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By Kokopelli23209
#195168
Hello Everyone,

I'm trying to determine if a phenomenon my team and I are observing is normal.

Let me describe our setup first. We are working with two of these flex sensors laid end to end and wired in series. We did this because the area we need to measure is longer than what one 4.5 inch sensor could handle and couldn't find any that were longer. If I understand correctly the sensors are basically just variable resistors so there shouldn't be anything wrong with wiring them in series. So first question, is this true? Can they be wired in series to make a longer sensor?

FYI we are using the example code from the hookup guide.

Now a little back story for the weird readings we are getting. So this is part of a larger school project. Before we can move on to applying the sensors to the project our professor wants us to prove that we can get reproducible results in fixed positions. So we printed out the angle diagram that was on the end of the first page of the datasheet and turned it into a crude test fixture out of toothpicks and florist foam as you can see in the image below.

Image

When we first put the sensors into the track with the sharpest bend the serial data monitor gives us a reading around 90 degrees but if we let it sit in the track the angle reading slowly drops. We let it sit for 10 minutes and the reading had dropped to around 80 degrees. So we did a little troubleshooting and removed the sensors from the Arduino and just hooked them straight into a multimeter to see if it was a problem with the sensors or the arduino. The resistance readings started around 30k and proceeded to slowly drop over time just like the data in the serial monitor was doing.

So now the main question. Is it normal for a flex sensor's resistance value to drop over time?

Is it possible we have bad sensors or damaged them when we soldered them together? I'm inclined to believe that we didn't damage them because we measured their resistance values in series on a breadboard before we soldered them together and had the same results. We wondered if we got bad sensors because the resistance range we got for measuring one sensor was approximately 10k-20k ohms. These values seem to be much lower than what other people have reported in their tutorials. One tutorial said the resistance range should be from 20k-70k ohms.

Thanks for taking the time to read this. Any and all answers are appreciated.
User avatar
By darrellg
#195176
The datasheet (https://cdn.sparkfun.com/datasheets/Sen ... RREVA1.pdf) shows a recommended circuit that includes an op-amp that is used as an impedance buffer, which is not included in the hookup guide. Have you tried that?

The same datasheet also shows a resistance tolerance of +/-30%. It doesn't specify under what conditions, so I can't tell if it means total resistance of the strip or of the measurements.
By monstah
#195191
Hey, I am facing the same "problem", so I ended up having the same question. Couldn't find any informations on that in the internet (for the dropping resistance problem). I also bought some 4.5" bend sensors and measured 10-11k ohm if unbended and something between 20k and 25k ohm if bended. According to the datasheet I found from sparkfun this is not right but I figured out that there is the company name "spectra symbol" printed on the back of my sensors, so I managed to find another datasheet providing some other informations:

http://www.spectrasymbol.com/wp-content ... -Rev-A.pdf
-Flat Resistance: 10K Ohms ±30%
-Bend Resistance: minimum 2 times greater than
the flat resistance at 180° pinch bend (see
"How it Works" below)
but yea, my main question still and also is:
Is it normal for a flex sensor's resistance value to drop over time?
I am happy, that I am not the only one, who came up to this..