Tutorial: Op-amp circuit for force sensor signal conditioning--- make money online

hey everyone in this video I'm going to

show you a circuit that you can use to

interface this force sensor with a

microcontroller or some other analog

input device so this is a flexi force a

4:01 force sensor and it's a sort of

like a variable resistor so let me zoom

in and show you how to how to make this

circuit okay so this force sensor is

built by laminating two pieces of

plastic with some conductive ink on the

inside so the ink is facing the two

inked surfaces are facing each other so

when you apply force this way on the

sensor the two conductive inks are

pressed together and the resistance goes

down so if we wanted to interface this

with a microcontroller 18 mega or an

Arduino or any other sort of analog

capture device we need to figure out how

to get this signal into the analog

digital converters so you might be

thinking well if it's a variable

resistance we can just draw that as a

resistance that's variable and to

measure this variable resistance what we

could do is just add another fixed

resistor to ground and put this up to

some plus voltage source and then take

our output here so we have a voltage

divider but there's a problem

this sensor actually has a curve that

looks like this so if this is force and

this is resistance the the graph

actually looks like this so if we were

if we were to make a voltage divider

like this it would work we could measure

the voltage using a simple divider like

this but in our software in the firmware

we have to compensate for this

weird-lookin curve so if we actually

wanted to make real meaningful force

values force measurements with this

sensor we'd have to compensate for the

this strange-looking curve however lucky

for us here's there's a way to get

around this so the manufacturer of the

sensor has given us this helpful chart

showing us that here's there's the

resistance curve and if we happen to

take one over the resistance well

we get is this line which is nice and

linear so one over R is the conductance

of the sensor and the conductance is

actually very linearly related to the

applied force so as we add more and more

force the conductance goes up linearly

so the trick is how do we make a circuit

that will actually measure the

conductance of that force sensor that

way we can just sample the output

voltage or sample the conductance and

have a linear representation of how much

force is being applied so again lucky

for us the manufacturer has given us

this little chart here or this little

diagram and this is a very simple op-amp

circuit if you're familiar with op amps

this is a non-inverting amplifier and

what they've done is put the variable

resistance of the force sensor between

the inverting input and a negative

voltage source now in this example they

show the feedback resistor being

variable but it could be fixed and they

even helpfully say the feedback resistor

should be about 1k to 100k and the

reason for that is when the force sensor

is getting near its limit in terms of

how much force you can apply the

resistance across the sensor will be you

know in the range of 10 to 20 kilo ohms

so you want to get this value fairly

close to this value when it's at its

maximum and the reason for that is this

formula here so the output voltage is

equal to negative VT which is this

negative voltage that we're applying

times our feedback over our series so if

the feedback resistor and the series

resistor are about the same value then

we're just going to get this negative

voltage times negative 1 so a positive

voltage coming out here so let me hook

the sensor up to the meter and show you

this ok so when no load is applied to

the sensor it's essentially open more

than 40 mega ohms on this meter and if

we apply some force to the sensor the

resistance comes down so we've got about

800 K there and if I push really hard

the resistance goes down further now

we've got about 80 K and if I use a

bigger area the resistance also goes

down so for the same force that spread

over a bigger area there will also be a

lower resistance if I use my thumb to

push this down and I'm pushing pretty

hard I'd say I've got maybe 20 pounds of

force on there it's down to maybe 50 K

if I push really hard 30 K so it's tough

to get it below 30 K let's say with

finger pressure so one way to build this

circuit would be to desire like a 0 to 5

volt analog output so if your if we were

using an AVR chip with a 5 volt supply

rail it's analog/digital converter would

be comfortable with with input voltages

between 0 and 5 volts so let's just say

we want an output range of 0 to 5 and we

know that the sensor gets down to maybe

30 K ohms when it's fully depressed when

it's when it's experiencing the maximum

load for our application and also let's

just say that our negative supply

voltage here is negative 5 so if we

write out this equation we've got let's

go for the high case so let's say at

maximum we've got 5 volts out is equal

to negative times negative 5 volts in

times our feedback over the sensor

resistance and we just said that's about

30 K for for maximum load so if we look

at this obviously our feedback is also

going to be 30 K so we can cancel the

fives out and there you go so to give

this a little bit more Headroom I'm

going to choose a slightly lower value

so what would happen if this if this

were exactly 30 K and you pushed harder

on the sensor there would be no

additional output swing so you would

sort of top out and we don't want to run

out of measurement room so by charging

it by choosing a lower feedback resistor

the sensor will be

sure not to run out of a measurement

room here so I want to pick a value of

22 K for the feedback resistor 22 K ohm

the next question is where do we get

negative 5 volts now if we're building

this circuit and it's going to be

powered by USB or powered by a standard

wall wart type thing where do we get the

negative 5 volts one option is to use a

part like this this is a self-contained

DC to DC converter and it is part number

sr9851 of course Newark or mouse or any

of these other guys have it too and

inside here this is basically a 5 volt

to 5 volt converter but the the nice

thing is that the output is isolated so

if you put 5 volts in you can connect

the positive output to the negative

input and then get negative 5 volts out

so in my circuit I use this to supply

the negative 5 volts that we need to put

in over here okay so now we know the

feedback resistance that we want we know

how to get the negative 5 volts and

we've got the basic overall circuit

layout from from the manufacturer and

they recommended that we use an MCP 6004

or op amp which might be fine i've never

used one of those before i don't know

what the details are I actually happen

to like LM 324 s and I read somewhere on

the forums that someone said this is

like the worst op-amp that's still in

production today or whatever I I don't

know if that's true or not but I will

tell you why I like using this chip for

this application I'm going to be

measuring forces that are essentially

not changing I mean an AC is not really

a concern in this circuit I basically

just want to measure constant force

value and any rapidly changing forces

are not going to be an issue so this

op-amp I don't think has very great AC

characteristics but as far as DC

voltages it's fine and there's a couple

of the nice things about it one is that

the output can go all the way to zero

volts so if we're supplying the op-amp

with

v+ and zero the outputs will actually

get all the way down to zero which is

very handy if you started to work with

op amps you might have been you might

have realized that a lot of them require

very high voltages on the supply rails

both positive and negative and so having

an op amp it's able to go down to ground

is helpful

now the outputs won't go up to the v+

the I think they speck at it a volt and

a half under yeah so the output swing is

zero volts to V plus minus 1.5 volts so

for supplying the op amp with 5 volts

the output will never be higher than 3

and a half which is a problem because

we'd be losing signal you know this

thing would never supply a full 5 volt

signal to our ADC to our 18 mega or

whatever we're feeding so one solution

is to have a few different voltage

regulators in your circuit and you can

feed this chip 6 volts and the rest of

the circuit 5 you know five for the AVR

and that way you can get up to four and

a half volts out on this or use an 8

volt regulator or whatever you want just

just so it's able to have a 5 volt

output swing another benefit of this

op-amp is that it's a quad and it's

available in all kinds of different

packages you can still get this easily

in dip packages and I like through-hole

parts so that's that's nice and you know

it's reasonably okay I don't think it's

that bad of an op-amp but everyone has

their own opinions you can put that in

the comments now there's one more

addition to this circuit since I said

that I'm mostly interested in just

measuring steady-state force values I'm

not really interested in in high

frequency anything we're going to add

some low-pass filters to this circuit

and the easiest way to do that is to put

a capacitor here now what this does is

it creates a single pole low-pass filter

again not the greatest performing thing

but in this case we can really wipe out

I mean lots of stuff so I'm going to but

let's figure out the value so let's say

we wanted to wipe out most of the AC we

particularly want to wipe out 60

curves because this system is going to

pick up 60 Hertz noise from all the the

wiring it's actually going to have

fairly long wires on it and the

frequency that this is going to start

cutting is given by the formula 1 over 2

pi RC so RS here C is here and this this

formula will give us the cut frequency

in Hertz so the cut frequency looks kind

of like this if we had a frequency here

and gain or amplitude here and this is a

log-log scale

I think the curve would look something

like this and this is going to go down

it guy forget 10 DB per decade or

something I don't know you can look it

up single pole filter but it's going to

come on the specific frequency and this

frequency is the cut frequency given by

1 over 2 pi RC so if we were really

concerned about exactly what frequency

we wanted we could figure out exactly

what capacitor should be used there but

in this case we're kind of limited by

what capacitors are available so I'm

just going to say let's use 1 over 2 PI

R which is 22 K and C is going to be 0.1

micro farad's so 0.1 e negative 6 and

this comes out to be 72 Hertz so it's

not quite below 60 so ideally this

should actually be a little bit higher

we could have you know I don't have

anything quite bigger than this that

isn't already 1 micro farad and for

reasons that I'll get into later it

shouldn't be an electrolytic okay so I

did some testing and it looks like the

60 Hertz noise is not a problem so I'm

going to stick with this and maybe add

another capacitor between the inputs on

the op-amp if necessary so if you're new

to op amps you might be thinking well

what we could do to add some more

filtering is just

from the output here just you know add a

capacitor going right to ground like

that no you cannot do this most op amps

will freak out majorly if you do this

and the reason is that the output here

sets up an oscillating circuit with this

capacitor this op-amp does not have zero

output impedance like you would expect

from the theory I mean if you spend a

long time looking at op amp Theory you'd

figure well sure this would be okay but

the problem is that there's actually

output impedance with it with any op amp

with any real op amp and if you add a

capacitor here you there's a really good

chance you're going to set up a resonant

circuit so at the very best what you

could do is add a resistance and then

you basically have another single pole

low-pass filter here just like you have

here so if you add them together I think

then you have a two pole filter and you

can have them at the same cut frequency

or even different cut frequencies then

the only problem is your output

impedance going this way is going to be

affected by how big that resistor is so

in this case I don't want to do this I'm

just going to take the output straight

from the straight from the op-amp so

that I have a fairly good output

impedance and do all the filtering here

or even before ok just like on a cooking

show here's one that I've made

previously this actually has 10 channels

to it but it's the same circuit over and

over again just you know done 10 times

and these long cables are just that I

can have the four sensors relatively far

away from this circuit this is going

into a much larger installation and the

four sensors are going to be some

distance away from this circuit so

there's our negative 5 volt generator

and you might be thinking well this is

no good because it's it's i think the

switching frequency on that's 40

kilohertz or something and all that

noise is going to propagate right into

our output signal but actually if you

don't load this at all heavily the

output noise is very very low these

things will get noisy if you start

loading up the output but when they're

unloaded it's actually quite clean so

I'm going to feed this whole circuit

with an analogue a linear

wallboard I have had problems using

switching power transformer switching

power supplies with analog circuitry and

I would pretty much universally

recommend against that those those

switch mode power converters are great

for digital circuits but are really

terrible for anything analog so in this

case I have unregulated 9 volts coming

in from the wall wart and I've got a 5

volt regulator here which just drives

gives this power and I've got an 8 volt

regulator here which gives the op amps

power so 8 volts going to the op amps

means that the output can swing up to

six and a half volts so the output will

always be between zero and six and a

half volts and normally that would not

be preferable for interfacing to

something like an AVR because this thing

will freak out if you give it more than

five and you might run into some current

dissipation issues but in this case this

will actually be connected to this which

can handle negative 10 to +10 on its

inputs I would actually not recommend

this for hobby use just because it's

kind of overkill this is going into a

much larger installation with with many

other pieces but for hobby use National

Instruments makes a USB 6008 which costs

$150 and more or less has half the

functionality it's basically half of one

of these okay so let's plug it in and

try it out so here's the the power

supply that I'm going to use it's just a

9 volt unregulated 500 milliamp supply

okay plug that in and that's 2.1

millimeter barrel connector plug that in

and I've got the output from this

circuit wired to the scope and here's

the force sensor plugged into my three

miles of wire there so when I press on

the force sensor you can see the output

on the scope so the scope is in free run

mode and if I push on the sensor you can

see what the waveform looks like the

force waveform and if I tap on the

sensor like this you can see the the

step response there so what we could do

is measure that waveform and see how the

step response is um not quite ideal some

of that smoothing is happening because

my finger is soft and my step on this

thingy the force is not increasing

instantly fast but there's also some

smoothing that's happening because of

the filter that we implemented so step

responses on the force sensor will not

be perfectly square because that

low-pass filter is going to start

smoothing things down so if I just press

on this and hold it steady you can see

there I think I have it set for a volt

per division on the oscilloscope you can

see the response is pretty good it's

working normally so one other tests to

make is to switch this to AC coupling

and I'm going to press the auto setup

button just to see what we can see so

there is some noise on the on the output

what's happened here is I've switched to

AC coupling and it's the scope

automatically chose 5 millivolts for

division which is

as low as this thing can go and we've

got maybe you know 5 10 so 20 20

millivolts peak peak noise on there

which is you know not great but not bad

either

I've noticed in a lot of circuits just

adding the oscilloscope to to the

circuit introduces quite a bit of

measurement noise

however this spiking here is most likely

from the 5 volt 5 volt converter if we

had a true linear supply you could

probably get some of that noise down

here's the signal at 20 millivolts per

division and if I you know tap on the

sensor you can see how big the signal is

come here I'm just pushing on a sensor

very lightly you can see how big the

signal is compared to the noise so you

know the noise signal-to-noise ratio in

this system is quite high I could do a

measurement but just looking at it

graphically is usually the best way you

can tell that it's quite good

[Music]

okay well I hope that was helpful I'm

definitely not an op-amp expert but if

this was at all useful to you guys maybe

I'll do another one on on op amps that

are actually working in AC circuits

instead of just more or less a DC

conversion okay see you next time bye best way to earn money online, make money from home, make money online free, money making sites, earn from home, real ways to make money from home, online income ideas, make real money online, top online earning websites, earn money online fast, top 10 money earning websites, earn money without investment, earn money online for students, best sites to earn money, make money online 2019, earn easy money online, earn money online instantly, real money earning sites, make money from home online, make money online fast, earn dollar online free, online earning ways, best online money earning, earn money online with surejob, make money online, online money making sites, best sites to make money online, earn money online paypal, online earn money by typing, best survey sites, ways to earn money online, ways to earn money from home, make money online without investment, make money online surveys, 

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