Flex sensor doesnt work


Joined Jan 29, 2010
hi 123,
The 741 OPA is a old device with a very poor specification, use a LM324 or Lm358 as recommended in the datasheet.

Using the non inverting amp circuit from the d/s, with a 10k load for the sensor, the output voltage will decrease as the sensor is bent.

With no bending the Vout should be Vsupply/2.

Look at this LTSpice sim of the project, shows 3 flex states, note the Vout decreases with flexing.


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:confused: That's not what the bottom of page 1 of the datasheet says.
Yes, you are right. Please note that I had corrected by response before your message appeared and I apologized multiple times.

I feel bad about that and I will try to be more careful, but I can't guarantee that I will not make an error again.
OK, I want to try again. It's sometimes so difficult to do this interactively with a message board. Everybody comes into it with varying degrees of experience and knowledge etc...and it doesn't help when I use "decrease" instead of "increase" and we are talking about resistance sometimes and voltage at other times.

"so i have just started from the basics again. i found that with the sensor alone. the resistance increase as it shown on multimeter, but when i connect it with a 10k resistor. the resistance actually increases and from 5k to about 7k ohms when its fully bent. what could the problem be?"

When you wrote this, I know that you are describing the problem, but it is too difficult for me to tell just exactly how it is a problem.

What I propose is that you measure the resistance of the flex sensor out of the circuit and record the resistance in ohms of the flex sensor when it is lying flat and when it is bent about 45 degree and when it is bent approximately 90 degrees. From the data sheet, it should be about 10K flat, but you need to empirically establish the resistance when it is bent to various degrees.

Attach the meter just as though you were measuring a resistor. One meter lead to one flex sensor lead and the other meter lead to the other flex sensor lead. Set the meter for resistance. Do this just as though you were measuring a simple resistor. In fact, you attached a picture in an earlier message that had a 10K resistor shown. Use your meter in the same way as you would to measure that resistor, except the flex sensor is the resistor.

No voltage divider, no additional components or connections. No added voltage and no other connections period. One meter lead to one flex sensor lead and the other meter lead to the other flex sensor lead. I am not being condescending here at all, I just want to make sure that we are on the same page.

Please post the results. They are important because they will provide information on the range of resistance that you are getting out of the flex sensor.

From there, you can calculate the expected voltage out when you use the flex sensor in a simple voltage divider with one other resistor. Then you can calculate the value from the A2D converter that you can expect under various conditions of flex.

I think an appropriate op amp is important as an impedance buffer, but I don't think you have to address that part at this point. Not until you can characterize the behavior of the device in your hands and define what you need it to do.