A Digital Audio Player Hidden Inside a Cassette Tape

During the transition period from analog to digital audio, cassette adapters were used to play music from a portable CD or MP3 player through an old car stereo. Jonathan Rowny took that idea a few steps further with a device that eliminates the external player entirely. He built a complete battery-powered MP3 player inside a cassette shell, but instead of having its own headphone jack or speaker, it can only play music when inserted into a working tape deck.

The basic audio interface works much like those old cassette adapters. Rather than using magnetic tape, Rowny salvaged a stereo tape head from an inexpensive adapter and uses it in reverse. Audio from a PCM5102A DAC drives the coils in the head, producing a changing magnetic field. When the cassette is inserted, the tape deck's playback head picks up that field and treats it just as it would the signal recorded on magnetic tape.

The rest of the player is built around a Seeed Studio XIAO ESP32S3 development board, paired with the DAC and a microSD card for storing music. Power comes from a 600 mAh LiPo battery, which Rowny says provides around 10 hours of continuous playback. A physical button can also put the ESP32-S3 into deep sleep, allowing the battery to last for months while the player is sitting unused.

Before the MP3 player can do its job, it has to determine when someone has pressed Play on the tape deck. Rowny created a three-gear mechanism connecting the cassette's two take-up sprockets. The center gear has a tiny 1.7 mm hexagonal shaft that engages a 2-mm-thick rotary encoder mounted to the PCB. When the deck starts turning the cassette mechanism, the encoder detects the motion and tells the ESP32-S3 to begin playing.

Parts that small are not easy to manufacture. FDM printing worked for the cassette shell, but couldn't reliably reproduce the small hexagonal encoder shaft. Rowny eventually resin-printed the gears at a 30-micron layer height, producing parts precise enough to mesh smoothly without binding.

Space was tight enough that Rowny designed a custom two-layer PCB just 1 mm thick rather than using the more common 1.6 mm FR4. The board carries the ESP32-S3, DAC, microSD slot, encoder, and supporting components while keeping the analog audio traces away from noisier digital circuitry. Even the battery connectors had to be selected with the moving gears in mind.

The finished cassette has a few features no tape from the 1980s could offer. Its ESP32-S3 hosts a Wi-Fi web interface that can be opened from a phone or computer to upload MP3s and manage tracks, complete with metadata and album art. But once the music is loaded, operating it is appropriately old-fashioned: drop the cassette into a deck, press Play, and let the spinning gears bring the digital player to life.