Engineering Concept · Rev D
Groove — Fridge-Mount Vinyl Player
A 44 mm novelty record that clings to the fridge door and plays back a message on tap or button-press. No moving turntable parts — just a magnet, a speaker, and a record sleeve's worth of styling.
01Renders
The last two are per-part renders of the actual STL files, not the source CAD — a print/assembly reference for hand-assembling a prototype: print both shells separately, seat the electronics stack on the rear shell's standoff pegs, then press the front shell on until its alignment pegs seat in the rear shell's sockets. No glue or fasteners in the base design; friction-fit pegs hold the two halves together.
02Key specs
03Playback modes
No spinning platter — audio is either tapped onto the device via a phone, or fired back with a physical press. Two independent triggers keep the passive NFC side battery-free.
NFC tap
A companion app pushes a short audio clip over NFC induction into onboard flash. Fully passive — no battery draw, works even at 0% charge.
Side button
A tonearm-styled lever on the rim wakes the MCU, decodes the stored clip, and drives the edge-ported speaker. Battery-powered, ~6 s per play.
04Electronics block diagram
05Magnet & power budget
Magnet holding force
CR1620 battery life
06Bill of materials (illustrative, per unit)
| Component | Spec | Cost |
|---|---|---|
| Shell set (front + rear) | 2-shot injection molded ABS/PC | $0.50 |
| Ring magnet | N42 neodymium, 26×10×1.8mm | $0.15 |
| MCU + audio codec | QFN, onboard 128MB flash | $0.55 |
| NFC tag IC | NTAG213-class, etched antenna | $0.22 |
| Speaker | 10mm, 8Ω, 0.3W micro-speaker | $0.20 |
| Battery + holder | CR1620 | $0.18 |
| Side button | Tactile switch, IP-sealed boot | $0.06 |
| Status LEDs ×8 | 0603 amber | $0.14 |
| PCB | Rigid, 2-layer, ⌀30mm | $0.28 |
| Gasket + bumper feet | Silicone | $0.07 |
| Retail packaging | Printed sleeve + insert | $0.25 |
| Estimated BOM total | $2.60 | |
07Product break-even & margin (no marketing)
What the physical product itself needs to charge, independent of customer acquisition — the question "is this product economical to build and ship" answered on its own, before asking "can we afford to sell it."
| Channel | Break-even (0%) | Price @ 15% | Price @ 20% | Contribution @ 20% |
|---|---|---|---|---|
| Pre-order | $6.20 | $7.29 | $7.75 | $1.55 |
| Retail / wholesale | $6.85 | $8.06 | $8.56 | $1.71 |
On the product alone, Groove is fine — it breaks even around $6–7 and clears a healthy percentage margin well under $9. This sheet says nothing about whether you can afford to find a customer at that price, which is a separate question — see below.
08Marketing fund coverage (separate from product margin)
A fixed customer-acquisition cost per unit (M), checked against the contribution margin the product sheet actually leaves available — not blended into the same price calculation, because they answer different questions and a target hit on one says nothing about the other.
| Channel | Contribution @ 20% price | CAC (M) | Gap | Fully-loaded price to still hit 20% after M |
|---|---|---|---|---|
| Pre-order | $1.55 | $6.00 | −$4.45 | $15.25 |
| Retail DTC | $1.71 | $70.00 | −$68.29 | $96.06 |
The gap is the point of keeping these separate. At the $7.75 price the product sheet says is fine, neither channel's marketing cost is covered by the contribution margin available — not close, in the DTC case. Closing that gap means either raising price to the "fully-loaded" column, cutting CAC, or treating marketing as a subsidized growth investment rather than something each unit's margin pays for on its own — a real, common early-stage choice, but a deliberate one to make, not something a blended formula should paper over.
09Manufacturing notes
Shells split on the seam between the label face and the rim so both halves demold with straight pulls — the button boss is the only side-action, perpendicular to the main pull. Groove texture and label recess sit entirely on the front shell's outer face, so a single fine-texture core insert carries all of the cosmetic detail. Dropping the USB-C port removes its cutout and flap entirely, simplifying the seal to a single silicone gasket around the perimeter for the IP54 rating — the battery is swapped by twisting the shells apart rather than through a service port.
10Project files
- openscad/fridge_vinyl_player.scadParametric source — VIEW="assembled" | "exploded" | "cutaway" | "front_only" | "rear_only"
- openscad/renders/front_shell.stlPrintable front shell
- openscad/renders/rear_shell.stlPrintable rear shell
- openscad/renders/*.pngAssembled, exploded, cutaway, and standalone front/rear shell renders