Help Arduino To Measure Physical Tension

I have an embroidery machine that requires that the "top tension" and "bottom tension" of the threads involved be calibrated very carefully. Today, I do this with a small pen shaped device called a "Tension Gauge".

The way that it is used is that you disconnect the thread from the needle, tie it to the end of a hook that is attached to a spring with a needle on it, and you pull on it. The thread will pull down the spring until it reaches the point where the spring pulls back, and then the tension of the thread is measured.

I would like to try and build this with an Arduino and have it be digital. I don't believe that this is a complicated build, but I don't know what type of external sensor I can use to work with the Arduino board. Can someone provide me with some guidance on how to get started and any reference material.


Do you have a link to the sort of device you are currently using (it sounds like a miniature spring-balance) ?

Force sensors are often called “load cells” and for engineering and scientific use tend to be expensive (and extremely accurate). There are cheaper versions available that go into things like digital kitchen scales which are possibly a solution to your problem. Also I’ve heard of bending-force sensors which might be usable?

Yes, you are correct, it is a miniature spring. See here:

Are these other devices that you are talking about compatible with the Arduino?


The classical method of measuring tension would be a strain gage. These are very sensitive and accurate sensors, but do require low level amplification before being useable for reading with an arduino. You also must know the maximum force range you would require to obtain the proper gage.

Here is a 0-100 gram load cell

And a 0-1 kilogram unit

Does a strain gauge work in both push and pull? When I look at the photos that you sent, I can see how they would be used for a scale because you could put weight on them. But I need to "pull" on the thread. So if I tied the thread on one of these and pulled the thread, assuming that the gauge was secured in one spot. Would it read the "pulling" action? Would it read it as a negative number?

Also, when you say amplification, what do you mean?


Does a strain gauge work in both push and pull?

They typically respond to both pull and push forces. What direction voltage change depends on how the two sense wire are wired to a needed following op-amp. Strain gauges are usually wired up in a Wheatstone Bridge configuration, two wires for the voltage source and two wires are the measurement output. Strain gauges typically output a very small voltage, say 0-5 millivolts for those examples posted if powered with +5vdc. So a instrumentation op-amp is required to raise the voltage range to something more usable for a arduino analog input pin. Such a op-amp with it's gain set to X1000 would allow a full measurement range of 0-5vdc. For your application where you are not planning of needing a calibrated range of measurements, but rather just a repeatable value when the tension is properly reached, you may not need full 0-5vdc input to the arduino.


Argh! I came to this thread hoping for An Answer... but just got lots more questions. Did come across the following, aling the way, though....

Lots of good stuff at...

(Remarkably "similar" page at...

.. but with some op-amp stuff at the bottom.)

.. but it does everything with Wheatstone bridges... wonderfully simple, but are they temperature sensitive?

Maybe op-amp better, even if it means more learning...

(Richard is a reliable source... lots of good posts.)

Sparkfun sell the LM358 op-amp...

... not that this makes it the only suitable device... there are more op-amps than you can research in a lifetime... but Sparkfun are pretty well tuned to the needs of the average Arduino forum user, I think. (And, if one post is to be believed, there are two of them in the Duemilanove, for what that's worth!)

Here's a link to a 358 datasheet... yes, I know it says 158... the 358 is on the same sheet.

An excellent video at....

... gave me a much better understanding of various things... in particular what the 4 wires coming out of some strain gauges might be about, and a bit about the op-amp issues. (Turns out that Wheatstones may be integral to many strain gauges...)

(For an excellent 480 page .pdf ALL about op-amps, see...

The following circuit for a load cell amplifier may help, once you digest some of what's above...