Tuesday 26 April 2016

pic - Yet another Weighing Scale Project


I am taking some off the shelf weighing scales and would like to interface a PIC to them for display and wireless transfer... The scales have four 3-wire load cells, one in each corner, and look very like the ones on the SparkFun web site. I am guessing but I think the load cells have two resistors in them, one for tension and one for compression. After the bridge arrangement the output of the bridge goes into a capacitor, resistor arrangement. I am guessing that the next stage is an amplifier of some type but it's inaccessible to me as it's a custom controller covered by a black blob.


sketch of schematic


Please can someone confirm the picture is about right and also suggest a good, PIC friendly way, to amplify the output either before or after the discrete.



Answer



The picture looks right to me. Each strain gage is comprised of two resistance grids. 1k is a common value for a strain gage resistance.


You should amplify this with an instrumentation amp, such as AD623 or INA333.


I'm assuming you don't have the mechanical design specs of the scale, so start with a gain of about 200 and see if that gives you the right span.


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