[Curious Scientist] tried building an integrated strain gauge on a PCB, but ran into problems. Mainly, the low resistance of the traces didn’t show enough change under strain to measure easily. Even placing a proper strain gauge on the PCB had limitations. His new design uses a bridge design to make the change in the gauges usefully large. You can see a video of the project below.
Bridging strain gauges isn’t a new idea. However, the novelty of this design is that the PCB has cantilever beams that facilitate the weighing. Standoffs mount a plate to the beams so that weight on the plate cause deformation on the beam that the strain gauges can measure.
An HX711 module takes care of managing the whole thing. Rounding out the build is a CPU, a battery charger, and an OLED display.
By itself, the output of the device is raw counts, but thanks to the HX711, the output is linear so a simple linear regression can convert the counts to the units you want, such as grams. The resulting line equation is simple enough to implement in the microcontroller, and you can have normal scale units put out on the display.
While the display is nice, there’s something undeniably cool about an analog scale. You might think you don’t need to design a scale, but sometimes you want to weigh something as part of another device. Then it comes in pretty handy.