Getting higher resoultion from MPX4115

[ahmedragia21]

I'm trying to connect a pressure sensor MPX4144 inside a ballon so that when I pressure the ballon, it does change some color. I tried to hook up the sensor, but I get very small values, should I connect an amplifier to it or what ? I'm sure there is an amplifier inside that sensor, or that sensor can't work for that purpose, like putting it inside a balloon to sense if someone is pressuring or not ?

Data Sheet https://www.electro-tech-online.com/custompdfs/2012/10/MPX4115.pdf

 

[crutschow]

The pressure in a balloon is not very much above atmospheric pressure. I saw one reference that said it was a little over 2kPa which would give about a 100mV change in the sensor output voltage. If you want to amplify that you can use an op amp circuit, with offset to remove the atmospheric background pressure and gain to amplify the 100mV change.

 

[ahmedragia21]

Thanks for your reply. Can you explain me more about the amplification process ?

 

[ahmedragia21]

Can the following circuit work ? Can I put -V to be ground 0V ? and how can be the values of the resistors of the feedback?

 

[ericgibbs]

Can the following circuit work ? Can I put -V to be ground 0V ? and how can be the values of the resistors of the feedback?

hi 21,
This is one option.
Use a rail to rail OPA.

E

 

[ahmedragia21]

I will use LM358, is it fine?

 

[ericgibbs]

I will use LM358, is it fine?

Hi,
This is the best output voltage range you can get with a LM358,

About 50mV minimum, 3.5V maximum , with a 5V supply.

 

[ahmedragia21]

I tried the circuit, the output of the op-amo is always 3.71v whatever the variable resistor is, and the output of the sensor pressure is 3.91v. What could be wrong ?

 

[ericgibbs]

I tried the circuit, the output of the op-amo is always 3.71v whatever the variable resistor is, and the output of the sensor pressure is 3.91v. What could be wrong ?

Measure the voltage on the +input pin [ it has a 1K connected] what do you measure.??

 

[ahmedragia21]

The voltage on + input pin is 3.90v

 

[ericgibbs]

The voltage on + input pin is 3.90v

h21,
The MX4115 has an output of approximately 0.2V without any pressure difference, the 100mV change you stated is added to this 0.2V so it could be 0.3V at max pressure.

You should not have 3.9V on the +Vinp pin.

Can you post a picture of your project.?? also measure the voltage output of the MX [ not connected to the LM358] with no pressure and no pressure.?

E.

EDIT:
Mark on this plot image the range you are working with, min and max.

 

[ahmedragia21]

it measures the atomospheric pressure, that's why it gets 3.90v. How can I test then without any pressure ?
The sensor without any connection at all, it gets 3.90V, when connectiong just GND, VS, and check the output

 

[ericgibbs]

it measures the atomospheric pressure, that's why it gets 3.90v. How can I test then without any pressure ?

hi,
The ambient air pressure is approx 101kilopascals, which you measure on that sensor, what I need to know is when you apply the pressure equal to the pressure the balloon, what does it measure then.?

In Post #2, Carl said he has a value of approx 2kP above ambient inside an inflated balloon, so how much does the voltage change from outside the balloon to the inside of the balloon.?

 

[ahmedragia21]

Without using the op-amp configuration. The output voltage is 3.95 inside the balloon, and outside is 3.90v. When using the op-amp. output voltage inside balloon is the same without balloon which is 3.90V

 

[ericgibbs]

Without using the op-amp configuration. The output voltage is 3.95 inside the balloon, and outside is 3.90v. When using the op-amp. output voltage inside balloon is the same without balloon which is 3.90V

hi,
Checking with those voltages show that the Common mode input voltage of the LM358 is being exceeded when the supply voltage is +5V.

A solution would be to use a 9V supply to the LM358.

The change in voltage at the MX is +50mV, what amplified voltage level do you require.??

 

[ericgibbs]

hi 21,
Running a few simulations even with +9Vs shows that the MX4115 is not suitable for the project, due to the offset voltage of 3.9V at ambient pressure and the 50mV voltage change inside the balloon.

The ambient pressure can change by a few kP due to the weather.

E.

 

[ahmedragia21]

That's what I guessed. What other solutions do you think of ?
Can a difference amplifier work ?

 

[ericgibbs]

That's what I guessed. What other solutions do you think of ?
Can a difference amplifier work ?

hi 21,
Exactly what are you trying to do, it sounds interesting, but what is its purpose.?


A Difference amp could be a solution, but what would you use as a reference input.???
A second MX4115 outside the balloon would be required in order to provide the ambient pressure .

E.

 

[ahmedragia21]

I would like to sense the pressure inside a gynmanstic ball or balloon so that when someone sits inside it, it gets colored.

For difference Amp, I will have another pot fixed at 3.90V...

 

[ericgibbs]

For difference Amp, I will have another pot fixed at 3.90V...

That would not work correctly.
By using a fixed 3.9V you are assuming that the outside ambient pressure is approx 101kPascals.

Consider what would happen if the inside pressure was say 103kP and due to changes in weather conditions the ambient pressure changed to say 102kP, that would mean the OPA would only show 1kP differential pressure, not the original 2kP.

Or if the ambient fell to 100Kp, the difference would be 3kP.

When using a toy rubber balloon the size would change to try to compensate for the pressure difference, but a thick walled gymnastic ball may not.