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A 3D printed set of scales that uses a 5kg load sensor and HX711 module.

This project came about after reading an article on Load Cells on the Circuit Journal site by Indrek Luuk. A load cell is designed to measure the weight of an object. It is basically an aluminum bar contain 4 resistive strain gauges.

The 4 resistive strain sensors are connected together in a Wheatstone bridge configuration. At rest, the sensors have the same resistance (around 1000 ohms). By placing a 5 volts across the Red/Black wires, the voltage at the Green terminal is 2.5V and the voltage at the White terminal is also 2.5V. The difference in voltage measured by volt meter V is 0V.

As weight is applied to the load cell, the two opposite sensors will change their resistance equally. In the diagram, the red sensors reduce in resistance (less strain applied to them) while the blue sensors increase in resistance (more strain is applied to them). The volt meter measures the difference in voltage across the Green/White nodes. The voltage varies in proportion to the load being applied.

The change in voltage is small so it needs to be amplified and converted to a digital signal so that the microprocessor can read it. This is done via the HX711 module which contains an amplifier and 24 bit analog-to-digital convertor.

Load cells come in different sizes depending on the maximum weight they are designed to measure. Typical sizes are 1/5/10/20/200kg. The number of screw holes, their centers and thread sizes vary depending on the size of the Load Cell.”

Link to article