How to interface a digital scale strain gauge weight sensor with a microcontroller?
I Googled for your sensor, and this is the best I could find.



If this is your sensor, then you'll notice that it only has three wires coming out of it. This means it's probably a Half Bridge Load Cell. A Full Bridge Load Cell will have 4 wires, and it looks like this internally:



I have labelled the wires 1,2,3,4. You would apply a, say, 5v to wires 1&amp;2, and you would see about 2.5v on wires 3&amp;4. The voltages on 3&amp;4 should be very close together when there is no load applied. Ideally, they would be identical. When you apply a load to the sensor, the voltage on 3 would go down, and 4 would go up. This will be a tiny change in voltage, and so you need to amplify this by about 100x to 1000x so that an ADC can sample the value.

However, your Half Bridge one (probably) looks like this inside:



To properly amplify this voltage you need to amplify it relative to another voltage, in this case 2.5v. So you just need to set up a potential divider to create 2.5v. However, due to the tolerance of typical resistors, it's highly unlikely you'll be able to match your 2.5v to that produced by the load cell. This is where this chip comes in:

The instrumentation amplifier we always use is the AD8556. This is what's inside it.



It's a slightly expensive device, so not great for production except in high value products. The good thing about it is the ability to dynamically change both the gain, and the trimming, digitally from the microcontroller.

We use it to deal with some slightly dodgy strain gauges that aren't always trimmed properly. On boot up, the microcontroller automatically searches for the best trimming value so that we get the optimum range on the sensor.