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  1. #1
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    Force sensitive resistor controlling output voltage

    Hello. I'm planning a project which aims to regulate an output voltage from 0 to +5V DC by using a force sensitive resistor. I posted about this a while ago, and some forum members helped me create a schematic. Now I'm back. This is for a musical implementation: I own an effect pedal which accepts 0 to +5V voltage to control certain parameters. Thus, the harder I push on the sensor with my finger, the parameters will change.

    Originally I had planned a circuit using an op-amp (moderator action: removed link to external source)...missing some caps). In this schematic, the sensor is represented by the 68k resistor, as it starts at several million ohms with zero pressure and gets down to ~68k ohms with maximum pressure. The 1M resistor represents a pot I will place in the feedback loop to fine-tune the response. The 50k load represents the input of the effect pedal (this value sourced from the owner's manual).

    When attempting to breadboard the circuit, the op-amp malfunctioned and I didn't have a replacement. I didn't want to order parts just yet, so I tried coming up with a simpler circuit (moderator action: removed link to external source) to achieve the same thing. In this schematic, the 100k pot controls the incoming voltage to fine-tune the response. The 6.8k resistor in series is to lower the power running through the pot when it is at minimum resistance to avoid damaging the pot (I have since lowered this value to 2k), and the 50k resistor in parallel has a similar effect of allowing the current to flow elsewhere besides through the pot, and it also smoothed out the taper of the output voltage.

    My question is: Is there any downside to using the simpler circuit vs. the op-amp circuit? One issue that I'm aware of is that the load will have a significant effect on the output. However, because the load will always be the same (i.e. I'm always going to use the same effect pedal), does this matter? I thought it was weird that there was zero current flowing, but from the manufacturer: "the input is 0-5V, current won’t matter because there’s a >100k input resistor and the current is self regulating and protected." (He says 100k but the manual says 50k, not sure which is right, but could be compensated for with the pot.) He also told me the pedal has overvoltage protection, which alleviates another potential concern.

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    Last edited by KlausST; 26th March 2020 at 08:06. Reason: Removed links to ext. file servers

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  2. #2
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    Re: Force sensitive resistor controlling output voltage

    Your second circuit doesn't make a lot of sense to me. First of all, I think you may damage the FSR with such a high voltage. Secondly, what's the point of the cap on the wiper? Thirdly, why do you think you'll damage the pot "when it is at minimum resistance"? It's always a 100K; you've already shown there's less than a milliamp.

    Look at using a simple current source driving the FSR.



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  3. #3
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    Re: Force sensitive resistor controlling output voltage

    Quote Originally Posted by barry View Post
    Your second circuit doesn't make a lot of sense to me. First of all, I think you may damage the FSR with such a high voltage. Secondly, what's the point of the cap on the wiper? Thirdly, why do you think you'll damage the pot "when it is at minimum resistance"? It's always a 100K; you've already shown there's less than a milliamp.

    Look at using a simple current source driving the FSR.
    damage to FSR: yes, that's possible... i couldn't find anything in the datasheet about it though
    cap on wiper: to reduce noise
    damage to pot: it won't always be at 100k... i will adjust it to regulate voltage coming in to FSR; when close to 0 resistance and an 18v source, the simulator gives me 2.6mA

    anyway, i'm not set on this circuit, i figured it was weaker compared to the op-amp circuit. but i wanted to understand why. what do you mean by a simple current source? shouldn't a voltage source be the priority here?



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    Re: Force sensitive resistor controlling output voltage

    A constant current driving a varying resistance gives a varying voltage.

    Forgetting about that 50k, when the wiper is near the top of the pot you’ve got 18/110000= 0.16 ma through the wiper; when the wiper is near the bottom you get 18/210000=86 micro amps.



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    Re: Force sensitive resistor controlling output voltage

    Hi,

    I'd go with the Opamp solution.
    It is simple. Just a couple of parts. And it works perfectly. It's not the problem of the Opamp that it "malfunctioned".

    The drawback of the non Opamp solution is that it is very nonlinear.

    Klaus
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    Re: Force sensitive resistor controlling output voltage

    Quote Originally Posted by barry View Post
    A constant current driving a varying resistance gives a varying voltage.

    Forgetting about that 50k, when the wiper is near the top of the pot you’ve got 18/110000= 0.16 ma through the wiper; when the wiper is near the bottom you get 18/210000=86 micro amps.
    Ok, so something like (moderator action: removed link to external source)? The output voltage behaves necessarily as I sweep the value of the sensor... But this doesn't solve the issue of having too much voltage running through the sensor, right? And how do I fine-tune the response? Do I need a pot in the circuit? I apologize if I'm way off the mark here, I have no training whatsoever.

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    Quote Originally Posted by KlausST View Post
    Hi,

    I'd go with the Opamp solution.
    It is simple. Just a couple of parts. And it works perfectly. It's not the problem of the Opamp that it "malfunctioned".

    The drawback of the non Opamp solution is that it is very nonlinear.

    Klaus
    That's why I originally thought the op-amp was necessary. However, after the op-amp "died,' I breadboarded the simplest possible circuit: 9v running through the sensor. The sweep of output was actually quite linear, which surprised me and made me think I should just do a simple circuit. Perhaps this can be explained by the non-linear nature of the FSR being cancelled out by the non-linear nature of increasing pressure with your fingers?

    EDIT: Apologizes for external links, I will stop.
    Last edited by shivasage; 26th March 2020 at 08:19.



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