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Possible Duplicate:
How to interface a digital scale strain gauge weight sensor with a microcontroller
How to interface a digital scale strain gauge weight sensor with a microcontroller? It is a Load Sensor SEN-1638 (similar to the SEN-10245 from Sparkfun.
It can be used to Measure up to 110 Pounds. How can I interface the above sensor with a Microcontroller Component? Can I use the Load sensor directly to measure the load by means of using a power supply and Digital Multimeter?
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&2, and you would see about 2.5v on wires 3&4. The voltages on 3&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.
As Nick Johnson said - You need an instrumentation amplifier - the INA125 is a good choice, readily available in a DIP form factor and easy to hook up. See my blog post on my drinkbot for general instructions on how they work and how to hook one up.
And, as I said: Yes. Use a purpose built amp such as the one Nick suggests OR a version of the widely used standard circuit that has been used for about the last 50 years to in each case convert the VERY small signals to a value that can be handled with more usual instrumentation (a volt or few maybe) THEN use the analog to micro-controller interface method that suits your overall need. interface.
