# Calarin MagSafe geometry plan — 2026-04-12

## Planning basis
Use iPhone 15 Pro dimensions as the practical stand-in for review:
- phone body: `146.6 x 70.6 x 8.25 mm`
- MagSafe accessory ring reference commonly lands around:
  - outer diameter: `~56 mm`
  - inner opening: `~46 mm`

User intent:
- target future model behaviorally similar to iPhone Pro class
- attach to the back with MagSafe
- credit-card-ish footprint is acceptable
- v1 should be workable and iterated, not perfect

## Product-shape decision
Calarin should be a:
- flat vertical slab
- centered on the MagSafe zone
- visually more like a wallet / recorder badge than a square puck

This means:
- spend area before spending thickness
- battery should be oriented to preserve flatness
- `302540` is the right first geometry anchor

## Recommended v1 outer envelope
### Preferred envelope
- width: `55–58 mm`
- height: `80–90 mm`
- target total thickness: `9–10.5 mm` for a realistic v1

### Why this range
- narrow enough to sit comfortably on a Pro iPhone back
- tall enough to fit:
  - MagSafe attachment zone
  - battery-first internal stack
  - board / button / LED / mic routing
- feels closer to a wallet attachment than a random square block

### Harder but nicer stretch goal
- thickness: `<= 9 mm`

That probably requires:
- very disciplined stack layout
- very thin shell strategy
- avoiding accidental tall-component congestion

## Battery placement
### Primary battery orientation
- `302540` vertical orientation
- centered on the main body axis
- battery as the dominant flat component

That implies a battery footprint of roughly:
- `25 x 40 mm`
- with the `3 mm` axis contributing to thickness, not width/height

### Stretch battery orientation
- `302560` vertical orientation
- still viable if we decide we want more runtime and the extra height feels fine

### Fallback battery orientation
- `402030` centered as a bring-up battery only
- should not dictate final outer shape

## Rough stack logic
Current rough stack estimate:
- PCB: `~0.8 mm`
- ESP32-S3-WROOM module peak: `~3.1 mm`
- shell / adhesive / clearance / magnet stack budget: `~2–3 mm`
- battery thickness:
  - `302540`: `~3 mm`
  - `402030`: `~4 mm`

This means:
- truly matching naked-phone thickness is unrealistic for v1 unless layout gets unusually aggressive
- but a MagSafe accessory in the `9–10.5 mm` class is still coherent and product-plausible

## Layout recommendation
### Front / user-facing side
- button near upper-middle or upper-third
- LED ring integrated around button
- microphone port offset away from finger landing zone

### Internal mid-layer
- PCB shaped to avoid wasting area around MagSafe zone
- put tallest components where battery is not directly stacked if possible
- avoid stacking the thickest battery region directly under the tallest module region if the mechanical architecture can split them

### Rear / phone-facing side
- MagSafe magnetic structure centered to the phone axis
- soft-touch or thin protective rear surface to reduce wobble and keep the phone-contact side clean

## Camera / hand-feel guidance
- do not fight the camera bump by trying to occupy the entire phone back
- better to center Calarin below the top camera cluster and let it read like a wallet-height accessory
- preserve lower-half hand comfort over theoretical full-back coverage

## v1 geometry recommendation
If we want one clean geometry target for review, use:
- `56 mm` wide
- `86 mm` tall
- `~9.5–10 mm` realistic v1 thickness target
- internal battery target: `302540`

That is the best current balance of:
- phone-back comfort
- flatness
- battery realism
- prototyping simplicity

## ASCII sketch
Top-down back-of-phone concept (not to scale):

```text
+----------------------+
|  iPhone back         |
|  [camera bump]       |
|                      |
|      (MagSafe)       |
|    +------------+    |
|    |  Calarin   |    |
|    |  button    |    |
|    |   + LED    |    |
|    |            |    |
|    | 302540 batt|    |
|    |            |    |
|    +------------+    |
|                      |
+----------------------+
```

## Decision summary
Lock for review:
- geometry direction: vertical MagSafe slab
- preferred battery: `302540`
- stretch battery: `302560`
- fallback bring-up battery: `402030`
- target outer envelope for v1 review: `56 x 86 x ~9.5–10 mm`