The HMI board is the heart of the CTR2 system. The operator interfaces to it using the color touchscreen display and the rotary encoder mounted in the remote display enclosure. It interfaces to the radio I/O and antenna switch controllers through 10-conductor ribbon cables.

The HMI board comes fully assembled, as shown on the left in the photo below. The builder only needs to buy the development boards on the right in the photo, add .1″ pin headers to them, and plug them into the HMI board. The final board will look like the lead photo.

The HMI is based on the Teensy 4.1 development board from along with the Teensy 4 Rev. D audio adapter. WiFi is provided by an ESP-8266-01 module which must be running v1.74 AT firmware. I can supply programmed and tested ESP-8266 modules. The WiFi module is optional if you don’t plan on using WiFi.

NOTE: WiFi is required if you want to control a Flex radio or download log files using the web server built into CTR2.

HMI board with development boards

JP1 at the top right is a 2x .1″ pin header. It allows you to use an external SPST switch to control the power on the HMI. Connect two of the supplied female Dupont jumper wires from the switch to this header. Use heat shrink on the switch ends to provide strain relief for the wire.

The HMI is powered by a DC/DC converter that accepts any voltage from 9 to 36 VDC. By default, the input is negatively grounded. Cutting JP2 on the HMI board allows you to run the entire system from an ungrounded (floating) or positively grounded supply. You can also swap out the DC/DC converter for one that is rated for 18-72 VDC input. In this configuration the system could be powered by a 48 VDC positive grounded battery (common in the communications industry). The power input features reverse polarity protection.

JP3 (just above the ESP-8266 space in the bottom-left corner) provides a DC mic bias. Add a jumper here if you plan to use an electret microphone.

JP4, (just below the top right mounting hole) is used to route DC power from the input of the HMI board to the Antenna Switch Controller boards. This option should only be used if the HMI is powered by a 12 to 13.8 VDC supply as the relays on these boards are 12 volt relays. An option is provided on the switch controller boards to supply them with external power if needed.

The yellow/orange jumper wires shown on the audio adapter board must be added by the builder. These are 4″ Dupont female jumper wires and are supplied with the HMI board. The builder must remove the 3.5mm stereo jack installed on the audio adapter board and add these wires as shown. They bring the headphone output from the audio adapter to the Headphone 3.5mm jack installed on the HMI board (second jack from the bottom left).

WARNING! The headphone signals HPV and HPVGND must never be connected to ground. Doing so will probably destroy the SGTL5000 codec on the audio adapter board. Only plug headphones into the HMI’s headphone jack.


There are four pages of schematics for the HMI board. Only three will be presented here. Page 4 is just revision notes from previous versions. Schematic files can be downloaded here.

HMI Interconnect Schematic

This drawing shows the interconnections between the CPU and Audio board schematics in addition to option notes.

HMI signal routing and notes page

The second page shows the Teensy 4.1, Teensy audio adapter, ESP-8266 board and regulator, and the switch connector wiring detail. A separate regulator, U1, provides 3.3 volts for the ESP8266 board and the antenna switch controller boards. Ferrite beads and RF bypass capacitors are used throughout the design to minimize RF induced voltages.

HMI CPU, Audio Adapter, and ESP8266 Schematic

The third page shows the audio interface, Radio I/O line driver and connector, and RS232 interface.

JP3 provides DC bias if you are using an electret microphone.

Of interesting note it how the PTT_Radio and Key_Radio signals are sent to the Radio I/O module. Because of the limited number of wires available in CAT5 cables, these signals travel on the Line In and Line Out audio wires via center-tap transformers.

HMI Audio and CAT Driver Schematic

U2 is an 8 circuit Darlington Pair line driver. It’s main function is to isolate the Teensy processor from transients on the field wiring. It also provides level translation between the +5 and +3.3 volt logic levels.

IC2 is a dual port RS232 line driver. One port is used to interface with the remote display. The second port is connected to Serial6 of the Teensy CPU and is currently not used. I just thought it would be handy for future expansion.

J19 and J20 provide connections for a local rotary encoder and Nextion display. Local meaning that these devices would be mounted in the same enclosure as the HMI board since they use 3.3 volt logic levels. The firmware will detect when these are installed and adjust accordingly.

If you choose to run Node-RED in place of the Nextion display, connect a 3.3 volt USB TTL serial adapter from your PC to J20, the Local Display connector. Only TD, RD, and Ground connections are required.

NOTE: You cannot have both a local and a remote rotary encoder or display connected at the same time.