Pressure Sensor circuit help

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richacm

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Hi,

I'm just getting back in to Electronics and am not progressing as well as I would hope. I have been playing around with a pressure sensor I bought trying to come up with the correct output when pressure is applied.

Attached is the circuit I am currently running with.

What I am hoping is that the output I measure can be between 0 and 5 volts but I can't seem to get the 0 pressure reading to be 0 volts or the max pressure to be 5V. I have played around with the final 1k resistor on the output but it doesn't seem to do much. As you can see the current output is between 1.49V and 1.52V based on the pressure applied. Eventually the output needs to go to a PIC components ADC input so I can evaluate the pressure. I'm not sure if this small voltage range is going to work when the ADC converts.

Can someone give me some help as to what I should change on the circuit to get a better voltage range at the output?

Thanks,

Craig
 

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Please post the datasheet for the sensor.

Do you know that you can buy sensors with in-built amplifiers that output 0-5V signals?
 
Ooo, just checked out the price of some pressure sensors with built in op-amps....for me they are over $300 NZD. Bit too pricing for my simple project.
 
Hi,

I have updated the circuit based on a "0V - 5V Sensor amplifier which swings to ground" circuit I found.

Can someone check to make sure the circuit will work and my calculations are correct?

Calculations indicate a -1.23V supply after the 499 Ohm resistor.

The pressure sensor has a max pressure of 5psi which produces 115mV. I will be using in 3m of water which has a max rating of 4.267psi which will produce 98.1mV. To get to 5V at this rating I need a gain of 51. Based on the gain equation with R = 10k I then need a 408Ohm resistor. I used a 400Ohm in series with a 100Ohm variable one so I can calibrate the output to 5V.

There is also a 10k variable resistor so the 0V reading can be calibrated as well.

Thanks heaps everyone.

Cheers,

Craig
 

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hi Craig,
Did a circuit some time ago for a member, attached.

This is a an altimeter pressure sensor circuit, its been working in a rocket project for the past year.
 
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I'm sure the circuit you've posted is fairly simple but just looking at it gives me a headache. Please redraw it using triangle shaped boxes for the op-amps, just using a square box layed out how the amplifiers are in the IC makes it very difficult to understand.

Your sensor has an output impedance of 5k so your amplifier needs a much higher impedance than this to minimise the error. Going from your circuit (the parts I could understand), it looks like you've already catered for this as the two leads go directly to a couple of op-amps so I think you'll be alright.
 
Finally got around to drawing the circuit again.

I have also added in the PIC chip (which I have programmed) and the display output to LED's which kind of completes the first phase of what I want to do.

One thing to note is that the LED's are going to be about 2-3 meters from the PIC chip. Does that mean I will need to place a capacitor over by the LED's somewhere?

Cheers,

Craig
 

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hi.
The 1K0 from the +12V driving the 5.1V zener diode needs to be recalculated.
Especially as you are powering upto LED's off the same 5V line.

Consider one LED drawing say, 10mA, the voltage drop across the 1K0 will be 10V.!!

Also the power rail decoupling needs to be increased, a single 10uF is not enough, add some 0.1uF caps near the OPA's and the PIC.

Is the OPA section a proven design.?
Reason for asking is that its over complicated...
 
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Thanks,

I am delving in to LED configurations to get an understanding of the calculations I need to perform. I am, however, getting more confused than ever.

To get the current to flow through the LED I need a voltage drop across the diode - apparently this is about 1.4V from what I read. So the 330Ohm resistor afterwards will pretty much take the rest of the 5V if I have 10mA going through the diode. So if this is the case then does that mean I actually need 0V on the output pins from the PIC chip to light the diodes? I was programming it so 5V showed up on the output, which is what all the examples have done that I have seen. Does this mean all these examples are wrong? Should the diodes be around the other way and just be grounded on the cathode (after the 330Ohm resistor). Or could I whack some transistors in there so the Base is connected to the PIC pins?

Sorry for the ramble.

In terms of the circuit calculations I guess lowering 1khm: resistor down to 100hm: would reduce the voltage drop from 10V to 1V?

Thanks again everyone!
 
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You see this circuit makes a heck of a lot more sense to me, in terms of the LED section. 5V output from the PIC would drive the LED with a 10mA current. If I needed them a bit brighter I would reduce the 330hm: resistor down to say 165hm: to get 20mA through the LED.

Is my thinking correct?

The question I suppose is does the PIC chip handle a 20mA x 9 = 180mA supply to the LED's.

Cheers
 

hi Rich,
If the PIC could output 180mA and you have a 100R from +12V into a 5VZ, whats the voltage drop across the 100R.
Say 100R * 0.2A = 20Volts.!!!

I would not use a PIC to drive 180mA total, into LED's.
Why dont you lower the LED current to say 10mA for starters, also drop the 100R and 5VZ and use a 7805 1A Vreg for the +5V supply..?

I commented the other day on the over complexity of the OPA circuits, did you design it.??
 
Hi Eric,

Thanks for the input. I am no where near the state where I can design any of these things myself unfortunately - hopefully I will but I still find it very defficult to analyse complex circuits. I am an avid web user and thus find bits and peices everywhere and try and concoct something by putting them all together!

This circuit I got from Application notes on the pressure sensor from Honeywell (**broken link removed**). They said its a pretty common goal to have a 0-5V reading coming out of the pressure sensor. In order to do this it said to have a negative voltage supply (which is the lower left peice). It then goes through the Op-Amps to ramp the voltage up. The pressure sensor I have has 2 readings a V+ and V-. Looking at the design you put together I see it only outputted 1 reading hence I thought it wouldn't work with me.



I've been recommended another component to drive the LED's which is the ULN2003 which looks a treat as I can hook it straight to the 12V supply and drive it that way. I also eventually want to run a relay which apparently also sucks a lot of amps.

Sheez! I see what you mean by the voltage drop across the 100hm:! I didn't recalculate that one! If I add the ULN2003 in to the equation do you think I should still put in a 7805 1A voltage supply for the +5V? I wouldn't have a clue what the best way is to drop the voltage down from 12 to 5V...I want a constant 5V so a voltage divider wouldn't work. Thats why I decided on a Zener (having seen it on someone elses circuit )

All fun anyway

Craig
 
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hi,
Do you need a guide for the 7805, [ +5V from +12v]..?
 


Why not do as I do, and get FREE SAMPLES from Freescale, OnSemi etc etc etc...
Sometimes you'll need to pay the shipping, but the value of the freebies far outweighs any postage costs!
I've built a barometric altimeter, good to 50K feet, and has 1 foot resolution using these samples.. No op amps required!!
 
project help

Hi,
Eveybody, i hav to start my final degree project,
Area of research :-Micro Electronics
Title:- Data Acquisition system for industry
Bstract:- In industry parameters like temperature, like pressure and volume etc are essential to monitor and control as well. the signal can be collected in a central control room form locations where humans cannot go. And can be monitored and control as ell.


Its all about my project. I need ur help



Thanks
Zain
 
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