Connecting the load cells and the HX711 Amp
Now that we (hopefully) have figured out how to use the existing load cells we have to connect them to the amplifier and the amplifier to the ESP8266.
The mini PCB that came with the digital scale connected to the HX711
A look to the other side of the boards: mini PCB connected to the HX711. I drew the labels V+, V-, S+ and S- on the mini PCB. Sometimes you see this naming scheme
Again, how you connect the load cells to the HX711 requires a bit of educated guessing. Read through the previous chapter, read the SparkFun tutorial about load cells, use the color codes of the wires. Then connect the HX711 to the ESP8266. The following image is just a zoomed in version of the complete wiring below:
HX711 wiring. See full sketch below (Click to zoom)
Connecting the OLED display
The wiring of the OLED display should be straight forward. Just connect VCC on the display with 3V3 on the ESP8266, GND with GND, SDA on the display with D6 on the ESP and SCL with D7. In theory you can choose any D-pin for SDA and SCL, just make sure that they don’t have special functions. I think D8 is required for programming, others might be used for flash access. Also when you use the pins I suggest here you won’t have to change anything with my code. Please have a look at the complete sketch below for a graphical representation of the setup.
Connecting the power source
Now to my new favourite power source in the ESP8266 world: the LiFePo4 AA battery. After watching the excellent video below of my fellow (Swiss) country man Andreas Spiess about ESP8266 battery sources I immediately ordered a few from AliExpress.
The beauty is that their voltage range matches the 3V3 operation voltage of the ESP8266 perfectly. You don’t need any linear voltage regulator or boost up/down converter which just turn a considerable part of the battery’s capacity into heat. The discharge characteristic of the LiFePo4 also shows that it keeps providing a ESP8266 compatible voltage over a long time. For simplicity (and due to lack of necessary skills;-)) I decided to use a switch which was already built into the scale as on/off switch. An automatic power-saving circuit would have been nice but that would have complicated the project even more. In the original setup the built-in switch could be used to change between pounds and kilograms, now it will serve as the power switch.
