fsr circuit design feedback

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ketem13
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fsr circuit design feedback

Post by ketem13 » Wed Jan 13, 2021 1:59 pm

Hi there,
I made that fsr circuit that was based on the circuit by a fellow member from the topic in here

here is my adaption:
fsr circuit.jpg
I added a potentiometer to act as attenuverter for the fsr.
it seems that the output is between 0 to around 4V when fully cw and 0 to -4V when fully ccw.
when the pot at the middle no cv is output.

any feedback about that design ? what you would change? maybe some of the resistor values?
any suggestion to add a gate and/or trigger output? (I would like to keep it simple without adding a potentiometer)

many thanks!
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Re: fsr circuit design feedback

Post by The Peasant » Wed Jan 13, 2021 3:13 pm

I'm not sure exactly what you are looking for in terms of feedback. If you want more than 4 volts output, increasing the 100K feedback resistor in the final op amp stage will do that.

There are a few different options for FSR circuits in that other thread, including gate and trigger. You really do require a potentiometer to set the level where they activate, although you could use fixed resistors instead I suppose. If you have any specific questions I would be happy to help.

Take care,
Doug
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Re: fsr circuit design feedback

Post by The Peasant » Wed Jan 13, 2021 3:15 pm

The Peasant wrote:
Wed Jan 13, 2021 3:13 pm
I'm not sure exactly what you are looking for in terms of feedback. If you want more than 4 volts output, increasing the value of the 100K feedback resistor in the final op amp stage will do that.

There are a few different options for FSR circuits in that other thread, including gate and trigger. You really do require a potentiometer to set the level where they activate, although you could use fixed resistors instead I suppose. If you have any specific questions I would be happy to help.

Take care,
Doug
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Re: fsr circuit design feedback

Post by cackland » Wed Jan 13, 2021 3:42 pm

Is the actual FSR design a pcb interleaved pattern or the standard FSR component you can buy off the shelf?

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Re: fsr circuit design feedback

Post by ketem13 » Thu Jan 14, 2021 2:01 am

The Peasant wrote:
Wed Jan 13, 2021 3:13 pm
I'm not sure exactly what you are looking for in terms of feedback. If you want more than 4 volts output, increasing the 100K feedback resistor in the final op amp stage will do that.

There are a few different options for FSR circuits in that other thread, including gate and trigger. You really do require a potentiometer to set the level where they activate, although you could use fixed resistors instead I suppose. If you have any specific questions I would be happy to help.

Take care,
Doug
Thanks Doug, I will try your idea with fixed resistor instead of potentiometer for the gate output

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Re: fsr circuit design feedback

Post by guest » Thu Jan 14, 2021 2:44 am

if space is a concern, you can just tie the FSR to +12V, that would free up that other opamp to make your gate circuit. you can put another 6.8k resistor in series with it if the voltage goes to high for your tastes.
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Re: fsr circuit design feedback

Post by ketem13 » Thu Jan 14, 2021 3:20 am

guest wrote:
Thu Jan 14, 2021 2:44 am
if space is a concern, you can just tie the FSR to +12V, that would free up that other opamp to make your gate circuit. you can put another 6.8k resistor in series with it if the voltage goes to high for your tastes.
Actually space it is a concern.
But how can I make gate output from one opamp?
When looking at the schematics by Doug in the other thread I linked here, it uses lm311 and lm358 to make the gate output.
How can I make gate output with only one opamp? it is even possible?

Thanks

Edit: Anyway I will need to use the lm311 as comparator? So I could save the extra opamp from the fsr and use it to buffer the gate output from the comparator ?

Edit2: So I'm using the lm 311 and an opamp to make gate output like in Doug schematics. How can I limit the voltage to be around 6-8V and not +12V as it seems to be right now ?

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Re: fsr circuit design feedback

Post by ketem13 » Thu Jan 14, 2021 4:33 am

Here is the addition with gate out using lm311 and the extra opamp from the TL074:

What do you think?
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Re: fsr circuit design feedback

Post by cackland » Thu Jan 14, 2021 4:46 am

cackland wrote:
Wed Jan 13, 2021 3:42 pm
Is the actual FSR design a pcb interleaved pattern or the standard FSR component you can buy off the shelf?
ketem13 wrote:
Thu Jan 14, 2021 4:33 am
??

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Re: fsr circuit design feedback

Post by ketem13 » Thu Jan 14, 2021 4:52 am

cackland wrote:
Thu Jan 14, 2021 4:46 am
cackland wrote:
Wed Jan 13, 2021 3:42 pm
Is the actual FSR design a pcb interleaved pattern or the standard FSR component you can buy off the shelf?
ketem13 wrote:
Thu Jan 14, 2021 4:33 am
??
Sorry I don't think I understand your question..? If you asked what fsr it is so I'm using that

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Re: fsr circuit design feedback

Post by cackland » Thu Jan 14, 2021 5:00 am

Right ok, I thought you might have implemented a PCB interleaved copper design.

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Re: fsr circuit design feedback

Post by guest » Thu Jan 14, 2021 5:04 am

if you want to save a part, you can skip the LM311 and just put a diode on the output of the opamp to keep it from going negative. also, take the comparison signal from your first opamp, that way the attenuverter doesnt mess it up.
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Re: fsr circuit design feedback

Post by ketem13 » Thu Jan 14, 2021 5:36 am

guest wrote:
Thu Jan 14, 2021 5:04 am
if you want to save a part, you can skip the LM311 and just put a diode on the output of the opamp to keep it from going negative. also, take the comparison signal from your first opamp, that way the attenuverter doesnt mess it up.
I made it like that:
fsr w gate 2.jpg
it seems to output around 6.5V gate but I also have a constant offset of around 93mV at the output, is it normal ?
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Re: fsr circuit design feedback

Post by guest » Thu Jan 14, 2021 7:10 am

i meant that the opamp now becomes the comparator. so put your resistor divider on the inverting pin, and the fsr signal to the noninverting pin. then have the diode from the output of the opamp to a 10k resistor to ground. take your 1k from the diode to output.
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Re: fsr circuit design feedback

Post by ketem13 » Thu Jan 14, 2021 7:52 am

Thanks! it seems to work very fine!
fsr w gate 2(1).jpg
my next wish is making something like the tuned cv output of the pressure point.
for the moment I'm using a transistor and a potentiometer as a voltage divider in order to create tuned cv output:
tuned cv.jpg
right now it is not the most stable thing. I guess it is better to use lm4040 in order to create stable voltage source to connect to the collector?

for the moment as long my gate is high a tuned cv is presented at the output of the lug 2 of the potentiometer but the moment I release my finger the gate is of and there is no tuned cv at the output.

lets say I will make that circuit three times - how can I make the tuned cv to be presented as long I did not press different fsr ?
so if I press the first fsr and release my finger the tuned cv will be on and only when I press different fsr the tuned cv will be switched to the next fsr I pressed ? Like in the MN pressure points..
Any hint for that ?

Hope I made myself clear..

Thanks
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Re: fsr circuit design feedback

Post by guest » Thu Jan 14, 2021 8:09 am

if you want to switch output CVs, an easy way would be to use an analog switch. maybe a 1 of 4 selector (for your 3 CVs). you could have potentiometers to +V, with the wipers going through the switches to the output. perhaps you could buffer the output with an opamp to give a more stable output and protect the switch. then you need some logic that selects the switch based on the most recent rising edge. that could be done with D flip-flops (or another latch of some sort) that clears all other latches when set.

you might not need the switches if you take the output of the latches and drive the potentiometers with those. when off, they will be at 0V, so a mix of them should just show the one thats on.
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Re: fsr circuit design feedback

Post by ketem13 » Thu Jan 14, 2021 8:21 am

guest wrote:
Thu Jan 14, 2021 8:09 am
if you want to switch output CVs, an easy way would be to use an analog switch. maybe a 1 of 4 selector (for your 3 CVs). you could have potentiometers to +V, with the wipers going through the switches to the output. perhaps you could buffer the output with an opamp to give a more stable output and protect the switch. then you need some logic that selects the switch based on the most recent rising edge. that could be done with D flip-flops (or another latch of some sort) that clears all other latches when set.

you might not need the switches if you take the output of the latches and drive the potentiometers with those. when off, they will be at 0V, so a mix of them should just show the one thats on.
D flip flop like the cmos 4013 ?

is there any schematic out there that doing +- the above ? or it is time to dive into data sheets ?

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Re: fsr circuit design feedback

Post by guest » Thu Jan 14, 2021 8:49 am

yes, 4013. i dont know of any schematics offhand, but there might be. its sort of similar to an analog sequencer, except with latches instead of a shift register.
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Re: fsr circuit design feedback

Post by ketem13 » Thu Jan 14, 2021 10:49 am

guest wrote:
Thu Jan 14, 2021 8:49 am
yes, 4013. i dont know of any schematics offhand, but there might be. its sort of similar to an analog sequencer, except with latches instead of a shift register.
any other cmos I can use instead of the 4013?
of course I will have many other cmos but not the one I needed... :despair:

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Re: fsr circuit design feedback

Post by johnstilton » Thu Jan 14, 2021 11:03 am

The Peasant wrote:
Wed Jan 13, 2021 3:13 pm


Take care,
Doug
Off Topic: Thank you Doug for maintaining your website full of really great DIY builds. It is an inspiration!"

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Re: fsr circuit design feedback

Post by The Peasant » Thu Jan 14, 2021 12:33 pm

ketem13 wrote:
Thu Jan 14, 2021 4:33 am
Here is the addition with gate out using lm311 and the extra opamp from the TL074:

What do you think?
Be careful with this type of circuit, maximum specification is 1mA FSR current per square centimeter of applied force, and your circuit potentially comes close to that.

This design guide can be very helpful: https://cdn.sparkfun.com/assets/4/d/0/f ... _0.5in.pdf

johnstilton wrote:
Thu Jan 14, 2021 11:03 am
The Peasant wrote:
Wed Jan 13, 2021 3:13 pm


Take care,
Doug
Off Topic: Thank you Doug for maintaining your website full of really great DIY builds. It is an inspiration!"
Thank you for the nice comment John, I'm glad that I am able to help others on their DIY journey!

Take care,
Doug
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Re: fsr circuit design feedback

Post by ketem13 » Thu Jan 14, 2021 12:45 pm

hi,
where do you see that? in what part of my circuit I am getting close to that?
(I'm still learning and probably jump above my understanding ..)
The Peasant wrote:
Thu Jan 14, 2021 12:33 pm
ketem13 wrote:
Thu Jan 14, 2021 4:33 am
Here is the addition with gate out using lm311 and the extra opamp from the TL074:

What do you think?
Be careful with this type of circuit, maximum specification is 1mA FSR current per square centimeter of applied force, and your circuit potentially comes close to that.

This design guide can be very helpful: https://cdn.sparkfun.com/assets/4/d/0/f ... _0.5in.pdf

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Re: fsr circuit design feedback

Post by The Peasant » Thu Jan 14, 2021 12:51 pm

Current through the FSR is limited by the two 6K8 resistors in series, with 12 volts/13K6 ohms = 0.88mA. Probably safe but if you are using 15 volts then the resistor values should be increased.

There is a bit more circuit detail here:

https://www.sparkfun.com/datasheets/Sen ... rguide.pdf

Take care,
Doug
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Re: fsr circuit design feedback

Post by The Peasant » Thu Jan 14, 2021 1:03 pm

This is assuming an approximate 1 square cm contact area. Using your finger on the sensor would probably result in an area approximately half of that, so if you push relatively hard on the sensor so that it's internal resistance gets below 1Kohm, current per square cm could approach double that, which would be above maximum specification. My original circuit uses a 0.6 volt reference for driving the FSR, which is low enough to stay well out of danger under all circumstances.

Take care,
Doug
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Re: fsr circuit design feedback

Post by ketem13 » Thu Jan 14, 2021 1:19 pm

The Peasant wrote:
Thu Jan 14, 2021 12:51 pm
Current through the FSR is limited by the two 6K8 resistors in series, with 12 volts/13K6 ohms = 0.88mA. Probably safe but if you are using 15 volts then the resistor values should be increased.

There is a bit more circuit detail here:

https://www.sparkfun.com/datasheets/Sen ... rguide.pdf

Take care,
Doug
in my case the two 6k8 in series you mean to those highlighted in green ?
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