a load cell
A load cell is a finished, ready-to-use force sensor built around strain gauges. On its own a strain gauge is just foil that needs a beam to stretch it; a load cell packages that beam and gauges into a single rugged metal block with mounting holes and a cable. Press or hang weight on it and out comes a small voltage proportional to the force. It is the part inside almost every electronic scale.
Inside a typical load cell, a precisely machined piece of metal (often aluminum or steel) flexes by a controlled, tiny amount under load. Four strain gauges are bonded to it — some where the metal stretches, some where it compresses — and wired as a full Wheatstone bridge so all four work together. This full-bridge arrangement quadruples the signal and cancels temperature drift, because heat affects all four gauges alike. The output is specified as a sensitivity in millivolts per volt of excitation at full scale: a common rating is 2 mV/V, meaning a cell excited with 5 V gives 10 mV at its rated maximum load. Feed it a stable excitation, amplify the bridge output with an instrumentation amplifier, and digitize.
Why this matters: load cells are how the world weighs things electronically — bathroom and kitchen scales, supermarket checkouts, truck weighbridges, hopper and tank batching in factories, and force feedback in test machines. The honest realities: the full-scale signal is only millivolts, so amplifier offset and noise matter; the reading is ratiometric (it scales with the excitation), which you can exploit by powering the bridge and the converter's reference from the same source so supply drift cancels; and every cell needs calibration with known weights, plus attention to off-center loading and creep. The cell sets the accuracy, not the converter.
A 5 kg load cell rated 2 mV/V is excited with 5 V, so at its full 5 kg load it outputs 2 mV/V times 5 V = 10 mV. A 1 kg object then gives one fifth of that, about 2 mV. An instrumentation amplifier with a gain of 100 lifts that 2 mV to a comfortable 0.2 V for the converter.
Four gauges in a full bridge make a packaged, calibrated force sensor.
A load cell's output is ratiometric: it is a fraction of the excitation, not a fixed voltage. Power the converter's reference from the same supply as the bridge excitation and supply drift cancels out — and always calibrate the finished system with known weights.