Cantilever Beam Load Cell Components

Strain gauges measure deformation and deformation is proportional to load, so you can use strain gauges to measure applied forces. (video 2:21)

The changes in resistance are very small for practical elastic deformations. Read about strain gauges in your textbook or online to find out about different gauge geometries, typical gauge resistances, and the definition of the gauge factor. What other influences in the measurement environment can change the resistance of a strain gauge? How can you minimize or compensate for that extraneous change?

Full Bridge Circuit Configuration

A Wheatstone bridge circuit with strain gauges will provide a millivolt level output and you will need some amplification capability. You will set up the INA125 Instrumentation Amplifier in pseudoground mode, so that the results can go positive of negative relative to a 1.24 volt reference.

When I tested my load cell, measuring with the multimeter, I found it had a voltage of 1.04 mV across the bridge unloaded, 1.42 mV with my bucket, and 2.11 mV when I added a 1 kg jar of peanut butter to the bucket. It went up 0.69 mV for the extra kilo, with an excitation voltage of 3.3 V on the bridge, so the overall sensitivity is

0.69/3.3/1.0 = 0.209 mV/V/kg

for this particular load cell. Yours is probably different, depending on just where the strain gauges were attached. (video 4:37)

Setting up the Bridge on a Breadboard

Connecting the four gauges on the breadboard will look like spaghetti, so it will be good to have a plan in advance. (video 9:54)

Alternately, you can set up the same circuit to run at 3.3 volts on the Itsy Bitsy, although you will need to switch to pin 13 for the 1.24 volt reference on the amplifier.

Calibrating the System

There are a lot of variables in the configuration of the system, introducing a lot of uncertainty, so the best way is to measure the performance. Find analogRead() values for no load, and for a full load of several kg, and assume a linear response between those points to remove much of the uncertainty. (video 13:33)