First off, I know enough about electronics to get hurt. Second, talk down to me .. I don't mind. I get lost in the jargon.

Here's my situation:
I'm using a pressure sensor using 5-12volts input and .008 to .050mV output. The output is going to an Arduino Uno analog. The problem (and you probably already see this coming) is that the analog in on the Arduino requires 0-5VDC.

I have two hurdles to get over:
1) I need to amplify the .050mV to 5V.
2) I need to connect that output to the Arduino. (one leg to the analog in is obvious, but the other is not at all clear)

What I've tried:
LM741 OP-AMP but, apparently the mV input to the OP-AMP is too low.

Your help would greatly be appreciated.
Ken

 

You need a gain of 100. That's not too difficult.
You need to give us the make and model of the pressure sensor.
What are you using to power the pressure sensor?
LM741 will not cut it. Show us a circuit diagram.
We can recommend an op-amp circuit once you provide us with more information.

 

Oh, and the output of the sensor is likely 0.05V, which is 50mV

 

You're right... output is 50mV .. range is 8mV to 50mV... pressure sensor is 24PCEFA6G .. Power to the sensor is 5VDC. I was going to take it off the Arduino.

Not able to insert circuit..??

 

LM741 will not work with 5V supply. It also requires a dual supply.
You need a bridge amplifier and a method of zeroing (or balancing) the bridge.
LM324 is a quad opamp that will operate from a single 5V supply.
I will try to post a circuit for you later unless someone else on AAC can help out.

 

yeah! Every year these showers on the playa get more high-tech.

Thanks buddy.

 

So do you want 0.00V out when the input is 8mV and 5.00V out when the input is 50mV?

 

Here is a circuit, not tested.

I have chosen LM358 op-amp because it is a dual op-amp in an 8-pin package. The LM324 would work also but it is a quad op-amp in a 14-pin package.

 

So do you want 0.00V out when the input is 8mV and 5.00V out when the input is 50mV?

Yes sir. Following the input.

 

Here is a circuit, not tested.

I have chosen LM358 op-amp because it is a dual op-amp in an 8-pin package. The LM324 would work also but it is a quad op-amp in a 14-pin package.

Nicely done, my friend. I really appreciate it.

 

Yes sir. Following the input.

Two points I would like to make since I have done a little bit of interfacing sensors to Arduino.

1) Is the range of the sensor linear? I mean is the progression from 8mV to 50mV, is it a straight line?

If it is straight line, you could implement correction in the software.
So let say you have 0 PSI pressure. Your sensor outputs 8mV. This 8mV goes to amplifier with Gain=100. The output of the amplifier is .8V. This .8V goes to ADC on the Uno. Let say output of ADC on Uno is 0-255, so that 0V is 0, 5V is 255. So when you measure 0 PSI, ADC receives .8V, converts it into 40 or 41. In software you can easily manipulate the readings. In hardware you would need to build extra circuits.


2) The supplies for the op-amp are +5V and -5V. That only works in perfect world of simulation. The reality is that your output will never reach +5V. From small experience, you will be able to reach 4.7 to 4.95 volts. So lets say you are measuring maximum pressure, the output of the sensor is 50mV. You feed this to the amplifier with Gain=100. You expect the amplifier to produce 5V. Not going to happen. If you buy some really good amplifier (something in 20-30 dollars range), you will get close to 5V, something like 4.9-4.95 volts, but you will never reach 5 volts.

To get around this problem, you need your supplies to be slightly higher then 5V. Something like 5.5 or even 6 volts.

 

... you could implement correction in the software.

True for a non-linear response also, even more so. Point #2 is a very good one, often missed by noobs. Don't forget that op-amps may not get their outputs very near the power rails unless they are carefully chosen for that capability. That's another common boo boo.

 

I only just noticed the resistance of the sensor is in the 5kΩ range.

In that case I would multiply all of the resistances in my circuit by a factor of 10, i.e. 1kΩ becomes 10kΩ, 560Ω is 5.6kΩ, 10kΩ becomes 100kΩ.

 

I'm surprised nobody has mentioned that these op amp circuits are a very bad way to amplify the output of a sensor of this type. What you want is an instrumentation amplifier, like the AD623. It runs off a single 5V supply and its output is "rail to rail" meaning that it can swing (almost) to the full extent of the power supply and the A/D range. And you can set its gain with a single resistor without having to match ratios. If you do use an instrumentation amp, set its offset voltage so that the output is some small voltage like 0.25 for zero pressure, and then the full range of the A/D is available for measurement. That would be preferable to having the zero reading coming in at half the power supply voltage, which would mean that half the resolution of the A/D would be wasted. At least that would be the way to do it if you only measure positive pressure, which is the most likely case. If you need to go + and -, then centering the output might be right.

 

Your preesure sensor has an output impedence of 5k ohms. You should be able to connect this directly to your analog input.

Also, your DEFAULT Voltage range for ANalog input is 5 volts. You can, instead, use the A-ref pin to input an arbitrary voltage - use 2.048 volts (use an LM317 or similar). That way, your 10-bit (1024 increments) analog to digital converter value equal 2 mVolts unit.

A-ref pin is in red. See tutorial to see how to tell the board to use the ref pin instead of the 5v default.

http://tronixstuff.com/2010/12/07/tutorial-arduino-and-the-aref-pin/

 

Hey MrChips, why do you need the R9 to zero or balance the bridge? Why not just connect R8 to GND?

 

You need a way of trimming out any offset in case the bridge is not perfectly balanced.