The HMI board is the core of the CTR2 system. It ties the Teensy 4.1, Teensy Audio Adapter, and ESP8266 WiFi development boards together in addition to providing interconnections to the various field equipment like a microphone, headphones, key, PTT switch, radio I/O, automation control, and etc.

Schematics and an interactive bill-of-material for this board are available in the File category for your reference.

This board is supplied completely built and tested as shown on the left in the photo below. There are a few things the builder will need to do. They are described below.

WARNING: Always use precautions to guard against electrostatic discharge when working on this board. A wrist strap and ground mat are highly recommended.

HMI Board with development boards


All three development boards need to have .1″ header pins soldered to them so they can plug into the header sockets on the HMI board as shown in the lead photo. This is not difficult to do, just make sure the pins are aligned at a 90 degree angle to the board and that there are no solder bridges when you’re done. When you plug the development boards into the HMI every header socket should have a header pin in it.

NOTE: Be sure to install the short 5-pin header in the USB Host pins of the Teensy 4.1 board. This header plugs into the J1 socket on the HMI and brings the USB-A Host signals out to the USB-A Host connector on the HMI.


To prepare the Teensy Audio Adapter board for installation, remove the 3.5″ headphone jack that was factory installed on this board. Use two of the Dupont female jumper wires supplied with the HMI board to make jumpers from the top two pads where the headphone jack was mounted as shown in the photos above. This jumper will plug into 2-pin header J14 just above the headphone jack on the HMI board as seen on the lead photo.

WARNING: Both headphone output pins on the Teensy Audio Adapter have voltage on them. NEVER connect either pin to ground. Doing so will probably destroy the SGTL5000 codec on the audio adapter.


The last task to complete before plugging in the development boards involves isolating the USB +5 VDC and +5 VDC power sources on the Teensy 4.1 board.

By default, the Teensy 4.1 board takes power from its micro-USB socket. Unfortunately, USB cannot provide the current the CTR2 system requires for operation. You also don’t want CTR2 to backfeed it’s +5 VDC into the USB cable connected to your computer. To isolate these power sources it is necessary to cut a jumper on the Teensy 4.1 board.

This jumper can be found on the bottom of the Teensy 4.1 board, just below the micro-USB connector. Use a razor knife to cut the trace between the two pads. Use a multi-meter to verify the jumper is open.

Also, note the 5-pin header installed in the USB Host location (at the bottom of the photo). As described above, this header must be installed in order to extend these signals to the USB-A connector installed on the HMI board.

Cut this jumper on the HMI

A couple of final notes:

  1. If you plan on using a remote power switch, wire the switch with two remaining Dupont jumpers supplied with the HMI board and plug the female ends into Ext Sw (J9 at the top left corner in the lead photo). You can solder jumper JP1 to bypass the switch if you don’t want to use this option.
  2. Pin 1 of the 10-pin ribbon connectors is marked with a small triangle as shown below. Use this as your reference when building the ribbon cables that connect the option boards together.
#1 pin mark on 10-pin ribbon cable connector

This completes the construction of the HMI board. Let me know in the comments if you have any questions.