A round bar scale that reads the bottle as well as weighs it

One 100 mm PCB — the diameter of the largest common spirits bottle base — carrying an ESP32-S3, a 24-bit bridge ADC, USB-C charging, a LiPo and a camera that looks straight up through the glass. A 10 kg load cell bolts on, a tempered glass disc goes on top, and you have a scale that knows which bottle is standing on it: show a barcode or QR label to the window in the sticker and it decodes it locally, no display and no scanner gun.

What is where

Numbers on the render match the table. Coordinates are millimetres from the top-left of the board's bounding box, y downwards, centre (50, 50). Generated from placement.json.

The empty band across the middle is not wasted space — the load cell bolts down there, and the two M4 holes take the fixed end. Everything else is packed into the two half-discs it leaves.

How it weighs

A TAL220B at 2 mV/V on 3.3 V excitation puts 6.6 mV across the ADC at 10 kg. The HX711 at gain 128 spans ±20 mV, so full load uses about a third of the range and the 24-bit converter still leaves roughly 0.1 g of noise-free resolution at 10 SPS. Grams, in other words, are an averaging and mechanics problem here — not a converter problem.

Two details do the real work. First, the ADC reference is AVDD and the bridge excitation comes from that same node, so the measurement is ratiometric: rail drift cancels instead of arriving as grams. Second, the excitation is switched by a P-FET, because a 1 kΩ bridge sitting across 3.3 V draws 3.3 mA — more than everything else on the board draws asleep.

Blocks — what each part is doing here

Against the obvious alternatives

The closest published thing to this board is SparkFun's OpenScale — an ATmega328P, an HX711 and a temperature sensor on a breakout that speaks serial. It is a good design, and it is the reason there is a temperature sensor on this board at all. The gap is everything after the measurement: OpenScale hands you grams over a USB cable and stops there, and the cheap HX711 module kits do not even do that much.

Where the difference lands: this is an appliance, not a component. It ships as the platform a bottle stands on, it knows which bottle that is, it runs on its own battery and it reports over the network — so a bar gets pour-level inventory instead of a number on a serial terminal.

Mechanical stack

The load path is glass → stud → cell live end → cell → fixed end → spacers → PCB → base ring → feet. The board carries the full 10 kg in bending, which is why it is 1.6 mm rigid with a 60 × 20 × 3 mm aluminium stiffener under the fixed end: FR4 flex shows up directly as non-linearity.

• The sticker printed on the underside of the glass carries the branding and leaves a 30 mm clear circle over the camera plus a small hole over the status LED. Both positions come out of placement.json, so the artwork is generated, not measured off a photo.
• The battery lives in the base ring, not on the board — that keeps the whole top face available for electronics and the load cell.
• USB-C sits on the south rim. The plug clears the board because the receptacle's opening is above the board plane; the base ring carries the cut-out.

Schematics (KiCad 10, ERC 0 errors)

Vector — zoom in the browser. Every sheet is emitted by generator/gen_0scale.py; the generator refuses to write a sheet where two wires cross without being one net, which is the failure mode that makes generated schematics untrustworthy.

01-powerPower — USB-C, charger, rails

02-mcuMCU — ESP32-S3R8 + flash

03-usbUSB-C — connector, CC, ESD

04-loadcellLoad cell — HX711 + bridge

05-cameraCamera — OV2640 DVP

06-ioIO — buttons, LED, buzzer, antenna

Bill of materials

Unit prices in USD at small quantity. lcsc = verified LCSC/JLCPCB listing, est = estimate (±30 %). 12 lines are marked verify: they have no confirmed part number yet and must be checked against a live listing before ordering.

Off-board — hand assembled, not in the SMT run

Build estimate — JLCPCB, 5 prototypes

• Standard PCBA is required: the 0.4 mm pitch QFN-56 and the DSBGA-8 charger are past Economic assembly.
• Hand steps per unit: load cell and its spacers, the glass and its sticker, the battery, the camera ribbon, the base ring. Call it 15 minutes once the jig exists.
• At 100 units the SMT parts drop roughly 35 %, the PCB lands near $3 each and the fixed PCBA cost amortises — order of $45–60 per scale including the cell and the glass.

Files and how it is generated

• hardware/0scale/generator/gen_0scale.py — all six schematic sheets
• floorplan_0scale.py — placement: circle blockers, the load cell band, anchors
• pcbgen_0scale.py — the board: round outline, zones, keep-outs (runs under KiCad's python)
• bom_cpl_0scale.py — JLCPCB BOM / CPL / hand-assembly list
• annotate_0scale.py — the numbered render and legend.json above
• pipeline.sh build | route | finish | all — the whole chain, reproducible

Nothing here is drawn by hand. Changes go into the generator and the chain is re-run; the .kicad_sch and .kicad_pcb are outputs, not sources. That is the same discipline as the 0Card project this borrows its schematic engine from.

Open questions