Stefan Wagner's Low-Cost Stereo FM Transmitter Is Driven by a WCH CH32V003 RISC-V Chip

Maker Stefan Wagner has put together a pocket-sized battery-powered FM transmitter, designed to accept analog audio on a 3.5mm hack and spit out a stereo FM signal on a user-selectable frequency.

"With the portable Li-ion battery powered Stereo FM Transmitter you can transmit audio of any kind," Wagner explains of his creation. "Simply plug your audio source into the 3.5mm audio jack, set the desired frequency, and then tune any standard FM radio receiver to receive the signal."

The pocket-sized device is built around what Wagner describes as "cost-effective components," including its central microcontroller: the WCH Electronics CH32V003 32-bit RISC-V microcontroller, one of a family of parts which launched in October last year for under 10¢ a piece. To this, Wagner has added a KTMicro KT0803 FM transmitter chip, a UMW TP4054 lithium ion battery management chip connected to a USB Type-C port for charging, and a 128×32 OLED display with three buttons for the user interface.

Said user interface allows the frequency of the transmission to be shifted up and down, to avoid existing radio channels, and to adjust the gain of the audio. Transmission power is limited, both for technical and legal reasons, while a simple wire antenna is used to cut down the bill of materials still further.

This isn't Wagner's first experiment with pocket-sized radio devices, though the last was built to receive signals rather than transmit them: back in July 2022 the maker showed off PocketRadio, an FM radio with integrated display and speaker built around a Microchip ATtiny402 or ATtiny412 microcontroller and an RDA Microelectronics RDA5807MP FM radio chip.

The project's design files, source code, and a 3D-printable case are all available on GitHub under the reciprocal Creative Commons Attribution-ShareAlike 3.0 Unported license.