Since the FlexTrak boards are self-contained with their own processor, they can be used standalone rather than connected to a Raspberry Pi. This saves weight and increases run time on battery, though at the expense of course of the functions provided by the Pi, such as photography.

If you wish to use FlexTrak standalone, you still need to program it with the balloon callsign, radio frequency etc., and that can be done in two ways:

  1. Temporarily connect to a Raspberry Pi with running the FlexTrak Python software with flextrak.ini containing your settings.
  2. Connect to a Windows PC via a USB-TTL Serial Cable, and use our Windows configuration program.

This article is about the second method, which is recommended as it’s easier and quicker.

FTDI Cable

There are various USB-TTL serial cables available, and generally these use a converter chip either from FTDI or Prolific. The following assumes the use of an FTDI adapter using the standard FTDI wire colours, however you can use others so long as you take care to use the correct wires.

There are two standard sets of colours, but please refer to the documentation that came with your device:

A: Red-Vcc , Black-Gnd , White-Rx , Green-TX , Yellow-RTS , Blue-CTS

B: Red-Vcc , Black-Gnd , Yellow-Rx , Orange-Tx , Green-RTS , Brown-CTS

Connect as follows:

FlexTrak AVR Board Shown; STM board has the same pins in the same place except RST is not present

Finally, connect the RED wire to the power-in pin as shown above. This will happily accept 3.3V or 5V from the USB-TTL adapter. Or you can run from battery as usual.

Connect the cable to a Windows PC, and you should see the red LED flashing on the FlexTrak board (and later green if it has a GPS lock).


Download the configuration program from:

After connecting the FTDI cable, run the program and you will see:

Choose the serial port allocated to the USB-TTL adapter, and the program will automatically communicate with FlexTrak and show you the current status, e.g.:

and you will see periodic messages from it in the log window:

If you see neither of these, check the FTDI connections to FlexTrak and make sure that FlexTrak is running (flashing LED).

To set the payload callsign etc., click the Common tab. For the current (STM) board, the current settings are filled in for you, but this is not the case for the earlier (AVR) board. Fill in the callsign, field list, flight mode altitude and “include field list” boxes as described in the FlexTrak manual, then hit “Set”.

You will then see the new settings in the log as radio packets are sent:

To set the frequency and mode, choose the LoRa tab:

Click Set to send to the board. If you monitor the radio signal with an SDR, that will show the new frequency:

The new settings are stored in flash memory in the CPU on the FlexTrak board, and will be retained through power cycles, until they are changed again in the configuration program or the board is connected to a Pi and programmed from there.


This firmware has a bug which causes the saved settings to be replaced with default settings on startup. Please update to V1.21 using the instructions in the manual. Or, if you wish to program from the Arduino IDE or with AVRDUDE, you can get the source code and HEX files from

V1.2 is still OK to use if you have the board attached to a Pi running our Pi tracker software, since that replaces the settings when the Pi boots anyway.