Force sensor voltage divider vs adjustable buffer

[shivasage]

Hello everyone. I'm planning a project which involves using a force sensing resistor (FSR) to convert force to voltage, but I'm confused as to some details (I'm a total newb, so bear with me). I'm considering using the Interlink FSR 408, and I found a useful integration guide for it. On pages 18 and 19 of the guide, there are schematics for a voltage divider and two types of adjustable buffer. I have some questions about these:

1) Based off of the schematics, these circuits require: FSR, opamp(s), resistor(s), potentiometer(s), and... what else? A 9V battery? Anything else?
2) According to the guide, one difference between the voltage divider and the adjustable buffers is that the adjustable buffers allow adjustment of the gain. In this case, what is 'gain'? Is it simply referring to a further increase of the output voltage, perhaps a multiplier of the applied force?
3) The adjustable buffers also allow adjustment of 'output offset.' What is this? The guide states that "offsets resulting from the non-infinite FSR resistance at zero force (or bias currents) can be trimmed out with a potentiometer." Is this a long-winded version of saying that I can use a pot to eliminate extreme voltage outputs that happen when I'm not applying any force to the sensor? Or perhaps it has to do with a attaining a better force-to-voltage curve by eliminating the response of the sensor when no force is applied?

Any help would be greatly appreciated!

 

[Pommie]

I would buy the cheapest one from sparkfun and have a play with it. Read the hookup guide on the sparkfun website, google "Arduino FSR" and you'll find lot's of example circuits/code.

Mike.

 

[DrG]

Agree with what Pommie has said. I have used these before and they work well and are easy to understand. Tell us what your project is - that is, what are you trying to do. That will help us help you.

1) Based off of the schematics, these circuits require: FSR, opamp(s), resistor(s), potentiometer(s), and... what else? A 9V battery? Anything else?

You would want a meter to measure voltage out. Use of a 9V battery for power may or may not be what you need as that will depend on the OP amp you are using. For understanding, those three graphs are basically assuming V+= 5V and GND = 0V. Since the OP amp is not given, let's assume that you chose an MCP601 and V+=5V and GND=0V. Again, many different OP amps could be used.

Stick with the first circuit initially. Once you understand that circuit, you will be able to understand the other two.

Look at this portion of that circuit:

That is a simple voltage divider using two resistors; the top resistor is the FSR (which is a variable resistance, in Ohns, depending upon the force applied) and the bottom resistor is a fixed resistor of some value (in Ohms). Look here for a decent tutorial.

When you add the op amp, you are buffering the voltage out.

At this point, some recommendation for an introduction to OP amps is probably called for. Look online for a simple one. Many times you will see these circuits used with V+ and V- (e.g., +9V and -9V) rather than V+ and GND (e.g., +5V and 0V). That might confuse you at first. I suggest that, at least initially, use an OP amp with V+ and GND (there are ones [like the MCP601] specifically for that, called single supply op amps - although EE's can take the mystery out of the +/- aspects).

Looking at the graph for the voltage out for the circuit, you can see that it depends on the value of the fixed resistor (RM) and the resistance of the FSR.

You should be able to see that so well that given the value of RM and the voltage out, you should be able to calculate the force applied to the FSR.

Now, you have to reveal what you want to do with it because it is going to determine how you use it and whether you use circuit 2 or 3 or something else.

Say, for example, that you want the circuit to "trigger" when the force applied is, at least, some amount. But you do not exactly know how much - say a tap by a finger or when something is dropped on the FSR. In that case, you want a Yes/No response from the OP amp, but you may want to adjust the threshold of the Yes/No. Take a look for an example of such a circuit (first schematic, middle). Note that there is the addition of R2 (look up hysteresis or use the link in that article - but dont worry about it at this point). That is a simple application that is used to provide an input to a microcontroller to detect when some threshold force was applied to the FSR - what's more, the threshold force could be set using the potentiometer.

As for your other two questions, you have that right, sort of, kinda...get the first part of the understanding down first - for better or worse, that is what helped me.

 

[Pommie]

What I tried to say and Dr G said much better is start playing and learning. This is all complex stuff and you'll need time and experience to fully understand it. However, once you fully undestand it you'll have a very valuable feather in your cap.

Mike.

 

[shivasage]

Agree with what Pommie has said. I have used these before and they work well and are easy to understand. Tell us what your project is - that is, what are you trying to do. That will help us help you.



You would want a meter to measure voltage out. Use of a 9V battery for power may or may not be what you need as that will depend on the OP amp you are using. For understanding, those three graphs are basically assuming V+= 5V and GND = 0V. Since the OP amp is not given, let's assume that you chose an MCP601 and V+=5V and GND=0V. Again, many different OP amps could be used.

Stick with the first circuit initially. Once you understand that circuit, you will be able to understand the other two.

Look at this portion of that circuit:
View attachment 121884
That is a simple voltage divider using two resistors; the top resistor is the FSR (which is a variable resistance, in Ohns, depending upon the force applied) and the bottom resistor is a fixed resistor of some value (in Ohms). Look here for a decent tutorial.

When you add the op amp, you are buffering the voltage out.
View attachment 121886

At this point, some recommendation for an introduction to OP amps is probably called for. Look online for a simple one. Many times you will see these circuits used with V+ and V- (e.g., +9V and -9V) rather than V+ and GND (e.g., +5V and 0V). That might confuse you at first. I suggest that, at least initially, use an OP amp with V+ and GND (there are ones [like the MCP601] specifically for that, called single supply op amps - although EE's can take the mystery out of the +/- aspects).

Looking at the graph for the voltage out for the circuit, you can see that it depends on the value of the fixed resistor (RM) and the resistance of the FSR.

You should be able to see that so well that given the value of RM and the voltage out, you should be able to calculate the force applied to the FSR.

Now, you have to reveal what you want to do with it because it is going to determine how you use it and whether you use circuit 2 or 3 or something else.

Say, for example, that you want the circuit to "trigger" when the force applied is, at least, some amount. But you do not exactly know how much - say a tap by a finger or when something is dropped on the FSR. In that case, you want a Yes/No response from the OP amp, but you may want to adjust the threshold of the Yes/No. Take a look for an example of such a circuit (first schematic, middle). Note that there is the addition of R2 (look up hysteresis or use the link in that article - but dont worry about it at this point). That is a simple application that is used to provide an input to a microcontroller to detect when some threshold force was applied to the FSR - what's more, the threshold force could be set using the potentiometer.

As for your other two questions, you have that right, sort of, kinda...get the first part of the understanding down first - for better or worse, that is what helped me.

Thanks very much for your response. I read up on some things, including what you suggested. Regarding what I'm using this for, I want to use it as a pressure sensor on a guitar which uses the output voltage to interact with effect unit that accept 0 to +5V CV. I'm thinking I don't even need to bother with Arduino in this case, right? I more or less get the circuit now, so I plan on buying the sensor, the LM358 opamp as per the guide (or maybe there's another you recommend?), some pots/resistors, and some breadboard, and then start testing things out.

So to simply the adjustable buffer circuit, it's basically a voltage divider using a resistor chosen to get the voltage output in the right ballpark, then there's a potentiometer to fine tune the gain, i.e. fine tune how much force is needed to get the desired output. Is that correct? The schematic in the link you sent me also has a hysteresis resistor, but it doesn't seem I need this because this isn't a toggle on/off situation.

The only thing left that I don't understand is the second potentiometer in the guide I linked, which is used to trim output offsets. I don't see this in the link you sent, unless it's functioning similarly to the hysteresis resistor. Is it the same thing? And whatever this thing is, why would I want to isolate it from the gain pot, as in the 3rd schematic from the link I sent?

Oh yeah, and for powering the circuit via the breadboard, I can just do this with a 5V battery, yeah? The opamp operates from 3V to 32V.

 

[DrG]

Glad you are getting in to it some more. I think that if you get some parts and start to see how they act, you will get a feel for the circuits and the characteristics. I recommended the MCP601 OP Amp and +5V and 0. The LM358 is a little bit different, but you can experiment with that as well.

No, hysteresis is different than the third circuit. The link I posted was to give you an example, but as I said, its use was as a simple switch with an adjustable threshold. You should be able to build all three circuits and see how they are working.

I appreciate your attentiveness to accuracy, but I would think through what you are asking the circuit to do with that same attentiveness...

From the guide (p. 8): Start with Reasonable Expectations (Know Your Sensor) The FSR sensor is not a strain gauge, load cell or pressure transducer. While it can be used for dynamic measurement, only qualitative results are generally obtainable. Force accuracy ranges from approximately ± 5% to ± 25% depending on the consistency of the measurement and actuation system, the repeatability tolerance held in manufacturing, and the use of part calibration.

...and what you want to do:

Regarding what I'm using this for, I want to use it as a pressure sensor on a guitar which uses the output voltage to interact with effect unit that accept 0 to +5V CV.

So, discuss this a bit because it is integral to being successful....you want to press on the FSR with your finger (or pedal or ?)) to, produce a voltage from 0-5V. That output will provide input to another device which will vary some parameter...some guitar effect. Whatever the effect is, 0V=no effect and 5V=maximum effect. Is that basically it?

So, go back to the Voltage vs. Force graph in the guide. Ideally, I think you would like the slope of that function to be 1 (nice linear diagonal line with 0g force=0 volts and 1000g force=5 volts). Note that none of the functions in the graph have a slope of 1 for that first circuit.

You also want to be able to "press" on the sensor to get that full range - can you do that reliably and with some precision? - I don't know. Given as much as a 25% deviation in accuracy, that means that the same force applied by your "press" (let's say 400 g) can give a voltage results comparable to a force range of 300-500g - regardless of how skilled you are at pressing - at least that is the way I am reading it....and you need to be skilled enough to produce the target force repeatedly.

Bottom line is that you need to "play" with this device for a while and see if and how you can hook it up to meet your needs. I don't want to be discouraging, but It may not be your best choice for what I think you want to do.

 

[shivasage]

So, discuss this a bit because it is integral to being successful....you want to press on the FSR with your finger (or pedal or ?)) to, produce a voltage from 0-5V. That output will provide input to another device which will vary some parameter...some guitar effect. Whatever the effect is, 0V=no effect and 5V=maximum effect. Is that basically it?

Yes, that is correct. With my finger.

So, go back to the Voltage vs. Force graph in the guide. Ideally, I think you would like the slope of that function to be 1 (nice linear diagonal line with 0g force=0 volts and 1000g force=5 volts). Note that none of the functions in the graph have a slope of 1 for that first circuit.

Yes, I noticed that as well. I was hoping that I would be able to fine tune it (either via the circuit or via the effect pedal) to eliminate the quick jump at the beginning of the curve, at which point the rest of the curve is fairly linear. I also found a circuit which purportedly linearizes the curve. I can't quite tell if it is basically the same circuit as the one from the guide.

I am unfortunately tied to this sensor because of it's length. I was unable to find anything of a similar shape. I did, however, find a series of sensors that supposedly has a more **broken link removed**. The shape is different, but I might be able to change my idea to accommodate (at which point I'd be using a spot on my arm to push on a 2" x 2" square sensor). I dunno, there's really nothing else that will work besides one of these two sensors. What do you think? I don't need to have a perfect response or repeat the same force repeatedly, but I would like the sensor to, at the very least, respond when it's expected to respond, even if the curve isn't the smoothest possible.