Mixxtape

Mixxtape — a cassette you can actually record on

A 2-layer PCB in the form factor of a compact cassette (99.5 × 63.5 mm) that slides into a real Norelco cassette case. The bare board is the object — no enclosure, no bezel.

An onboard MEMS microphone records three 4-minute tracks. Hold REC to record, release to stop. Pair it to any Bluetooth earbuds or speaker and press PLAY.

Cassette-shaped Bluetooth players exist. Nobody ships a cassette-shaped recorder — and recording is the part that made a mixtape a mixtape.

_{3D render from KiCad. The mask reads green here because that is the viewer's default — the board ships matte white, which is what makes the sharpie label work.}

_{Artwork mockup of the B-side design and the J-card insert. The board is the object; this shows where the artwork is going.}

How you use it

• Hold REC — records. Release stops. Recording always overwrites the current track from zero — recording is erasing, same as tape.
• PLAY — plays all three tracks in order. It's a tape side, not three voice memos.
• TRACK — cycles track 1 → 2 → 3.
• MODE (hold 2 s) — pairing. Hold the board against your speaker; it picks the closest device by signal strength. No phone, no app, no list.
• Plug it in and it auto-reconnects to the last thing it paired with.

Notable design features

• Snap-off write-protect tab. A mouse-bitten break-off PCB tab carries a trace to a GPIO. Snap it and the firmware refuses to record — permanently. Handing someone a tape becomes a ritual with a physical, irreversible step.
• The reels show tape position. Twelve RGB LEDs ring each routed-out reel window. During playback the left ring empties and the right ring fills, like tape transferring between reels. Red pulse tracks recording level.
• Matte white solder mask so you can sharpie the label on, with a ruled label block silkscreened where a cassette's paper label sits.
• No battery. USB-C only, and there is no charging circuit on the board at all. A cassette's promise is that you find it years later and it still works.
• Jig-programmed. No USB-UART on the board — USB-C is power only. Firmware loads through six pogo-pin pads via a programming jig.

Hardware summary

Every fitted part is stocked at JLCPCB. The project rule is a minimum of 5,000 in stock, so a run never stalls on one line item — with a single documented exception: the microphone sits at ~3,500, because no digital MEMS microphone at JLCPCB clears the bar, and the one that does is analogue and would wreck the audio path.

Passives are all JLCPCB basic-library parts: 10 µF 0805 ×2 (rail bulk), 10 µF 0402, 1 µF (the ESP32's EN delay), 100 nF ×12, 10 k ×7, 5.1 k ×2 (the USB-C CC pull-downs) and 100 Ω (microphone supply isolation).

Full detail, with live stock and pricing, is in the generated BOM — it is derived from the schematic, so it cannot drift out of step with it.

Project status

Design locked, schematic and firmware core underway. hardware/ holds a KiCad 9 project with a first-pass schematic (ERC-clean) and JLCPCB part libraries. firmware/ has the ESP-IDF skeleton plus the host-tested core — crash-safe slot manager, ADPCM codec, playback sequencer, reel display, pairing policy, bond table and record limiter — all proven without hardware, since there is no devkit. Target: a prototype run of 3–5 boards, then a full run of 20. What is left needs a board in hand: the SPI flash, I²S capture, the A2DP plumbing, LED output, and A2DP against real earbuds.

Documentation

Before ordering boards:

• Pre-production checklist — the case-fit test, open decisions, and what to re-quote
• Case-fit test sheet — print at 100%, cut out, put in a real cassette case

For people using one:

• Cassette insert template — double-sided J-card for the case, 1:1, print at 100% (side 1, side 2)
• Documentation index — everything below, organised
• Quick-start pictograms — wordless instructions, one printable sheet
• Manual — how to record, play, pair, and what the lights mean
• FAQ — including the honest limitations

For people building or hacking one:

• Project write-up — the pitch, design rationale, and open questions
• Agent handoff brief — the authoritative spec: locked decisions, GPIO map, behaviour spec, phase order
• Bill of materials — generated from the schematic with live JLCPCB prices and stock, so it cannot drift
• Line-out option — rejected; kept for the reasoning and the verified part numbers
• Prior art — research on cassette-form-factor projects
• Firmware plan — architecture, milestones, and hardware interface for the firmware agent

Prior art

• Mixtape Alpha (Open Music Labs with Jie Qi / MIT Media Lab, 2012) — a cassette-shaped ATmega328p synth; the bare-PCB-as-object philosophy comes straight from it.
• Mixxtape (Mixxim) — the commercial cassette-shaped Bluetooth player. The thing this project is not.