I2C FM-Transmitter Board For Sending Audio Input To Car Radios

HW-158 is a Wide-FM transmitter that can send audio input to a radio frequency in the range of public broadcasting 70-116MHz.

It is controlled by a I2C interface, uses the latest KT0803L “Radio-Station-on-a-Chip” and requires an external microcontroller to operate.

Transmitting information on public broadcast bands typically prohibited and requires a broadcast license. Some countries (i.e. US) allow transmissions as long as the transmitted distance is low.

Boards like HW-158 take this into account and emit with very low RF power sufficient for distances of just a few meters. With these boards, external audio sources can i.e. be fed into car radios that have no auxiliary input:

An external audio source is connected via 3.5mm cinch plug into the HW-158 module. The module then sends the audio to a preset VHF radio frequency that can be received by any FM radio set in close vicinity as if it were a public broadcasting radio station.

Always make sure whether operating this board is legal in your country before you power it on. There may be substantial fines.


The board comes with four solder pads labeled on the front.

Pin Description
GND negative voltage
VCC 3.0-5.0V
SDA I2C Interface
SCL I2C Interface

Technical Data

The actual transmitter chip used by the board requires a voltage of 1.6-3.6V.

The breakout board adds a 662K (XC6206P332MR) 3.3V voltage regulator that takes 1.8-6V and converts it to 3.3V.

Item Description
Voltage 1.8-6V
Operational Current <17mA
Standby Current <3uA
Frequency 70-108MHz
RF Power max 1.3mW
Interface I2C, 5V tolerant
I2C Address 0x0111110
Modulation Wide-FM
Chip KT0803L from KTMicro
Features 20bit Audio ADC, DSP Core, 24dB PGA
Size 27.7x26.4x7mm


The chip that drives this board is the KT0803L from KTMicro.

The picture below shows the main chip, and close to the VCC connector, you see the 662K 3.3V voltage regulator that takes the 1.6-6.0V input voltage and converts it to the 3.3V required by the chip:

KT0803L is a hardware-compatible enhanced version of KT0803K and KT0803M which in turn base on the KT0801.

These are the added features found in KT0803L over its predecessors:

  • Software Standby
  • Automatic Amplifier Power-Down On Silence
  • ALC (Automatic Level Control)
  • Better SNR (Stereo 66dB)
  • Improved Audio Frequency Response
  • Software-controlled XTAL selection

Software Libraries

Here are a few software libraries on github that produce the I2C control messages required to set up the transceiver chip:


Data Sheet

KT0803K Radio-Station-on-a-Chip
KT0803L Radio-Station-on-a-Chip
662K 3.3V Voltage Regulator


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(content created Apr 19, 2024)