ │ ~ │ 1.3V   1.5V │ ~ │ │
│ └───┘             └───┘ │
│       4.9V   3.2V       │
└─┌─────┐─────────┌─────┐─┘
  └─────┘         └─────┘

Switch pin numbers:
          ┌─────┐
┌─────────└─────┘─────────┐
│         1     4         │
│ ┌───┐             ┌───┐ │
│ │ ~ │   2     5   │ ~ │ │
│ └───┘             └───┘ │
│         3     6         │
└─┌─────┐─────────┌─────┐─┘
  └─────┘         └─────┘

When the INPUT is pulled to GND, the switch's output is the HDMI Input A. When the INPUT is pulled to 3.2V, the switch's output is the HDMI Input B. When the INPUT is floating, the switch's output is unstable and may not output anything.

The INPUT control pin will be driven by a simple latching switch and a 1kΩ pull-up resistor.

To modify the HDMI switch from stock, pins 1, 2, and 3 should be bridged together. The pull-up resistor is placed between pins 5 and 6.

There are two LEDs to indicate which computer is being used as the source. These can be tapped into in order to get the current state to be read by the ESP32 and displayed on the OLED screen. When active, the LED has a 1.8V potential difference. However, relative to a shared GND, these are the observed voltages in the various states:

LED A State	LED A Pin 1	LED A Pin 2	LED B State	LED B Pin 1	LED B Pin 2
ON	0V	2.5V	OFF	0V	0V
OFF	0V	0V	ON	0V	2.5V

Note

Due to lack of pins on the ESP32-C3, the state of the HDMI Switches will not be monitored by the ESP32-C3. The latching switches should be sufficient for state UI.

USB Hub Switch

The USB hub switch directs 4 USB ports between upstream computer A or upstream computer B.

There are two control pins with a 4.75V potential difference. When these two control pins are bridged, the hub toggles the USB source. These will be driven by a momentary button switch.

There are two LEDs to indicate which computer is being used as the source. These can be tapped into in order to get the current state to be read by the ESP32 and displayed on the OLED screen. When active, the LED has a 1.9V potential difference. However, relative to a shared GND, these are the observed voltages in the various states:

LED A State	LED A Pin 1	LED A Pin 2	LED B State	LED B Pin 1	LED B Pin 2
ON	0V	1.9V	OFF	5V	5V
OFF	5V	5V	ON	0V	1.9V

Since all pins go to 5V in some state is necessary to leverage a voltage divider in order to read the 5V signals with a 3.3V ESP32. The voltage divider could consist of 10kΩ/20kΩ resistors:

V_out = V_in * (R2 / (R1 + R2))
V_out = 5V * (20kΩ / (10kΩ + 20kΩ))
      = 5V * (20,000 / 30,000)
      = 5V * 0.6667
      ≈ 3.33 V

I_total = V_in / (R1 + R2)
        = 5V / (10kΩ + 20kΩ)
        = 5V / 30kΩ
        ≈ 0.000167 A
        = 167 µA

P_total = V_in × I_total
        = 5V × 0.000167 A
        ≈ 0.000833 W
        = 0.833 mW

Note

Since the USB hub switch is triggered with a momentary switch rather than a latching switch, the ESP32-C3 will monitor state for presentation to the user on the OLED screen. The monitored pins are LED A Pin 1 and LED B Pin 1.

Speaker Channels

There is currently two toggle switches that are manually spliced into the 3.5mm audio cables from two computers to direct the output from each computer to the speaker left and right channels. This implementation will remain the same as this simple analog switching is working well.

Note

The toggle switches are sufficient state UI and therefore does not require monitoring by the ESP32-C3.

• Refine design of enclosure
  • Orient inputs and outputs on the same side
  • Use PETG instead of PLA for better durability
  • Use fuzzy skin to camouflage layer lines for improved aesthetics
• Redo connections to use hot-swappable DuPont connectors and longer, more flexible stranded wires

USB Power

Control USB power using MOSFETs. Planned USB-powered peripherals include:

• Pyle PAD43MXUBT Audio Mixer (500mA @ 5V)
• Meeting Sign (200mA @ 5V)

To be triggered by an MCU, this should be accomplished with a logic level P-Channel MOSFET. The MOSFET should be logic level in order to be driven by a 3.3V ESP32 directly. The IRLML6402 is widely available and cheaper but without features like short-circuit and thermal protection of a dedicated USB Switch IC.

Using an N-Channel MOSFET is not ideal because the USB spec assumes GND is always connected and stable.

Using the P-Channel MOSFET should include

• A 1kΩ inline series gate resistor to reduce inrush current and EMI when switching the gate. Source
• A 10kΩ pull-up resistor to ensure the MOSFET stays off during MCU boot/reset, while the GPIO is floating.

Meeting Sign

An existing project that is USB-powered and utilizes an Arduino Nano to countdown a timer and indicate the remaining duration on a series of LEDs. The Meeting Sign will be powered via one of the USB Power MOSFETs controlled by the MCU.

• Rewrite firmware
  • Use the ESP32-C3 instead of the Arduino Nano in order to use consistent esp-hal and embassy tooling
  • Allow the timer to be controlled via UART
  • If no UART commands are received, there should be a default timer
  • There should be a SENSE connection between the Meeting Sign ESP32-C3 and the Control Panel ESP32-C3 that allows the Control Panel to detect if the Meeting Sign is online
    • This can be a Meeting Sign output that is set low when the Meeting Sign is online and high when it is not
    • The Control Panel can use this signal on an input with a pull-up resistor to determine if the Meeting Sign is online and display the status on the OLED screen -> This turned out to limit the functionality of the control panel output and sensing the value of the SENSE connection was cumbersome
  • Once a timer completes, the ESP32-C3 should go into deep sleep
  • Archive the previous version of the repository with a link to this new version
• Refine design of enclosure
  • Shrink footprint thanks to smaller size of the ESP32-C3
  • Use USB-C socket instead of Arduino Nano's micro USB for power
  • Use power switch for manual operation if not using the Meeting Sign with the Control Panel

Important Concepts

• Make sure to connect the grounds of all the peripherals.

High Level Wiring Diagram

Back Module Distribution PCB Schematic

Firmware Development

This project is built in a no_std environment utilizing the esp-hal crate in conjunction with the Embassy framework.

Commands

Flash binaries

cargo run --release --bin control_panel
cargo run --release --bin meeting_sign

Run tests

cargo test --config .cargo/probe-rs.toml

Reference

ESP32-C3 GPIO Summary

ESP32 C3 SuperMini Pinout