Note! Please follow the attached interactive BOM. It will help you to find the correct components and it will also guide you with the placement.

I recently published a project where I tried to utilize the traces on a PCB as a strain gauge in order to use the PCB as a weighing scale (or “bending load cell”). The concept was simple: I created a cantilever beam section on the PCB and I placed a pattern of copper traces on this bending part that is similar to the pattern of the conductive traces in a strain gauge. Unfortunately, the concept did not work, because the pattern had too low resistance and an even smaller change in resistance when the PCB was bent. However, when I put a strain gauge on the cantilever beam part, the concept worked perfectly. But, due to structural limitations, I could not measure more than 200-250 grams.

Therefore, I decided to reiterate the project and create a better version of this concept. Since I knew that the PCB traces won’t work, I kept using strain gauges in this design. I also replaced most of the circuitry. The whole Wheatstone bridge is now made of strain gauges (full bridge configuration). This bridge is then managed and measured by an HX711 module. The module is read by an ATTiny85 microcontroller and the results are printed on a small OLED display. The PCB also has a built-in LiPo battery with a charging circuit (MCP73831T).