Digital Scale (Custom Load Cell + Analog Front-End)

Commercial load cells and amplifier modules are widely available, but they hide the core sensing and analog design challenges. This project was a ground-up attempt to design a load cell + amplification + digitization chain and achieve useful scale performance with quantified accuracy and repeatability.

A custom steel load cell concentrates stress using drilled holes. Four strain gauges are placed where deformation is greatest and wired as a Wheatstone bridge. The bridge’s differential millivolt-level output is amplified by an instrumentation amplifier into a single-ended signal suitable for digitization. A stable mid-rail reference (~2.5 V) is generated via a buffered divider so the amplifier output stays in-range. A 12-bit external ADC provides higher resolution than the Arduino Nano’s onboard 10-bit ADC, and the MCU converts ADC codes to mass using an experimentally obtained calibration curve. The system supports taring via a command from the connected computer.

Key hardware elements include: (1) steel-beam load cell with strategically drilled holes to localize strain; (2) four strain gauges bonded above/below the holes and wired in a Wheatstone bridge; (3) instrumentation amplifier with adjustable gain; (4) bridge balancing via a small series trim potentiometer to minimize offset that would otherwise consume amplifier headroom at high gain; (5) buffered reference voltage to meet amplifier requirements; (6) external 12-bit ADC feeding the Arduino Nano over digital communication.

Firmware reads the external ADC, applies a calibration mapping (derived from measured mass vs. output), and streams mass readings to a connected laptop for display. A tare command allows the user to zero the scale without hardware changes.

Performance was characterized using an experimental calibration curve showing strong linearity across 0–1000 g. Theoretical maximum resolution is ~±0.25 g based on ADC scaling; practical resolution is ~±1 g due to hysteresis. Accuracy compared to a laboratory scale is ~±1 g in the tested range.

A key limitation is temperature dependency: warm airflow over the electronics could increase the measured mass by up to ~5 g at higher masses and cooling reverses the effect. A working hypothesis is that instrumentation amplifier gain varies with package temperature. Future iterations would focus on improved thermal stability, mechanical isolation, and/or ratiometric measurement strategies.

Improve thermal stability (component selection, layout, shielding), reduce hysteresis via mechanical refinements, and consider enclosure-level temperature sensing/compensation to maintain accuracy in variable environments.