Teensy TX Bridge

Teensy TX Bridge is a firmware that allows you to control RC transmitters via the Trainer port using PPM, over a USB Serial connection.

This allows you to make your own control scheme for your aircraft, by sending commands from a computer in real-time. This can be used, for example, to control an aircraft using a non-standard controller such as a flight simulator setup, or a custom-built controller.

Caution

This project involves working with RC transmitters and aircraft. Improper use can lead to loss of control of the aircraft, which may result in damage or injury. Always test in a safe environment and ensure you have a way to regain control of your aircraft. I am not responsible for any damage or injury caused by the use of this project. Use at your own risk and liability!

Hardware Requirements

• A Teensy board (this is specifically designed for Teensy, I might add support for Arduinos in the future)
• A mono or stereo 3.5mm TRS cable to connect the Teensy to the Trainer port of your RC transmitter.
• A USB cable to connect the Teensy to your computer.

Wiring Diagram

Cut off one side of the 3.5mm TRS cable and strip the wires, then connect them to the Teensy as follows:

TRS Pin	Teensy Pin	Function
Sleeve	GND	Ground
Tip	PPM_PIN	PPM Signal Output

Valid pins by board type:

Teensy 3.6, 3.5:  5, 6, 9, 10, 20, 21, 22, 23
Teensy 3.2, 3.1:  5, 6, 9, 10, 20, 21, 22, 23
Teensy LC:        6, 9, 10, 20, 22, 23
Teensy 4.0:       6, 9, 10, 11, 12, 13, 14, 15, 18, 19

Protocol

The Teensy listens for commands over the USB Serial connection at a specified baud (115200 by default).

These are the available commands:

Command	Description
CH:1500,1500,1000,...	Set channel pulse widths (up to 16, comma-separated)
FS	Force failsafe immediately (manual trigger)
RST	Reset failsafe and resume normal operation

In the firmware, a default failsafe configuration and timeout are configured. If no valid channel data is received within the timeout period (500ms by default), or if the FS command is received, the Teensy will output the failsafe channel values.

To reset the failsafe, and resume normal operation, send the RST command. Always send this command in the beginning of your client script, after connecting to the Teensy and when your control source is ready to send valid channel data.

Software Setup

• Install the Arduino IDE (version 2.0 or later)

• Install the Teensyduino add-on for Arduino IDE

• Open the RC.ino file in the Arduino IDE

• Select the board in Tools > Board > Teensy

• Select the correct USB type in Tools > USB Type > Serial

• Set Optimize in Tools > Optimize > Fastest

• Adjust any of the following parameters in RC.ino as needed:

  • PPM_PIN: The pin used for PPM output (default 23)
  • MAX_CHANNELS: The number of channels to output (default 16, max 16)
  • SERIAL_BAUD: The baud rate for the Serial connection (default 115200)
  • SERIAL_TIMEOUT: The timeout in milliseconds before triggering failsafe if no serial data is received (default 500)
  • failsafeChannels[]: The default failsafe channel values (modify as needed)

• Upload the code to the Teensy board

Usage

• Connect the Teensy to your computer via USB
• Connect the Teensy to the Trainer port of your RC transmitter using the 3.5mm TRS cable
• On your radio, turn on the Trainer mode (in EdgeTX, this is under Model Setup > Trainer > Mode: "Master/Jack"; then adding a Special Function "Trainer" to activate the Trainer output)
• Use a script of your choice to send commands over the Serial connection to control the channels

Acknowledgements

• PulsePosition Library by Paul Stoffregen for generating PPM signals