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CW Flea Transmitter documentation

Tel. 123-456-7890

Fax. 123-456-7890

500 Terry Francois Street, 
San Francisco, CA 94158

Flea Top.png


Source code

Link to Sourcecode on GitHub

The CW Flea Transmitter is an Open Hardware/ Open Source CW transmitter with the following characteristics  

  1. Covers the 40, 30 and 20m bands.

  2. 300mW RF power output.

  3. Powered by 8 to 15V DC (Can be "Hacked" for 5V operation)

  4. Frequency generation with a Si5351 and a 1.5ppm TCXO that can generate any frequency from 0.1 to 14MHz.

  5. Two 9 pole low pass filter built in. One covers 40m and the other covers 30 and 20m. 

  6. Sturdy Power Amplifier that will not be damaged by high SWR.


  • Size:

  • Current usage:
    200mA at transmit.

  • Weight:

Theory of operation:

The CW Flea is a CW transmitter with an Arduino Pro Mini compatible micro-controller that controls a Si5351 PLL and a switchable low pass filter.

The band switching and tuning is done using three buttons.

It is meant to be used together with a suitable CW receiver.

To keep the the hardware as simple as possible there is no frequency display - the user will instead use his/her hearing to hear when the transmitter is tuned to the same frequency that the receiver is tuned to.

The software in the Flea makes it operate in one of three possible states:

  1. Receive

  2. Tune

  3. Transmit


If the CW Key is not pressed and no tune button is pressed it will be in receive mode.

In this mode the Flea has a relay that is passing RF signal coming in from the antenna directly to the Receiver.  E.g the Antenna BNC connector and the Receive SMA connectors center pins are connected to each other.

This is the default mode that it will start up in.


When one of the tune buttons are pressed the Flea will go in to Tune mode.

This mode is similar to the Receive mode in that the antenna signal is passed on to the receiver but a weak signal generated by the PLL is also added to receiver port.

This signal can be heard in the connected receiver.

The CW Flea will switch back to the Receive mode after a short delay when there is no tune button pressed.


In transmit mode the Flea will disconnect the receiver from the antenna and connect the receiver center pin to ground.

After short delay to allow the Antenna relay to do it's job it will generate a transmit signal and send out on the Antenna port. 

This transmit signal is keyed by the Morse Key.



To operate the CW Flea first connect it to antenna, Receiver and Power:

  1. Connect  8.5 to 15V DC to the power connector. The 5V regulator will run cooler at a lower voltage.

  2. Connect an antenna the BNC connector.

  3. Connect a receiver to the SMA with a suitable cable and/ adapter - see pictures for an example​​

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Hold down one of the Up or Down buttons until you can hear the signal in the receiver. To fine tune, use short single presses of the up down button until the tone has the required pitch for you to start the transmission.

Calibrate Ref

Changing the firmware.

The Flea can  be reprogrammed from the Arduino IDE.

How to reprogram it using the Arduino IDE.

  1. Connect a USB cable to the Flea.You can power the flea from the USB connector so you don't need to have the DC in on the regular power connector

  2. Download the latest firmware from Github and open it in the IDE. 

  3. In the Arduino IDE, set the correct serial port using the menu "Tools, Port:"

  4. Set the board type using the  "Tools" menu and submenu "Board:"and choose the board "Arduino Pro or Pro Mini"

  5. Use the "Tools" menu, submenu "Processor:" and set it to ATmega328P (3.3V, 8MHz)

  6. You may have to download an Arduino library, read the source code for tip how to do that.

  7. Recompile and upload the source code to the on-board Arduino.

  8. Check the boot messages in the Serial monitor and make sure it identifies a CW flea when it boots up.

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