In each corner of the scale is a load cell, which looks remarkably like the load cell found at SparkFun. These load cells have a rated capacity of 50kg which given that the scale has a total rated capacity of 200kg and there are four cells with evenly distributed weight makes this a very good candidate.
The load cells each have three wires, Blue, White and Red. I don’t know much about load cells but I am guessing that these comprise two strain gauges; the first strain gauge is located on the ‘top’ surface and is subject to tension when the load plate is loaded, whereas the second gauge is subject to compression. The strain gauges themselves are variable resistors; with tension the increase in resistance (the path gets longer) and in compression their resistance decreases. Finally it should be noted that these changes will be very small but more of that later.
According to SparkFun the specification for these devices (if they are the same) is…
Capacity kg 40-50 Output Sensitivity mv/v 1.0±0.1 Input Resistance Ohms 1000±20 Output Resistance Ohms 1000±20
So we should expect that from one wire to common (White) we will get a resistance of around 1k Ohms and then when a weight is applied this resistance will change. It seems that Load Cells are described in terms of a mv/v sensitivity. i.e. When a voltage is applied across the cell (the excitation voltage) a voltage difference will be observed when the cell is loaded. This is all new stuff to me so I’m not sure if 1mv/v is for 50kg or for 1kg… I guess I will find out later.