 # Dog Weight Scale Part 7: Choosing Matching Resistors

In my previous post, I worked through the calculations of weight and center of gravity when using four Load Cell Amplifiers instead of one.  In this post, I build the circuit for the first of the four Load Sensor / Load Cell Amplifier combinations I’ll be using.

The Sparkfun Load Cell Amplifier is designed to connect a Load Cell to an Arduino.  A Load Cell contains a full (4 resistor) Wheatstone Bridge, but a Load Sensor contains only half of a Wheatstone bridge.  To connect a Load Sensor to a Load Cell Amplifier, I need to add two resistors: R3 and R4 in the following diagram.  The dotted box represents the Load Sensor. The triangle in the middle of the diagram represents the Load Cell Amplifier.  As the weight on the Load Sensor increases, R1 decreases, which causes the voltage V1 to increase, causing the digitized amplifier output to increase.

Once I’d built the circuit, the next task was to find the right pair of resistors, R3 and R4. In the photo below, R3 and R4 are the two resistors at the left; the Load Cell Amplifier is the small red board in the center, and of course the Arduino is on the right.

R3 and R4 need to have a ratio that is close to the ratio of the two (matched) resistors that are in the corresponding Load Sensor, so that the digitized voltage difference, V1 and V2 in the above schematic, is near zero.

The tricky part is that a normal multimeter shows that the two resistors of the Load Sensor are equal, and that R1, above, changes by only an ohm or so with pressure.  That multimeter isn’t accurate enough to show the differences in resistance that you need to measure.

So either you can buy a high-precision multimeter (expensive), or you can use the Load Cell Amplifier to measure the resistances. I decided the use the Amplifier.

I ordered a bunch of 1Kohm, 0.1% resistors.  Then I picked 4 of them (I could have used more, but that’s more work), labeled them A, B, C, and D, then started measuring the Arduino ADC (Analog Digital Converter) values of various pairings of those resistors.