November 28, 2021

All boards are now available completely built, programmed, and tested. Complete turnkey systems are also available. Visit the Systems and Boards page for more information.

The functional diagram above shows the major components of the CTR2 system.

The core of the system is the HMI board. It contains the brains of the system. The system is designed such that additional option boards stack on the HMI board using M3 25mm+6mm male/female standoffs. Each board added to the stack adds a little over an inch (26.6mm) to the overall height. Boards are slightly less then 4″ square. Power, USB connections, radio I/O, antenna switch wiring, microphone, headphone, key paddles, a remote PTT switch, and the remote display all connect to this stack. That’s A LOT of cables! The remote display allows the stack to be placed in a convenient (even hidden) location for optimal cable routing. Boards can be stacked in any order or laid out on a flat surface. The only consideration is the length of the 10-conductor ribbon cables used to interconnect the boards.

CTR2 Front CTR2 Rear

The stack above is my current system. It is capable of controlling 12 radios and 8 antennas. Four additional radio’s can be added by adding another RJ45 switch expansion board and replacing the side panels to allow for the additional board.


The HMI board is the core of the system. It typically mounts on the bottom of the stack. This board contains the Teensy 4.1 development board, Teensy Rev D Audio Adapter board, and the ESP8266-01 WiFi module. All boards plug into .1″ pin header sockets so they can easily be removed if needed. This board comes completely assembled (minus the Teensy and ESP boards) and tested.

Additional details on the HMI board can be found in the HW: HMI Board blog post.


The HMI board is controlled using a Nextion 3.5″ or 5″ Enhanced color touchscreen display and a rotary encoder. Both are mounted in a wedge shaped acrylic enclosure which connects to the HMI using a single short CAT5 cable. (Use CAT6 cable if your display will be more than 6 foot away from the HMI.) This arrangement allows the display to be placed at any convenient location in your station. I keep mine near my key and keyboard.

The display requires a display driver board to convert RS232 to 3.3V logic levels.

For more information on the display hardware, visit the HW: Remote Display Driver Board blog post.

There are two no-cost alternatives to using a hardware display. The first option is implemented using Node-RED flows. You can read more about this option here. The second option is to use the simulator in the Nextion IDE. Find out more about this option here.


A Radio I/O module is required to interface the HMI to each radio. If you are just starting out and only need to control one radio, order the Auxiliary-Option 1 board with the integrated Radio I/O circuitry. A photo of this board is shown below. This configuration allows you to control a single radio without the need of any additional hardware. Order the Auxiliary-Option 2 board if you plan on connecting to 2 or more radios. This option has just J3 and J4 installed and acts as a 10-pin ribbon cable to RJ45 adapter. This option requires a Radio I/O module for each radio.

More information on this board can be found on the HW: Auxiliary Board blog post.


When connecting to two or more radios, each radio requires a Radio I/O module. This module routes the radio’s transmit and receive audio (transformer isolated), CAT, Key, and PTT signals back to the stack using common CAT5 cable. The Radio I/O module includes a configurable level converter for the CAT data signals. It can interface CAT signals at 5 volt normal and inverted levels and supports the Icom CI-V 2-wire interface.

The Auxiliary-Option 1 board contains a Radio I/O circuit so a separate Radio I/O module is not required. This is a good option if you’re just starting with CTR2. To control more than one radio you should order Auxiliary-Option 2 (or disable the onboard Radio I/O circuit on the the Auxiliary-Option 1 board and install J3, an RJ45 jack). Use a CAT5 cable to connect the RJ45 jack on the Auxiliary board to the common port of a manual RJ45 switch then connect all of your Radio I/O modules to the switched ports on that switch, again using CAT5 cables. This allows you to select a radio with just a touch of a button.

NOTE: The Radio I/O is not Ethernet based – Ethernet switches, hubs, and router CANNOT be used to select a Radio I/O module.

A photo of an external Radio I/O module is shown in the photo below. Wiring schematics for various radios are published under the Schematics category.

For more information about the Radio I/O module check out the HW: Radio I/O Board blog post.


The first step in automating your station involves replacing the manual RJ45 switch with a fully automatic switch controlled by the HMI. This eliminates the need to remember to switch the manual RJ45 switch when changing radios.

The RJ45 switch, like all other options for CTR2, is modular. It consists of one master board (shown below) and up to three extension boards. Each board is capable of switching four radio ports. Together they switch 16 radio ports. You only need to build the capacity you need. You can always add additional switch boards as your needs change.

Each board contains a simple relay select circuit and 16 DPDT miniature relays. The logic selects one, and only one, bank of 4 relays to switch the 8 lines on that radio port to the HMI‘s radio I/O port. The board is supplied with the necessary SMT components installed. The builder needs to supply and install the relays and connectors. A photo of the master switch board is shown below. Additional information on this board can be found here.


Continuing with the automation options, the next requirement is to automatically connect the selected radio’s antenna port to the station antenna (or antennas). CTR2 accomplishes that in two parts. First, the HMI commands the Radio Antenna Switch Controller (RASC) to select the antenna assigned to the selected radio. Second, the RASC selects one of it’s 16 output relays to energize a remote antenna switch such as a DX Engineering RR8B-HP.

The RASC is designed such that one and only one relay can be energized at a time. This eliminates the danger of connecting two or more radio antenna ports in parallel… which is bad! LEDs on the front of the card illuminate to indicate which relay is active as a secondary level of protection.

Always use the best remote antenna switch that you can afford. The goal is to minimize the power coupled from the active port to the inactive ports. Unless you are always going to run QRP power levels, go for a remote switch with a port isolation rating above 60 dB between all ports. The RR8B-HP is rated at 70 dB. Other switches are available, just make sure you buy one from a reputable company that publishes the isolation specs. Your receivers (and your pocketbook) will thank you.

The RASC can be strapped to output + voltage or sink the output to ground. You can also power the external relays from the power supply powering the HMI or from an external power supply.

The photo below shows a RASC board that has been populated with 8 relays since I only have one RR8B-HP and control 8 radios. I can always add the additional relays, LEDs, and the second connector (TB2) when I’m ready to expand my system.


The final automation option is the Antenna Switch Controller (ASC). It is similar to the RASC above in that it controls an external remote antenna switch. However it is designed with latching relays to allow one or more relays to be active at the same time. This allows phased arrays to be switched in any combination. There is an option on the Antenna Select page to allow or block multiple antenna selection. This controller includes MOVs on its outputs to aid in noise and lightning suppression since the remote antenna switch is generally located outside.

This controller also has the option to power your remote antenna switch from the HMI‘s power supply or use an external supply. Strapping also allows external power that is brought into this card to be shared with the RASC. Also like the RASC this card can source + power or sink outputs to ground depending on strapping and has indication LEDs to show relay status.

The photo below shows the ASC board.