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Load-cell scale front end

Embedded (ESP32) · Advanced · about 20 minutes · part of Embedded & IoT

Four strain gauges on a metal bar form a Wheatstone bridge whose output is a millivolt or two at full load. The HX711 excites the bridge, amplifies the difference 128 times and converts it to 24 bits; the ESP32 removes the offset (tare) and scales the counts to grams.

Load-cell scale front end: the circuit on the simulator's bench

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How it works

Full scale (5 kg) gives 1 mV per volt of excitation: about 4.3 mV at 4.3 V, 860 nV per gram.

The HX711 maps ±0.5·E/128 to ±2²³ counts, so one gram is 429.5 counts: that is the set_scale() factor in the sketch.

Try it on the bench

  1. Put 2.5 kg on the cell (select it).
  2. Change set_scale() to a wrong number and see why every scale needs calibrating with a known weight.

Parts on the bench

In the course

Embedded & IoT, module “Buses and sensors”.

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