SO2R Mini: SO2R Box In Your Palm

The SO2R Mini is a do-it-yourself construction project that is simple to complete, which yields a fully functioning SO2R control box. The SO2R Mini allows an operator to listen and transmit on two radios, transmitting on one and listening on another, or listening on two radios at the same time. It has switching for headphones and a microphone, and a built-in CW keyer. It connects to the computer through a USB interface.

The kit includes all parts needed to build the project, including a heavy metal case. The project is based around an Arduino Nano, which runs a modified version of the K3NG Winkey software. Support has been added for SO2R and can communicate with logging software natively as a Winkey device, or using the SO2R Mini Connector. The connector is a small piece of software that translates your logging software OTRSP (Open Two Radio Switching Protocol) and Winkey into a single stream for the device.

The hardware and software are fully published, allowing the end-user to customize the device as they see fit. For more technical users, I2C and Aux buses are available, allowing for inter-device communication, as well as GPIO, for extra features, like interlocking, multi-paddle, band data and antenna outputs, RTTY generation, and more. There are also pins available for LED indicators of transmitting and receive status.

The SO2R Mini is designed to be an inexpensive, yet complete and flexible solution for SO2R contesting. Similarly, the SO2RM can act as a Winkey by itself. This tiny box measures only 10cm long and 5cm tall. All input connectors are 3.5mm TRS jacks. Separate jacks are provided for all inputs, with paddle and PTT outputs combined on a common jack, allowing for a stereo splitting cable to be used for most traditional radios.

The SO2R Mini is powered by USB and requires a computer connected for full functionality. When no power is applied, the box automatically shifts transmit and receive audio focus to radio one, although PTT and CW signals are not passed. If the user desires the ability to forgo the computer, the SO2R Mini can be expanded to support transmit and receive control switches and a potentiometer for CW speed control. These features are supported on the circuit board and in software. Just add your own pot and switches.

Assembly is easy and quick. From start to finish, the average kit builder should be able to complete the kit in an hour. Modest skills are required, there are no surface mount ICs or RF connectors. The kit contains everything needed to build a functioning unit and costs 50 USD + 10 USD shipping and handling. If you would prefer to have the unit built by the SO2R Mini team, a 50 USD build fee applies.


  • Inexpensive, easy to build, compact, powerful.
  • Small, portable, USB-powered.
  • Headphone audio switching: AA/AB/BB Modes.
  • Headphone latch mode.
  • Microphone audio switching.
  • PTT and CW line steering.
  • Rugged RFI-resistant metal enclosure.
  • Audio transformer isolation on receive.
  • Winkeyer protocol support.
  • Easy expandability using provided auxiliary headers.
  • Construction requires basic kit-building tools and no drilling required!
  • Communicates with most major logging software (OTRSP support required).

Setting Expectations

  • What’s missing from this miniature design?
    • For Interaction – No speed potentiometer, switches, or LEDs come standard, you must add your own
    • For Operating – Audio switching uses relays instead of a clickless audio mux. There is a tiny and barely noticeable click when RX audio selection changes.
    • For RTTY – While FSK support exists in software and board hardware, you must add your own external connectors to the case.
  • Degree of mechanical difficulty – low
    • The SO2R Mini comes with an extruded aluminum case. The board slides perfectly into the case and is secured using custom laser cut end panels, which are included. Final assembly is just a matter of screwing on one end panel, sliding the board in, and installing the other.
  • Degree of soldering difficulty – low
    • The audio transformers are technically surface-mount parts, but the pads are quite large and are very easy to solder with any sort of iron. To test this, the hardware designer successfully soldered these parts with a Weller soldering gun! While this is not recommended, it can be done.
    • It is important to follow the published directions in building the board. Components go on in a certain order to keep the build process easy.
    • Parts are well spaced to make for easy work.
    • If you would like to expand the board using the auxiliary bus, you can, but this is an advanced feature.
  • Computer skills needed – low
    • No programming required.
    • Published connector software has one pane, and configuration only takes a few seconds.
    • The Arduino sketch is available on the downloads section of this website. If you would like to modify the sketch for your own use, or flash a new Arduino Nano, you may. 

About The Team

Marty Sullaway, NN1C is the proprietor of the SO2R Mini, designing the first prototype in early 2019 for W1UE to take to HQ9X. He manages all of the hardware development, case design, and PCB and case hardware production and fabrication. He also manages the website and is super lucky to work with two other excellent hams in developing this project. He is an improving SO2R operator and frequent contester, having gotten the chance to operate at many different stations in the last five years. An automation and control nut, Marty got sick of not being able to control everyone else’s SO2R boxes, so he designed his own. 

Paul Young, K1XM is a software engineer by trade. Paul wrote all of the software for the SO2R Mini, as well as the Arduino sketch modifications allowing for the SO2R Mini to communicate using the Winkey 1 protocol. Paul is a serial ham radio product designer, having been largely responsible for the YCCC SO2R Box, MOAS, and OTRSP (Open Two Radio Switching Protocol). Paul is a frequent traveler and dxpeditioner, having been part of teams at 6Y1V, KL7RA, HQ9X, W1KM, KL7RA, 4X0W, PJ4X, and other world-class stations. Paul was also the original inventor of the SO2RDuino, the motivation for this project. 

Dennis Egan, W1UE is an avid contester located outside of Boston, Massachusetts. He handles all of the order fulfillment, building documentation, and hardware testing for the SO2R Mini project. He is a successful single op and multi-op contester, winning several CQ WW RTTY contests, and holds at least one US LP record. Dennis is a frequent op at HQ9X, W1KM, K1VR, KC1XX, and K1LZ. Dennis handled similar duties for the YCCC SO2R Box project. Dennis serves on the ARRL Contest Advisory Committee and has brought significant developments to ARRL digital contesting and contest activity involvement.