Assembly

Assembly is mechanical: you seat the phone in its corner, swap the plotter’s pen for the stylus, clamp the camera straight overhead, and run everything to a powered USB hub. No soldering, no custom boards — about an hour, mostly fiddling with the camera angle.

        camera (gooseneck, ~25 cm up, looking straight down)
            │
            ▼
   ┌─────────────────────────┐
   │  pen-plotter arm        │
   │  X/Y gantry             │
   │  Z = solenoid + stylus  │
   │                         │
   │   ┌───┐                 │   ◄── phone seated in the corner,
   │   │ ▓ │  phone          │       TOP-LEFT registered to the
   │   └───┘                 │       holder corner
   └─────────────────────────┘
        │            │
     USB serial   12 V power      camera ─USB─┐
        │            │                        │
        └──── powered USB hub ── USB ──────────┘──→ your Mac

The order matters a little: seat the phone first so the camera has something real to aim at, and wire last so nothing tugs as you position parts.

1. Seat the phone — corner first. Lay the anti-slip pad on the plotter bed and set the two L-shaped blocks to form a corner. Place the phone face-up and push it firmly into that corner so its top-left corner registers against the holder’s corner. This one detail is what makes calibration reusable: the phone returns to the exact same spot every session, so the arm’s learned screen position stays valid.

2. Fit the stylus in the Z holder. Remove the plotter’s pen and slide the capacitive stylus into the Z carriage holder. Set its height so the soft tip sits just above the glass when the solenoid is released, and presses the glass when it fires. The tip should touch flat, not at an angle.

3. Hang the camera straight overhead. Clamp the gooseneck to the desk and bend it so the camera looks straight down at the screen center, about 25 cm up. Frame the whole screen with a little margin. Straight-down matters: the agent reads on-screen positions from this image, so a tilted view skews where it thinks things are.

4. Wire everything to the powered USB hub. Plug the camera and the control board into the powered USB 3.0 hub, and the hub into your Mac. A powered hub keeps the board and camera from browning out a laptop port. Run the phone’s screenshot bridge over the same hub if you use a wired connection.

5. Power the control board. Give the control board its two connections: USB to the hub (data, G-code) and the 12 V barrel jack (motor and solenoid power). The board’s LED should light on 12 V. Power 12 V first, then USB, matching the firmware flashing order.

Rigidity is everything — a bumped gooseneck means re-calibration

The single most common cause of misaimed taps is camera drift. Flex in the gooseneck, a nudged clamp, or a phone reseated even slightly off-corner all move the screen out from under the positions the arm learned. Tighten every clamp, keep the gooseneck short, and treat any bump as a reason to re-calibrate.