Smart Body Composition Scale Product

Overview

A smart body-composition scale measures two unrelated things in one step-on: weight, by strain-gauge load cells, and an estimate of body composition, by passing a tiny alternating current through the body and measuring its electrical impedance. Results sync to a phone app over Bluetooth, which is the actual product: a longitudinal log of weight and composition trends rather than a single morning number.

Weighing

The user stands on the Tempered Glass Top, a 6 mm tempered plate that spreads the load to four corner feet. Each Foot Cap bears on a Strain-Gauge Load Cell — an aluminium spring element carrying foil strain gauges whose resistance changes by parts per thousand at full load. The four half-bridge cells are wired into one full Wheatstone bridge, a trick that sums the corner forces electrically, so the reading is correct no matter how the user's weight is distributed across the platform.

The bridge output at full scale is a few millivolts; resolving 50 g steps in a 180 kg range means resolving microvolts. That is the job of the 24-bit Weighing ADC, a 24-bit delta-sigma converter on the Mainboard that integrates away mains hum and noise. The Load Cell Brackets and the corner pockets of the Base Moulding matter as much as the electronics: any mechanical shunt path that lets force bypass a gauge beam shows up directly as error, which is why scales misread on carpet — the soft pile loads the pan instead of the feet.

Body impedance

The composition estimate uses bioelectrical impedance analysis (BIA). Lean tissue is mostly electrolyte-rich water and conducts well; fat conducts poorly. The BIA Front End drives a fixed 50 kHz current of under a milliamp — imperceptible and safe for skin contact, though contraindicated for pacemaker wearers — from the AC Current Source into one foot and reads the resulting voltage at the other through the Sense Amplifier. The Electrode Multiplexer swaps drive and sense roles among the four ITO Foot Electrode zones, transparent indium-tin-oxide coatings on the glass that contact the bare soles via sprung Electrode Spring Contacts underneath.

Measured impedance — typically 400–700 Ω foot-to-foot — feeds a regression formula along with the stored height, age, and sex of the recognised user. Firmware on the Microcontroller outputs body-fat percentage, muscle mass, total body water, bone mass estimate, BMI, and basal metabolic rate. The honest caveat is built into the physics: foot-to-foot current travels up one leg and down the other, sampling the lower body and inferring the rest, and hydration shifts the reading day to day. BIA scales track trends well but sit several percent from DEXA on absolute body fat, which is why consistent conditions (same time of day, before eating) are recommended.

Electronics and sync

The Compute SoC Module provides Bluetooth Low Energy 5.0; after each measurement the scale advertises, the phone app collects the record, and the BLE Trace Antenna trace handles the 2.4 GHz link. Multiple household users are distinguished automatically by weight proximity to their stored profiles. With no phone present, readings buffer in the scale and upload on the next sync.

The readout is the Hidden LED Display: white LED Segment Matrix digits shining through a printed Display Mask in the glass coating, invisible when off. Weight shows in about two seconds; impedance and the composition screens follow while the user is still standing.

Power budget

Everything runs from three AAA cells in the AAA Battery Holder. The design constraint is standby: the scale spends 99.9% of its life asleep, drawing single-digit microamps while watching the load-cell bridge for the step-on disturbance that wakes it. An active measurement with the BLE radio up draws tens of milliamps for a few seconds. Averaged over typical once-daily use, the cells last about a year. The glass is bonded to the Base Pan with perimeter Perimeter Adhesive, and the boards sit on Board Standoffs clear of the slightly flexing pan.

Accuracy practice mirrors any precision scale: hard level floor, bare feet centred, and a re-zero (auto on power-up) if the scale has been moved.