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Hardware: 3D Printed Case & Mounting

Case Design

The OpenSCAD file case.scad generates a snap-fit case that holds:

  • Raspberry Pi (3B+/4 or Zero 2W — configurable)
  • 2-channel relay module

Both components mount on M2.5 standoffs inside the case.

Features

  • Snap-fit lid (no screws needed for the case itself)
  • Ventilation slots on both sides
  • 3 cable exit holes: USB power (left), bike button wires (back), speed sensor (back)
  • No supports needed for printing
  • Parametric — adjust wall thickness, standoff height, tolerances etc. in the config section

How to Use

  1. Install OpenSCAD: https://openscad.org/ (free)
  2. Open case.scad
  3. Configure:
    • Set rpi_model to "3" (RPi 3B+/4) or "zero" (RPi Zero 2W)
    • Set part to "base", "lid", or "both" (exploded preview)
  4. Render (F6) and Export STL (F7)
  5. Print base and lid as separate STL exports

Print Settings

Setting Value
Layer height 0.2mm
Infill 15-20%
Supports None needed
Material PLA or PETG
Orientation Flat (open side up for base, flat side down for lid)

Approximate Dimensions

RPi Model Case size (W x D x H)
RPi 3B+/4 ~150 x 64 x 34 mm
RPi Zero 2W ~130 x 46 x 34 mm

Component Mounting Dimensions

Reference for designing your own case or modifying the OpenSCAD file.

Raspberry Pi 3B+/4

PCB: 85 x 56 mm
Mounting holes: M2.5, 4 corners

    3.5mm    58mm
    ├──┤├───────────┤
    ●───────────────● ─┬─ 3.5mm
    │               │  │
    │   Raspberry   │  49mm
    │     Pi 3/4    │  │
    │               │  │
    ●───────────────● ─┴─ 3.5mm

    Hole positions from bottom-left:
    (3.5, 3.5)  (61.5, 3.5)
    (3.5, 52.5) (61.5, 52.5)

Raspberry Pi Zero 2W

PCB: 65 x 30 mm
Mounting holes: M2.5, 4 corners

    3.5mm    58mm
    ├──┤├───────────┤
    ●───────────────● ─┬─ 3.5mm
    │  RPi Zero 2W  │  23mm
    ●───────────────● ─┴─ 3.5mm

    Hole positions from bottom-left:
    (3.5, 3.5)  (61.5, 3.5)
    (3.5, 26.5) (61.5, 26.5)

2-Channel Relay Module (typical)

PCB: ~50 x 38 mm
Mounting holes: M3, 4 corners (varies by manufacturer — measure yours!)

    3.5mm   43mm
    ├──┤├─────────┤
    ●─────────────● ─┬─ 3.5mm
    │             │  │
    │   [Relay1]  │  31mm
    │   [Relay2]  │  │
    │             │  │
    ●─────────────● ─┴─ 3.5mm

    Screw terminals along one long edge
    Signal pins (IN1, IN2, VCC, GND) along the other

Note: Relay module dimensions vary by manufacturer. Measure your actual module and update the relay_pcb and relay_holes values in case.scad if needed.

A3144 Hall Effect Sensor

The sensor is tiny (4 x 3 x 1.5mm) and mounts on the bike frame near the flywheel, not inside the case. Route its 3 wires through the back cable hole into the case.

Assembly Inside the Case

Top-down view (lid removed):

┌─────────────────────────────────────────────────┐
│                                                 │
│  ┌──────────────────┐    ┌───────────────┐      │
│  │                  │    │ ○ Relay 1     │ ←cables
│  │   Raspberry Pi   │    │ ○ Relay 2     │  out
│  │                  │    │               │      │
│  │              USB→│    │  IN1 IN2 V G  │      │
│  └──────────────────┘    └───────────────┘      │
│  ↑                                              │
│  USB power in                                   │
└─────────────────────────────────────────────────┘

Dupont wires connect RPi GPIO header to relay module pins.
No breadboard needed — wires route directly between the two boards.

Speed Sensor Mounting

The A3144 and magnet mount on the bike itself, not in the case:

  • Magnet: Any magnet, attached to the flywheel rim with hot glue, tape, or epoxy
  • Sensor: Mount on the bike frame 2-8mm from the magnet path. Hot glue, cable tie, or a small 3D printed bracket

Optional: Sensor Bracket

A simple bracket can be printed to hold the A3144 at the right distance from the flywheel. The design depends on your bike's frame shape — a small L-bracket with a slot for adjusting the gap distance works well. If you want, you can add a parametric bracket to this folder.

Bill of Fasteners

Fastener Qty Purpose
M2.5 x 6mm screw 4 Mount RPi to standoffs
M2.5 x 6mm screw 4 Mount relay module to standoffs (or M3 depending on your module)
M2.5 nut (optional) 4 If standoffs aren't self-tapping

If you prefer not to use screws, friction fit with slightly taller standoffs (add 1mm) works for a case that won't be moved often.