Our own circuit board, shaped to the stump

All 48 parts and where they sit

Height is in mm above the grass. Across is mm from the stump’s centre line. Side A faces the window and the heat bar; side B faces the back.

1. The shape: one board, folded into the stump

Unfolded, the board is a spine of five rigid and flexible sections:

Why this shape:

• The bore is 31.75 mm and the carrier’s core is 28.4 mm, so the brain is 28 mm wide on the stump’s centre line, with room for chips on both sides.
• The heat bar runs z 477–557, so the antenna end (z 558–570) sits above it, where the glass-fibre tube passes radio.
• The cells stay low in the stump, and the tail flex reaches them.
• The disc lies on the base, where the charge pads and the tamper pocket are.

2. What is on the brain

Every board starts from the microSD card (owner, 4 Nov 2026): put the card in, switch on, and it starts our program. How the design makes sure of it:

• The RV1126B tries its start devices in a fixed order: SPI flash, then eMMC, then the card, then USB. An image on the eMMC would start before the card, so the eMMC ships empty and the loader on our card is set to read the card first. The eMMC is kept for recordings and logs.
• The card has its own power switch (U12), so the processor can restart a stuck card without restarting the stump. It also has ESD protection (U13) and 22 Ω series resistors, because the slot is the one place a person touches the electronics.
• The lid is hinged and latched, so a ball hit cannot eject the card. The card is an industrial pSLC one (Kingston Industrial or Swissbit), rated −40 to 85 °C.
• The card holds a read-only system with a separate data partition, so a power cut cannot damage the system. Updates over Wi-Fi write the second system copy, and a bad update rolls back by itself.
• On the card: the start loader at sector 64, U-Boot at sector 16384, then the system and data partitions. Writing a card is one step on a PC.
• Signed calls do not depend on the card: the key stays in the security chip (U8), and the card’s system is signed.

3. The camera, built on the board

• The OmniVision OV9281 global-shutter camera chip (5.24 × 4.46 mm, datasheet v1.22) is soldered on the head section, in the same slot the CAD kept for the camera board (24 × 25 mm at 662 mm).
• An M8 lens holder with a Ø7 lens goes over it. An M12 lens would not pass the head’s Ø9 window bore. The lens is chosen by the camera-view simulations for the 12° view from 662 mm.
• The camera’s 2.8, 1.8 and 1.2 V regulators and the 24 MHz clock sit on the back of the head.
• The camera’s signal reaches the RV1126B through the board’s own flex section. There is no camera cable to fit, reverse or shake loose, which removes the failure that overheated our bench board on 3 Nov 2026.

4. The battery system, built on the board

Power budget (rev T, board review):

• The cells: three 3.4 Ah LiFePO4 26650 cells (1S3P, the third bay of the cell carrier).
• A 10-hour day: 8 h of play (700 balls) and 2 h of breaks uses about 13 Wh. That is 2.8 W in each delivery, 1.3 W between balls (camera at 60 fps, AI engine asleep) and 0.55 W in breaks.
• Run time, worst case (aged cells at 5 °C): 13.8 h.
• Why three cells: two A123 cells gave only 6.1 h worst case and 9.1 h new.

The dev kit and rev A measurements replace these estimates.

5. Heat

• The RV1126B draws about 1.5–2.5 W at full camera and AI load (est.).
• Its heat path: a 1 mm thermal pad, then the aluminium heat bar (z 477–557), then the tube wall.
• No bought heatsinks: the pad and the bar are in the CAD.
• Rev A logs its temperature from the first power-on.

6. Factory test (every board, every time)

1. Current-limited first power-on (100 mA). A short trips in about 1 s, before anything heats.
2. Every power rail within ±3 %, and the charger at its set voltage into a test load.
3. A factory card goes in and the board starts from it, with the eMMC left empty. The card power switch is tested by cutting and restoring the card supply.
4. Camera frame from a test chart (focus, exposure, frame rate). Then Wi-Fi, the motion sensor, the lights read by a light sensor, both bail sensors with a magnet, and the tamper switch.
5. Cell protection trips at low voltage and on a short; the gauge and thermistor read correctly.
6. The unit’s own key goes into the security chip, the ID chip is written and the label is printed. Every result is stored against the serial number.

7. Who makes what

8. Cost per stump, electronics (est.)

Rev A is dear because a few rigid-flex boards cost a lot to make. From about 1,000 boards the single board is cheaper than any bought-board build, and it needs no hand assembly.

9. Timeline

10. CAD check (rev T, 3 Nov 2026)

The board is placed in the stump CAD (products/cricketone/cad/cricketone-t-cad.mjs). That script loads every rev S printed part and every bought part that stays, puts the folded board in all three stump types (middle, leg, off), and checks every board part against all of them:

• No board part touches a bought part: cells, heat bar, window, light strips, button, pads, vent.
• Where the board needs room in a printed part, a 0.3 mm clearance pocket is cut:

• Rev T changes found by the check:
  1. The base’s spring pins move from x −14.2 to x −11.8, onto the disc’s charge pads.
  2. The Hall-tamper post goes; the board’s tamper switch does that job.
  3. The heat bar shortens to 75 mm (477–552 mm), so no metal is within 5 mm of the antenna. It is bonded to the processor with a thermal adhesive pad.
  4. The bar’s old screw holders, which the brain cut loose, are gone. The brain is held in card guides.
  5. A sounder port goes through the top carrier and sleeve.

11. Still to check

1. The RV1126B-P’s power rails, from Rockchip’s hardware design guide (the Boardcon module runs it from a single 3.3 V input, which this design follows).
2. The camera driver for the OV9281 in Rockchip’s SDK, on the development kit.
3. The LiFePO4 protector chip and its exact cut-off voltages.
4. The rigid-flex maker’s limits for the 453 mm tail.
5. The card-guide hold of the brain under the drop and ball-hit tests (rev T).
6. The antenna’s range with the aluminium collar 2.6 mm above it (RF test).

Sources

• OV9281 datasheet v1.22 (package size)
• TI BQ25070 LiFePO4 charger
• Boardcon CM1126B-P module manual (single 3.3 V input)
• JLCPCB flex PCB capabilities
• RV1126B modules for comparison: Boardcon Tiny1126B
• Parts audit and the bench camera short: the first stump, section 9