Yaaar! Become your very own pirate radio station with this FM radio transmitter. This breakout board, based on the best-of-class Si4713, is an all-in-one stereo audio FM transmitter that can also transmit RDS/RBDS data!

Wire up to your favorite microcontroller (we suggest an Arduino) to the I2C data lines to set the transmit frequency and play line-level audio into the stereo headphone jack. Boom! Now you are the media. Listen using any FM receiver such as your car or pocket radio receiver - this is an easy way to transmit audio up to about 10 meters / 30 feet away.

This transmitter even has RDS/RBDS support - that's text/data transmissions that many modern FM receivers support. (It's how some car radios can display the FM station and current song playing). You can transmit just about any text you want, set the station identifier as well as the 'freeform' buffer.

Best of all, you'll be up and running in minutes with our awesome Arduino library, example code and tutorial!

There's a couple pins on this here breakout, lets cover them all in groupings by 'type'

Audio Inputs

• LIN - this is the line level LEFT input. Its connected to the headphone jack as well but in case you want to wire directly without a chunky cable, pipe line level (~0.7 Vpp) audio into here. There's an AC blocking capacitor on board so it can be DC biased
• RIN - same as LIN but the RIGHT input.

Power Pins

• Vin - this is the power input pin. You can power the chip from 3-5VDC. Ideally you should use the same voltage you use for logic levels. For an Arduino, that's usually 5V
• GND - this is power and logic ground, connect to your microcontroller's ground pin
• 3Vo - this is the output from the onboard regulator, 3.3V nominal. You can use this if you need up to 100mA of 3V regulated voltage
  

Interface Pins

The FM transmitter chip requires a microcontroller for setting it up unlike pure-analog solutions that have a tuning potentiometer. The trade off is some code is needed, but the output is digitally tuned so its much more precise.
Our codebase uses I2C to communicate. The chip supports SPI as well but it was annoying enough to support just I2C so we don't have code examples for SPI!

All the interface input pins are 5V friendly, and can be used with 3-5V logic

• RST - This is the Reset pin. You must have this pin toggle before starting to communicate with the chip. When at logic 0, the chip is in reset.
• CS - This is the Chip select pin, used in SPI mode. It also determines the I2C address. When pulled high (it is by default) the I2C address is 0x63. If this pin is shorted to ground, the I2C address is 0x11
• SCL - this is the I2C clock pin, connect to SCL on your microcontroller.
• SDA - this is the I2C data pin, connect to SDA on your microcontroller.

Extra GPIO Pins

There's also two "GPIO" pins, you can use these to blink LEDs. The initial state of these pin sets up the chip for Analog Mode so don't short them to ground or VCC during reset. They are 3V output only!

• GP1 - this is GPIO #1
• GP2 - this is GPIO #2

GPIO #3 is used for the 32Khz clock generator onboard.

Arduino Wiring

You can easily wire this breakout to any microcontroller, we'll be using an Arduino. For another kind of microcontroller, just make sure it has I2C, then port the code - once the low level i2c functions are adapted the rest should 'fall into place'

Download Adafruit_Si4713

To begin reading sensor data, you will need to download the Adafruit si4713 library from the Arduino library manager.

Open up the Arduino library manager:

Search for the Adafruit Si4713 library and install it

We also have a great tutorial on Arduino library installation at:
http://learn.adafruit.com/adafruit-all-about-arduino-libraries-install-use

Load Demo

Open up File->Examples->Adafruit_Si4713->adaradio and upload to your Arduino wired up to the sensor

You may want to update the FM station transmission. By default the library transmits on 102.3MHz FM, but that might be 'taken' in your area.

Find this line

#define FMSTATION 10230 // 10230 == 102.30 MHz

And change it to an unused frequency. This number is in 10KHz so for example 88.1MHz is written as 8810

Upload it to your Arduino and open up the Serial console at 9600 baud

As long as you get to the RDS On! message that means everything works, pipe some audio into the 3.5mm jack and make sure you see the InLevel audio volume range from 0 to about -10 (dB)

The fastest way to test the RDS message sending is using an RTL-SDR (that's how we debugged the breakout!) or a phone/radio that can do RDS decoding

Using the RPS Scanning function

The Si4713 has the ability 'scan' the FM band and measure the input power. You can use the RPS functionality to locate a good unused station. Find this section in the adaradio demo and uncomment the for loop:

  // Uncomment below to scan power of entire range from 87.5 to 108.0 MHz
/*
  for (uint16_t f  = 8750; f<10800; f+=10) {
   radio.readTuneMeasure(f);
   Serial.print("Measuring "); Serial.print(f); Serial.print("...");
   radio.readTuneStatus();
   Serial.println(radio.currNoiseLevel);
   }
*/

  // Uncomment below to scan power of entire range from 87.5 to 108.0 MHz
/*
  for (uint16_t f  = 8750; f<10800; f+=10) {
   radio.readTuneMeasure(f);
   Serial.print("Measuring "); Serial.print(f); Serial.print("...");
   radio.readTuneStatus();
   Serial.println(radio.currNoiseLevel);
   }
*/

Reupload and look at the serial console:

The larger the number the higher the transmission power. For example, 96.3MHz is a higher number than the others (FYI, its Univision 96.3 FM!) whereas 95.1 MHz is nice as low, that's not used for any transmission. Try to find a number that's also not surrounded by high numbers, since it can get 'drowned out' by the nearby frequencies.

Library Reference

Radio Transmitter control

Start out by initializing the Si4713 chipset with

begin()

This will return true if the radio initialized, and false if the radio was not found. Check your wiring if its not 'showing up'

Then you can turn on the radio transmitter with

setTXpower(txpwr)

the txpwr number is the dBμV transmission power. You can set this to 88-115dBμV or 0 (for off)

Of course, you'll want to tune the transmitter! Do that with

tuneFM(freq)

That will set the output frequency, in 10's of KHz. So if you want to tune to 101.9 the frequency value is 10190

You can check in on the radio with

readTuneStatus()

Whcih will set the currFreq currdBuV adnd currAntCap variables in the radio object. The first two are the frequency and power output, the third variable is the tuning antenna capacitor it set for the best output. This number will vary with antenna size and frequency.

RPS (Radio Power Sensing)

This function is used with two procedures.

readTuneMeasure(freq)

begins the measurement, freq is in units of 10KHz so 88.1MHz is written in as 8810
Then you have to call

readTuneStatus()

which will wait until the chip has measured the data and stick it into the currNoiseLevel variable

RDS/RBDS (Radio Data Broadcast)

The Si4713 has great support for sending RDS data and we made it real easy too. Initialize the subsystem with

beginRDS()

Then you can set the "station name" with

setRDSstation("AdaRadio")

The radio station name is up to 8 characters
You can also send the main buffer which usually contains the song name/artist.

setRDSbuffer( "Adafruit g0th Radio!")

You can send up to 32 characters, but you can continuously send new data, just wait a few seconds before each data rewrite so the listener's radio has received all the data

GPIO Control

There's two GPIO pins you can use to blink LEDs. They are GPIO1 and GPIO2 - GPIO3 is used for the oscillator. To set them to be outputs call

setGPIOctrl(bitmask)

where the bitmask has a 1 bit for each of the two pins. For example to set GPIO2 to be an output use setGPIOctrl((1<<2)) to set both outputs, use setGPIOctrl((1<<2) || (1<<1))

Then you can set the output with

setGPIO(bitmask)

same idea with the bitmask, to turn both on, use setGPIOctrl((1<<2) || (1<<1)). To turn GPIO2 on and GPIO1 off, setGPIOctrl(1<<2)

Advanced!

We, by default, use the built-in AGC (auto-gain control) system so the audio level is maxed out. This may be annoying to you if have a good quality line level and the volume is fluctuating (it should be quiet, but isnt)

in the Adafruit_Si4713.cpp file find these lines

//setProperty(SI4713_PROP_TX_ACOMP_ENABLE, 0x02); // turn on limiter, but no dynamic ranging
setProperty(SI4713_PROP_TX_ACOMP_ENABLE, 0x0); // turn on limiter and AGC

and uncomment the first one, and comment the second. This will turn off the AGC

It's easy to use the Si4713 FM transmitter with Python or CircuitPython, and the Adafruit CircuitPython SI4713 module.  This module allows you to easily write Python code that controls the transmitter and sends RDS data.

You can use this sensor with any CircuitPython microcontroller board or with a computer that has GPIO and Python thanks to Adafruit_Blinka, our CircuitPython-for-Python compatibility library.

CircuitPython Microcontroller Wiring

First wire up a Si4713 to your board exactly as shown on the previous pages for Arduino using an I2C connection.  Here's an example of wiring a Feather M0 to the sensor with I2C:

Python Computer Wiring

Since there's dozens of Linux computers/boards you can use we will show wiring for Raspberry Pi. For other platforms, please visit the guide for CircuitPython on Linux to see whether your platform is supported. 

Here's the Raspberry Pi wired with I2C:

CircuitPython Installation of SI4713 Library

Next you'll need to install the Adafruit CircuitPython SI4713 library on your CircuitPython board.

First make sure you are running the latest version of Adafruit CircuitPython for your board.

Next you'll need to install the necessary libraries to use the hardware--carefully follow the steps to find and install these libraries from Adafruit's CircuitPython library bundle.  Our introduction guide has a great page on how to install the library bundle for both express and non-express boards.

Remember for non-express boards like the, you'll need to manually install the necessary libraries from the bundle:

• adafruit_si4713.mpy
• adafruit_bus_device

Before continuing make sure your board's lib folder or root filesystem has the adafruit_si4713.mpy, and adafruit_bus_device files and folders copied over.

Next connect to the board's serial REPL so you are at the CircuitPython >>> prompt.

Python Installation of SI4713 Library

You'll need to install the Adafruit_Blinka library that provides the CircuitPython support in Python. This may also require enabling I2C on your platform and verifying you are running Python 3. Since each platform is a little different, and Linux changes often, please visit the CircuitPython on Linux guide to get your computer ready!

Once that's done, from your command line run the following command:

• sudo pip3 install adafruit-circuitpython-si4713

If your default Python is version 3 you may need to run 'pip' instead. Just make sure you aren't trying to use CircuitPython on Python 2.x, it isn't supported!

CircuitPython & Python Usage

To demonstrate the usage of the sensor we'll initialize it and control the transmitter from the board's Python REPL.  Run the following code to import the necessary modules and initialize the I2C connection with the sensor:

import board
import busio
import digitalio
import adafruit_si4713
i2c = busio.I2C(board.SCL, board.SDA)
si_reset = digitalio.DigitalInOut(board.D5)
si4713 = adafruit_si4713.SI4713(i2c, reset=si_reset, timeout_s=0.5)

import board
import busio
import digitalio
import adafruit_si4713
i2c = busio.I2C(board.SCL, board.SDA)
si_reset = digitalio.DigitalInOut(board.D5)
si4713 = adafruit_si4713.SI4713(i2c, reset=si_reset, timeout_s=0.5)

Frequency Strength Scan

One interesting thing you can do with the Si4713 is measure the quality of an FM radio band.  This is handy for example to 'scan' the entire range of FM frequencies looking for possible radio station broadcasts (i.e. frequencies with a good quality signal).  This can help you find an unused frequency band to use for your transmitting.

The received_noise_level function can be called with a frequency (specified in kilohertz and only at 50khz steps) and will return the noise level, or signal quality, in dBuV units.  The radio supports a range of frequencies from 87.5mhz to 108mhz and you can scan them all with this code:

for f_khz in range(87500, 108000, 50):
    noise = si4713.received_noise_level(f_khz)
    print('{0:0.3f} mhz = {1} dBuV'.format(f_khz/1000.0, noise))

for f_khz in range(87500, 108000, 50):
    noise = si4713.received_noise_level(f_khz)
    print('{0:0.3f} mhz = {1} dBuV'.format(f_khz/1000.0, noise))

The higher the dBuV noise value the stronger the signal and better chance there's a real station broadcasting there.  The exact values will differ based on your area and the nearby stations but in general a small value under 32 is probably unused, and a large value above 40 is a strong radio signal.  Notice in the screen shot above 107.7mhz has a strong signal with noise value of 45 dBuV, this makes sense because a nearby FM station, Seattle's 107.7mhz FM The End, is broadcasting at that frequency.  However at 107.35mhz the signal has a much smaller noise value of 29 and likely indicates an unused frequency.

Transmitting

Once you find a frequency that's unused you can configure the transmitter to broadcast its audio input on it with the tx_frequency_khz property.  Set this to a value in kilohertz, again only within the range 87.5mhz - 108mhz and at 50khz steps, to change the transmitter frequency.  For example to use 107.35mhz:

si4713.tx_frequency_khz = 107350

si4713.tx_frequency_khz = 107350

You can also set the transmitter power with the tx_power property.  Set this to a dBuV value from 88 - 115, or set 0 to turn off the transmitter entirely.  You typically want the maximum power because these low power FM transmitters don't have much range by design.  For example to set the maximum 115 dBuV transmit power:

si4713.tx_power = 115

si4713.tx_power = 115

At this point the Si4713 should be transmitting anything sent to the audio jack or LIN & RIN inputs over the configured FM frequency.  Plug in an audio source and try tuning an FM radio nearby to hear the signal!  Remember the range of these low power FM transmitters is limited and you might need the radio in the very same room or close by to pick up the signal.

You might need to increase or decrease the volume of your audio source to ensure it's at a high enough level for the transmitter to pick up and send (or you might need to turn it down if it's too high and 'overmodulating' the FM signal).  You can actually check with the chip to see if it's getting a good audio signal and potentially overmodulating or running into other transmission issues.  Simply read the input_level property to see the audio level (in dB) and the audio_signal_status property to check if there are problems with the signal:

print('Audio level: {0} dB'.format(si4713.input_level))
print('Audio signal status: 0x{0:02x}'.format(si4713.audio_signal_status))

print('Audio level: {0} dB'.format(si4713.input_level))
print('Audio signal status: 0x{0:02x}'.format(si4713.audio_signal_status))

Notice the input level is around -16 dB which is a typical value for audio (lower values mean less volume / input level and higher values up to 0 or more mean very loud input).  The audio signal status is a byte that has a few bits to indicate status, in particular the 3rd bit will be turned on to indicate overmodulation (an easy way to check is if the value of the signal status is 4 or greater, that indicates bit 3 is on).

Try cranking up the audio source volume and notice the audio level value increases and the overmodulation bit turns on to indicate the volume is too high (remember a value 4 or greater means the input is overmodulating):

You should aim to keep the input level around -20 to -10 dB and ensure the overmodulation bit isn't set (if it is then the volume is too high).

RDS - Radio Data System

Finally the RDS features of the Si4713 allow you to transmit data along with the FM audio signal.  This is transferred in special side channels to the main FM broadcast and include data like the name of the station and the currently playing song.  If you've ever seen a car radio that prints the name of a song being played it's probably reading it over an RDS transmission from the radio station.

You can easily configure and set the RDS transmission of the Si4713 with the configure_rds function.  For example:

si4713.configure_rds(0xADAF, b'AdaFruit Radio', b'AdaFruit g0th Radio!')

si4713.configure_rds(0xADAF, b'AdaFruit Radio', b'AdaFruit g0th Radio!')

This call takes the following parameters:

• The station ID - This is a 16-bit value that the tuner can use to identify a station.  In this case we're sending the value 0xADAF.
• The station name - This is an optional keyword argument that can specify a byte string with the station name to broadcast.  You can only send up to 96 characters with this value.
• The buffer value - This is another optional keyword argument that can specify a byte string to broadcast as the current RDS buffer (typically shown as the song or now playing string on a radio).  You're limited to 106 characters with this value.

Once you call configure_rds it will enable the RDS broadcast and configure itself to do so at typical North American RDS radio broadcast values (i.e. a specific frequency deviation, repeat interval, etc.).  After RDS is configured you can update the station and buffer values by writing to the rds_station and rds_buffer properties respectively.  Remember each is limited to a length of 96 characters and 106 characters respectively:

si4713.rds_station = b'Mosfet Jams'
si4713.rds_buffer = b'Purrfect tunes!'

si4713.rds_station = b'Mosfet Jams'
si4713.rds_buffer = b'Purrfect tunes!'

That's all there is to the basic Si4713 usage with CircuitPython!

Below is a complete demo that will configure the board for FM transmission at a specified frequency (see the FREQUENCY_KHZ variable at the top).  It will broadcast RDS data and periodically print audio input level and signal quality status.  In addition it also shows basic usage of the GPIO outputs of the Si4713 with the gpio_control and gpio_set functions--these aren't commonly used but are available if you need a couple small outputs from the board.  Save this as code.py on your board.

Full Example Code

# SPDX-FileCopyrightText: 2018 Tony DiCola for Adafruit Industries
# SPDX-License-Identifier: MIT

# Simple demo of using the SI4743 RDS FM transmitter.

import time

import board
import digitalio

import adafruit_si4713

# Specify the FM frequency to transmit on in kilohertz.  As the datasheet
# mentions you can only specify 50khz steps!
FREQUENCY_KHZ = 102300  # 102.300mhz

# Initialize I2C bus.
i2c = board.I2C()  # uses board.SCL and board.SDA
# i2c = board.STEMMA_I2C()  # For using the built-in STEMMA QT connector on a microcontroller

# Initialize SI4713.
# si4713 = adafruit_si4713.SI4713(i2c)

# Alternatively you can specify the I2C address of the device if it changed:
# si4713 = adafruit_si4713.SI4713(i2c, address=0x11)

# If you hooked up the reset line you should specify that too.  Make sure
# to pass in a DigitalInOut instance.  You will need the reset pin with the
# Raspberry Pi, and probably other devices:
si_reset = digitalio.DigitalInOut(board.D5)

print("initializing si4713 instance")
si4713 = adafruit_si4713.SI4713(i2c, reset=si_reset, timeout_s=0.5)
print("done")

# Measure the noise level for the transmit frequency (this assumes automatic
# antenna capacitance setting, but see below to adjust to a specific value).
noise = si4713.received_noise_level(FREQUENCY_KHZ)
# Alternatively measure with a specific frequency and antenna capacitance.
# This is not common but you can specify antenna capacitance as a value in pF
# from 0.25 to 47.75 (will use 0.25 steps internally).  If you aren't sure
# about this value, stick with the default automatic capacitance above!
# noise = si4713.received_noise_level(FREQUENCY_KHZ, 0.25)
print(f"Noise at {FREQUENCY_KHZ / 1000.0:0.3f} mhz: {noise} dBuV")

# Tune to transmit with 115 dBuV power (max) and automatic antenna tuning
# capacitance (default, what you probably want).
si4713.tx_frequency_khz = FREQUENCY_KHZ
si4713.tx_power = 115

# Configure RDS broadcast with program ID 0xADAF (a 16-bit value you specify).
# You can also set the broadcast station name (up to 96 bytes long) and
# broadcast buffer/song information (up to 106 bytes long).  Setting these is
# optional and you can later update them by setting the rds_station and
# rds_buffer property respectively.  Be sure to explicitly specify station
# and buffer as byte strings so the character encoding is clear.
si4713.configure_rds(0xADAF, station=b"AdaRadio", rds_buffer=b"Adafruit g0th Radio!")

# Print out some transmitter state:
print(f"Transmitting at {si4713.tx_frequency_khz / 1000.0:0.3f} mhz")
print(f"Transmitter power: {si4713.tx_power} dBuV")
print(f"Transmitter antenna capacitance: {si4713.tx_antenna_capacitance:0.2} pF")

# Set GPIO1 and GPIO2 to actively driven outputs.
si4713.gpio_control(gpio1=True, gpio2=True)

# Main loop will print input audio level and state and blink the GPIOs.
print("Broadcasting...")
while True:
    # Print input audio level and state.
    print(f"Input level: {si4713.input_level} dBfs")
    print(f"ASQ status: 0x{si4713.audio_signal_status:02x}")
    # 'Blink' GPIO1 and GPIO2 alternatively on and off.
    si4713.gpio_set(gpio1=True, gpio2=False)  # GPIO1 high, GPIO2 low
    time.sleep(0.5)
    si4713.gpio_set(gpio1=False, gpio2=True)  # GPIO1 low, GPIO2 high
    time.sleep(0.5)

# SPDX-FileCopyrightText: 2018 Tony DiCola for Adafruit Industries
# SPDX-License-Identifier: MIT

# Simple demo of using the SI4743 RDS FM transmitter.

import time

import board
import digitalio

import adafruit_si4713

# Specify the FM frequency to transmit on in kilohertz.  As the datasheet
# mentions you can only specify 50khz steps!
FREQUENCY_KHZ = 102300  # 102.300mhz

# Initialize I2C bus.
i2c = board.I2C()  # uses board.SCL and board.SDA
# i2c = board.STEMMA_I2C()  # For using the built-in STEMMA QT connector on a microcontroller

# Initialize SI4713.
# si4713 = adafruit_si4713.SI4713(i2c)

# Alternatively you can specify the I2C address of the device if it changed:
# si4713 = adafruit_si4713.SI4713(i2c, address=0x11)

# If you hooked up the reset line you should specify that too.  Make sure
# to pass in a DigitalInOut instance.  You will need the reset pin with the
# Raspberry Pi, and probably other devices:
si_reset = digitalio.DigitalInOut(board.D5)

print("initializing si4713 instance")
si4713 = adafruit_si4713.SI4713(i2c, reset=si_reset, timeout_s=0.5)
print("done")

# Measure the noise level for the transmit frequency (this assumes automatic
# antenna capacitance setting, but see below to adjust to a specific value).
noise = si4713.received_noise_level(FREQUENCY_KHZ)
# Alternatively measure with a specific frequency and antenna capacitance.
# This is not common but you can specify antenna capacitance as a value in pF
# from 0.25 to 47.75 (will use 0.25 steps internally).  If you aren't sure
# about this value, stick with the default automatic capacitance above!
# noise = si4713.received_noise_level(FREQUENCY_KHZ, 0.25)
print(f"Noise at {FREQUENCY_KHZ / 1000.0:0.3f} mhz: {noise} dBuV")

# Tune to transmit with 115 dBuV power (max) and automatic antenna tuning
# capacitance (default, what you probably want).
si4713.tx_frequency_khz = FREQUENCY_KHZ
si4713.tx_power = 115

# Configure RDS broadcast with program ID 0xADAF (a 16-bit value you specify).
# You can also set the broadcast station name (up to 96 bytes long) and
# broadcast buffer/song information (up to 106 bytes long).  Setting these is
# optional and you can later update them by setting the rds_station and
# rds_buffer property respectively.  Be sure to explicitly specify station
# and buffer as byte strings so the character encoding is clear.
si4713.configure_rds(0xADAF, station=b"AdaRadio", rds_buffer=b"Adafruit g0th Radio!")

# Print out some transmitter state:
print(f"Transmitting at {si4713.tx_frequency_khz / 1000.0:0.3f} mhz")
print(f"Transmitter power: {si4713.tx_power} dBuV")
print(f"Transmitter antenna capacitance: {si4713.tx_antenna_capacitance:0.2} pF")

# Set GPIO1 and GPIO2 to actively driven outputs.
si4713.gpio_control(gpio1=True, gpio2=True)

# Main loop will print input audio level and state and blink the GPIOs.
print("Broadcasting...")
while True:
    # Print input audio level and state.
    print(f"Input level: {si4713.input_level} dBfs")
    print(f"ASQ status: 0x{si4713.audio_signal_status:02x}")
    # 'Blink' GPIO1 and GPIO2 alternatively on and off.
    si4713.gpio_set(gpio1=True, gpio2=False)  # GPIO1 high, GPIO2 low
    time.sleep(0.5)
    si4713.gpio_set(gpio1=False, gpio2=True)  # GPIO1 low, GPIO2 high
    time.sleep(0.5)

Datasheets & Files

• Si4713 Datasheet (this does not include any software interfacing details)
• Si47xx Programming guide - contains all the nitty-gritty details on command data packets etc.
• Fritzing object in Adafruit Fritzing library
• EagleCAD PCB files in GitHub

Layout Print

Dimensions in Inches

Schematic