# How to use a load cell as a "switch", more or less

Hey everybody, I’m trying to turn a load cell into a simple switch, i.e. if the load exceeds x amount, go to active state. If the load is below x amount, go back to inactive state.

I have a load cell with only two wires however, and I know they typically have three or four. I’ve done some research on it and the suggestion so far has been using a wheatstone bridge or a voltage divider. I have trid the voltage divider and it doesn’t see to work quite well, and additionally I don’t think I have everything needed for a wheatstone bridge and I’d like to avoid using one, but absolutely will use on if it’s the only option.

I guess all I really need to know is how to turn the values given by the load cell into something I can use to make a switch with a basic if, else loop.

I have included a picture of the load cell in question.

Thanks everybody!

Place it in a resistor bridge with 3 equal value resistors, add a trimmer potentiometer to give a small amount
of adjustment of one ot the resistors. Now just add a low-input-offset comparator between the
arms of the bridge to detect which has the higher voltage.

Some extra amplification of the voltage difference might be a good idea though, strain gauges
vary by a few tenths of a percent typically...

Have you measured the resistance of the sensor?

Hey MarkT,

I will try your suggestion of using the three resistors and a voltage comparator; if I’m not mistaken that’s sort of like an OP amp and I am a bit familiar with them. I don’t have them on hand, however.

Mind if I ask why this is necessary? I am very interested in this technique and want to know as much about this sensor as I can. Is it because the change in resistance due to load is very small, and needs to be amplified using a bridge?

During application, the cell will bend more than 90 degrees and then return back to 0 degrees. So it will flex a lot, and I could use the midpoint of the amount of load as a switch. It’s just difficult for me to find out what these values are.

#include <Servo.h>

int strain1AnalogPin = 0;
Servo servo1;
int pos1 = 0;

void setup() {
Serial.begin(9600);
servo1.attach(9);

}

void loop() {
pos1 = 180;
servo1.write(pos1);
}
else{
pos1 = 0;
servo1.write(pos1);
}

## }

EDIT:

I have measured the resistance of the strain gauge, it is 125.57

Does anyone know a lot about these types of strain sensors? I’d love to know more about them and why they need a Wheatstone bridge to work, and how you can tell by just looking at it aside from experience (I.e. how would you know what sensors require a Wheatstone bridge). Thank you! I’d like to post the fully completed solution in this forum thread so that future researchers could easily find out how to use these two-wire strain gauges. I’ll be using the suggestion above and posting the results, once I purchase the comparators and extra resistors.