CTR2 is a unique human-machine interface (HMI) that allows you to control and manage up to 16 radios and 8 antennas using a small touch screen user interface on your desktop, or from an Internet browser anywhere in the world. 

The CTR2 system is based on the Teensy 4.1 development board and audio interface provided by PJRC.com. The Teensy 4.1 features an ARM Cortex-M7 processor running at 600 MHz. Audio DSP processing is handled by a NXP SGTL5000 codec on the PJRC Rev D Audio Adapter board. The firmware is based on the Teensyduino 1.54 library, and uses the Arduino ecosystem. This library allows the Teensy 4.1 to be programmed in Arduino C/C++ and provides an extensive set of tools. It also provides a wide selection of easy to use audio tools that operate in the digital domain such as amplifiers, mixers, tone generators, tone detectors, equalizers, compression amps, and FFT frequency analysis. These functions are available for any radio connected to the system, from the latest all digital SDR radio to the oldest boat anchor.

There are two options available to control the system.


This option uses a Nextion 3.5″ or 5″ Enhanced HMI touch screen display and one rotary encoder to control the user interface. A photo of the Home page on this display is shown below. Provisions have been made to allow mounting the display and encoder in the same enclosure as the HMI or they can be remotely mounted in a small wedge shaped enclosure. Remote mounting lets you place the display anywhere it’s convenient. Tuning and parameter changes are managed using the touch screen, encoder, a USB keyboard, or a USB mouse.

Home page on Nextion display


The second option uses Node-RED, a free browser based user interface that replaces the Nextion display. The user interface is similar to the touchscreen except that most user interaction is handled using the mouse. This option also provides the capability to remotely control the HMI (and your radios) via the internet using any device with a modern browser. Just add an audio transport application such as SonoBus and you can take your station with you. A photo of the Node-RED Home page with the Tx Memory, VFO, Log Entry, and Auto Log panels enable is shown below.

Node-RED Home page

The functional drawing in the lead photo shows a large system with four radios and four antennas. Additional Radio I/O modules and antennas can easily be added to expand the system to control 16 radios. Conversely, a starter system with just the HMI and an Auxiliary Option 1 board can be deployed to control one radio.

The first CTR2 article covering the basic system was published by the ARRL in the September/October 2021 issue of QEX magazine. A follow-up article describing the radio I/O and antenna switching options supported by the system was scheduled to be published in the November/December issue of QEX but has been delayed for at least two months or more. With that in mind, I have released the information on the boards that will be covered in that article so builders that want to build a completely automated station control system won’t be delayed.

This is a experimental project and the schematics and source code are available under the Download menu. The source code for this project is copyright 2021 by Lynn H. Hansen. All code is open source and is subject to the terms of GPL3 (GNU GENERAL PUBLIC LICENSE, Version 3.0, 29 June 2007).

The design required extensive use of SMT devices to keep each board’s size to 4″x4″. I can supply bare boards or complete, programmed, and tested boards. I can also supply turnkey systems if you don’t want to assemble the system yourself.

Please note:
This is not a beginners project. If you choose to build individual boards you will need good soldering skills and the ability to fabricate ribbon cables for the board interconnections. In addition, experience with the Arduino IDE (or other supported IDE such as Microsoft Code or Visual Studio) is required as you will be downloading code to the Teensy 4.1 and the Nextion display.

I’m thinking about releasing videos on YouTube. Let me know in the comments if this would be of interest to you.

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