# Flex Sensor Dilemma! Help needed...

Discussion in 'The Projects Forum' started by MicroSprynter, Jan 16, 2012.

1. ### MicroSprynter Thread Starter New Member

Jan 16, 2012
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Right, so I bought 5 flex sensors (for quite a price) for my D.T AS coursework. However, after measuring the resistance ranges of each of them (about 2.8k), I have come to the conclusion that this is not great enough. The datasheet they provided on the website, states that the resistance varies from about 20kΩ-60kΩ (which would be good). Just for the record, the resting (flat) resistance is 20kΩ. I need the bent resistance to ideally reach double that flat resistance. I have tried and tried, but I cant find a way.
Also, the resistance change needs to be analogue.

Jul 9, 2011
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Jan 16, 2012
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4. ### praondevou AAC Fanatic!

Jul 9, 2011
2,936
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I don't see a parameter resistance change per degree of deflection.
What you measured, is it 2.8k at 90 degrees?

What exactly do you want to do with them?

Since there IS a change in resistance just adjust the following amplifier stages accordingly...

5. ### MicroSprynter Thread Starter New Member

Jan 16, 2012
11
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The 2.8k resistance is when it has been bent to 180 degrees of deflection (effectively the maximum).
I am using it in the timing section of a 555 monostable circuit (R1 being the flex sensor). This 555 timer has to create a pulse ranging from 1 and 2 ms. Ive done the maths, and determined that in order for this to happen, the resistance also needs to double. Therefore, the tiny change of resistance (compared to the overall resistance of the flex sensor) is not good enough; hence me asking the question in the first place.
Thanks again for taking the time to help out a fellow engineer!

6. ### THE_RB AAC Fanatic!

Feb 11, 2008
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Good luck with that bit!

Normally with those sensors you would read then with an opamp and microcontroller ADC, and part of the micro's job would be to use a lookup table to give you a linear response of bend angle to output number.

7. ### praondevou AAC Fanatic!

Jul 9, 2011
2,936
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You can apply a voltage that changes with the flexsensor resistance to pin 5 of the 555. You will even be able to more than double the pulse width.. just the basic monostable configuration.

Connect the flexsensor to an Opamp configuring it so that you get a good voltage swing at the output. The Opamp output goes to pin 5.

8. ### MicroSprynter Thread Starter New Member

Jan 16, 2012
11
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Thanks, I will try that as soon as I get a chance, and then report back.

9. ### MicroSprynter Thread Starter New Member

Jan 16, 2012
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Still not working guys!
I have measured the resistance across each one (bent and not bent), and found that one of the sensors has a much larger range of about 20k to 60k. This would be perfect, but the others do not behave like this. After inspection, it turns out that the sensor in question has more resistive strips on it. I think I will contact the company and trade in the 4 low range sensors for 4 more like the one I am talking about.
Speak later... ... ...

10. ### praondevou AAC Fanatic!

Jul 9, 2011
2,936
488
Ok, so they sent you the wrong sensors...

Good luck

11. ### John P AAC Fanatic!

Oct 14, 2008
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A pulse from 1msec to 2msec--do I smell a servo?

I really do think that running the signal through a microcontroller would be the best way to do it. You can get to the point you need with op amps and analog components, but the processor would give you better control over how it works.

A while ago I did operate a servo using a single 555, but I used (as Praondevou suggested) the control terminal. And I was only trying to drive my servo to 2 positions.

12. ### hgmjr Moderator

Jan 28, 2005
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Post the schematic of your 555 here so that we can see how you have connected up the sensor to control ITS frequency.

hgmjr