Qwiic FM Transceiver - Si4721

You're probably already familiar with Silicon Labs' brilliant 47xx line of radio devices. We've been carrying their FM tuners for a long time and find them reliable and simple to operate. That said, we thought it would be nice if you could transmit as well as receive, so we looked into it and we found this excellent FM Transceiver!

The Si4721 integrates the complete tuner and transmit functions from previous Si47xx devices into one chip for both FM broadcast reception and standards-compliant*, unlicensed FM broadcast stereo transmission. On top of stereo audio the Si4721 will also transmit and receive RDS/RBDS, allowing you to send and receive strings of text on the FM Band. This is the same technology that modern commercial stations use to broadcast things like call signs and song names but we think you could have fun using it to broadcast sensor data or other arbitrary text data.

In order to be compliant with most broadcast regulations, this transmitter is extremely low power and has a reliable range of maybe 10 to 20 meters. These types of transmitters are most often found in things like intercoms or car stereo audio input adapters. Within range, however, the audio signal is extremely clear.

The Qwiic FM Transceiver board includes not only the Si4721 IC but also a 3.5mm audio jack for line level input, a 3.5mm audio jack providing headphone level output via the on-board TPA6111 headphone amp, and a two position poke-home connector for easy antenna connection. The I²C address is jumper configurable between 0x11 and 0x63, allowing for multiple radio boards on a single Qwiic bus.

Check out our fork of Matthias Hertel's excellent Radio library for Arduino examples of both sending and receiving audio and text!

We do not plan to regularly produce SparkX products so get them while they’re hot!

NOTE: The I2C address of the Si4721 is 0x11 and is jumper selectable to 0x63. A multiplexer/Mux is required to communicate to multiple Si4721 sensors on a single bus. If you need to use more than one Si4721 sensor consider using the Qwiic Mux Breakout.

Experimental Product: SparkX products are rapidly produced to bring you the most cutting edge technology as it becomes available. These products are tested but come with no guarantees. Live technical support is not available for SparkX products. Head on over to our forum for support or to ask a question.
* In many parts of the world, using extremely low-power transmitters in the FM band for non-commercial purposes requires no license but you should always check the local laws just in case! You are responsible for complying with local RF transmission regulations.
  • Qwiic FM Transceiver Board
  • Uses I²C interface (Qwiic-enabled)
  • Transmit and receive stereo audio in the FM band!
  • Transmit and receive RDS/RBDS text strings
  • 2 jumper-selectable I²C addresses:
    • 0x11 (default)
    • 0x63 (jumper cut)
  • On-board TPA6111 Headphone Amplifier
  • 3.5mm audio jacks for line-level input and headphone output
  • 2-position poke-home connector for Rx/Tx Antenna and RF Ground
  • Operating voltage range
    • 3.3V to 5.5V
    • Typically 3.3V if using the Qwiic cable


Looking for answers to technical questions?

We welcome your comments and suggestions below. However, if you are looking for solutions to technical questions please see our Technical Assistance page.

  • BrentBoren / about 4 years ago / 3

    Nice product! Powers right up - although I had to slightly modify the Arduino code to get it working. The board default address is 0x11 hex - but the code is set up to use 0x63 hex. Changing either one works well. Also Nick (I think) looks like he accidentally uploaded a downlevel Transmitter test code. It was including the Si4705 library instead of the Si4721 lib. Changing that and adding in a few calls to turn on the transmitter had this working great and sending the RDS messages just fine.

    • Member #142544 / about 3 years ago / 1

      I'm having issues getting the SI4721 to find the chip during the TestSI4721 and TransmitSI4721 sketches. I've noticed that Matthias Hertel's updated transmitter library has " #include <si4721.h> but the file is called <SI4721.h> " (capital letters). Even after I change it, I'm getting the "Device NOT found at address 17" error, which doesn't make sense because its address should be set at 0x11 (hex for 17) and works at that address for the ScanRadio sketch. I've changed the address to 0x63 just to check, didn't work. What am I missing? It seems that the sample code is not initializing the chip?

  • datnguyen / about 4 years ago / 1

    Is anyone able to have transmitter work? I have used 104.1 MHz but it is tuned to 10.41 Mhz.

  • Member #235635 / about 4 years ago / 1

    Can you make any recommendations on good antennas for short range broadcasting?

  • Member #462246 / about 4 years ago / 1

    You can find an updated version of the radio library using the Arduino Library manager supporting the SI4721 FM Transceiver. The board has a good audio quality in contrast so other cheap boards and other chips. Well done.

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