IoT Industrial Scale
jenfoxbot 
Measuring Weight
How Do We Measure Weight??
Strain gauges! Also called load sensors, strain gauges measure electrical resistance changes in response (and proportional) to, well, strain! Strain is how much an object deforms under an applied force, or pressure (force per area). Check out this super awesome tutorial for more info on how strain gauges work.
Usually what you'll find in a bathroom scale is a load cell, which combines four strain gauges in a wheatstone bridge. This project uses four disc compression load cells rated at 200 kg, like the one pictured above.
OpenScale Overview
The OpenScale is a specialized board that allows a user to easily read and configure all types of load cells and strain gauges. It's designed for constant loads, includes an on-board temperature sensor, and is particularly helpful for places that are tricky to get to (e.g. under a beehive or conveyor belt).
The OpenScale combines an HX711 load cell amplifier with an ATmega328P, so you can program it just like an Arduino (Arduino Uno, specifically). The OpenScale uses TTL serial communication at 9600bps 8-N-1 to transmit and receive data. It comes with software that allows it to be controlled in, and print data to, the Arduino IDE serial monitor.
Here's the GitHub page for the OpenScale, which includes the Eagle schematic files.