Beanie, cameras and charging

The beanie

An off-the-shelf acrylic knit beanie with a turned-up cuff goes over the shell: the Oddjob Big Beanie in XXL, a hat for big heads, 64 cm around as made (Oddjob). The shell's hem is 775 mm around, so it stretches about 21 % and presses about 10 N on the shell (simulated, README § the beanie's cloth). An ordinary one-size beanie (a head of 56–58 cm) would stretch about 76 %. It hides the electronics and makes the helmet look like a hat. It does not hold the shell on: the pins push the shell off the head with far more force (150-430 N) than a knit holds (about 35 N). The headband (EXO-145) inside the shell does that (shell). The band is entirely under the shell, so the beanie, its hem and its cuff are as before. To loosen or tighten the band, lift the hem at the nape: the band's ratchet buckle is under the hub, inside the shell's rim. The cuff is a double layer about 6 mm proud of the body, tight enough to hold the hem against the head under the shell's rim. The hem runs at least 4 mm under the rim all round. Behind the ears, where the rim drops for the pod, the hem drops ahead of it so it never runs steeper than 1.6:1. The cuff stands 56 mm from the hem (hem_z in exo/shell_v1.py, exported to the 3D view).

In the 3D view the beanie is simulated as cloth: a knit sheet with stretch, shear and bend constraints, gravity and friction that settles onto the shell, the pod, a low swell over each camera and the head it's on, with its hem held to the hem line. The ribs, the crown's decreases and the folded cuff are built on the settled sheet. The sheet starts 4 mm off the layer it rests on, and each point is moved out past every shell part on its ray from the head's centre, the lift spread to its neighbours at 0.5 mm per mm, so it tents over the pod (it stands up to 18 mm out of the shell) instead of starting inside the back band: a cloth that starts inside a body it collides with holds its strain there (clear_shells in exo/cad_doc.py; 42 points were inside, 0 are, and the hem does not move).

Cameras

Five camera modules sit half-way up the cuff: three across the front (0° and ±72° from straight ahead) and two at ±122° on the slanted parts of the back rim, their long side along the slope. Each is a small board with an image sensor, a lens holder and barrel, a MEMS microphone, a status LED and a 12-way FPC to the hub. It clicks into a cradle on the shell (shell), and the lens comes through the knit in a grommet.

The lens grommet is the recording light (EXO-146). It is clear silicone, not black rubber: a ring round the lens on the outside of the knit and, under the knit, a 3 mm wide tab that reaches 8 mm to lie on the camera's status LED. A 45° face at the tab's end sends the LED's light along the tab into the ring, whose frosted outer face glows red while that camera's sensor or microphone is on (camera § the light seen from outside). A bystander sees a red ring round each lens that records; nothing else on the beanie lights.

Charging

USB-C, through a moulded eyelet in the back of the cuff, above a small woven label at the hem. The receptacle is on the hub's bottom edge; a tunnel runs straight down from it through the pod wall and opens low on the back of the shell, 11 mm above the hem (shell). The 3D view draws the eyelet half-way up the cuff. The hub holds the contacts' supply off while a cable is plugged in, so it charges only off the head.

Camera board

EXO-450, 19.0 × 17.0 mm: an RP2350A reads each camera's MJPEG frames and its microphone and sends both to the hub over the helmet's 12-pin FPC, which also carries its power, SYNC and a control UART. The camera has no radio; the hub's Wi-Fi is the helmet's one transmitter. See camera.