In this project I will show you how you can build yourself a fancy Withings-like body scale. I divided the article into two parts: Part 1 describes the hardware hack and Part 2 the software. In this project I used a cheap scale I bought in the local super market for 9 Swiss Francs (which is currently about USD $9.-) and I am optimistic that other digital scales will work for you as well. This project was a lot of fun since I had to do reverse-engineering, soldering, programming and (optionally) even 3D modelling and printing. I hope you will enjoy it as much as did!
Just a little word of warning: you might be taking apart a working digital body scale. I cannot guarantee that your retrofitting will actually work and you might end up having just a broken body scale. Because of this I recommend that you are using a cheap or old digital scale rather than an expensive working one…
The other thing I’d like to mention: it took me many hours of my scarce free time to build and document this project.
This is a write up of a project I started about a year ago. The project went back onto my “work-in-progress stack” and was waiting for a revival.
So here is the plan: you open up your pretty body scale, remove everything you don’t need and keep the load cells (the components which measure the weight) in place. Then you connect the load cells to your HX711 amplifier module and connect the amplifier to your ESP8266. Then you’ll need to attach the OLED display to the ESP8266 as well and bring in some kind of power source with an on-off switch. Easy, isn’t it?
For me the hardest part was to figure out how the load cells should be connected to the HX711. Luckily the load cells were attached to a mini PCB which in turn was soldered with only a few pins to the main board of the body scale. So I only had to figure out how to connect the four pins of the mini PCB to the HX711 module rather than figuring out how to connect the 12 wires of the load cells (3 wires and 4 load cells). I don’t know how your scale will look like. Maybe you will be lucky as well but it also could be that you have to do some hard core reverse engineering. Maybe the following pictures will help.
I suggest that you test the setup on a bread board before you take out the soldering iron. This way you can easily switch the wires until everything works. SparkFun has a very good introduction to load cells and the HX711: Sparkfun Load Cell Tutorial