Jonathan Rowny Built an MP3 Player That Only Plays on Tape Decks

Jonathan Rowny has been feeling nostalgic again, and this time the answer is a cassette that holds no magnetic tape. His player looks like a mixtape, fits a standard deck, and stays silent until the mechanism starts to turn. Songs live on a microSD card inside the shell. Sound leaves through a real tape head pressed against the deck’s own head, so the old machine treats digital files as if they were oxide sliding past at walking speed.

Commercial adapters from the early days of MP3s demonstrated that circuitry could be snuck into a plastic casing like that. Some were as simple as a 3.5 mm jack on a plate, while others had a full player or a Bluetooth module and managed to write analog music on the cassette using a spare head. Rowny had seen Clint’s Lazy Game Reviews (LGR) teardowns and decided to create something tighter, utilizing a bespoke board, a phone app, and some printed gears to keep the deck from shutting itself off.

The hardware starts with a Seeed Xiao ESP32-S3 module, which costs approximately $5 and includes Wi-Fi, Bluetooth, and a battery charger. The PCM5102A converter converts your digital files into a lovely, silent analog pair. That pair then feeds a stereo cassette head, which was chosen after two mono cassettes failed. As with a good adaptor, a spring-loaded retainer presses the head on the deck’s head. A 600 milliamp-hour battery powers the circuit for more than ten hours, and the files are stored on a microSD card, which is inserted into a special board from NextPCB that somehow fits all of the modules inside that thin cassette envelope.

The easy part is getting the audio into the deck; the tough part is keeping it happy. Auto-stop and reverse watch the hubs; however, the hubs still appear to be a blank piece of tape at the end, so the machine assumes it has finished with the tape and turns off. Rowny came up with a solution by printing a three-gear train on a SLA printer, allowing the take-up hub to drive the supply hub and maintain both sides of the tape rolling. There’s also a 2mm-tall Alps rotary encoder on the central shaft that tells the ESP32 if everything is spinning or not. Playback begins as it moves; when it stops, the machine does as well.

Resin printing was important since the 1.7mm hexagonal shaft required a surface with precisely flat, fused faces, which Fila couldn’t provide. PLA handled the rest of the shell after that. The first gears would bind severely until the middle wheel shrank by a full two tenths of a millimeter, which was not ideal. The gears were then given a dry lubrication on their teeth. One minor wiring issue with the SD card’s chip-select pin necessitated a little solder bridge before the card would respond, and to make matters worse, the encoder pins were placed on the incorrect wake lines, so the chip still cannot sleep when the deck comes to a halt. Just as he was finishing up the project, a somewhat larger gap appeared.

The capstan and pinch roller, rather than the hubs, govern the tape’s speed. The hubs are simply there to accelerate and slow things down when the tape pack diameter varies. However, the capstan speed must remain constant at around 47.6 millimeters per second. Fortunately, factory adapters that function properly are built to measure that exact shaft, but Rowny’s encoder is still configured to watch the wrong one, so fast-forwarding is out, and the player can’t copy wow and flutter yet. The next version is designed to read the capstan instead.

An app can now communicate with the ESP32 via radio, allowing playlists to be changed without opening the shell, although sleep remains out of the question until a new board revision is released. Despite this, the finished cassette is loaded into a functional deck, the gears mesh perfectly, the two heads align, and files that have never touched magnetic tape flow out of headphones that haven’t seen action since the good old hiss and mixtape days.
[Source]