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Topic: Simple op-amp circuit (Read 8588 times) previous topic - next topic

Belal1987

@mrsummitville, thank you!
I connected GND of the amp. to arduino's GND directly, i connected pin 3 and 4 directly, and connected pin 1 directly to arduino A0. The output is string of zeros, when i turn on a table lamp, it outputs 8,9,etc. So i think these readings are responsive to the amount of light that falls on the sensor.

Right now i only have one resistor in this circuit which is the one between input and output. Other op-amp circuits have two resistors, is that normal or do i need a second resistor?

Thanks again,
Belal


raschemmel

#31
Nov 14, 2015, 07:59 am Last Edit: Nov 14, 2015, 08:58 am by raschemmel
You're looking in the wrong place. This is an Arduino forum. Yes, we also know about op amps.
The question is : "Why are you asking an arduino forum about op amps instead of Googlng "op amp circuits" ? (/configurations/"current to voltage converter")
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

TomGeorge

Hi,

Quote
Why are you asking an arduino forum about op amps instead of Googlng "op amp circuits" ? 
Yes, should have googled, but it is about interfacing with ARDUINO, among other things what else would General Electronics in this forum be about.

We might learn something about how to use these sensors.
The OP isn't the first and won't be the last.

Tom.... :)
Everything runs on smoke, let the smoke out, it stops running....

CrossRoads

#33
Nov 14, 2015, 08:25 am Last Edit: Nov 14, 2015, 08:27 am by CrossRoads
Why are you not wired like so?
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

raschemmel

Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

CrossRoads

Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

raschemmel

#36
Nov 14, 2015, 09:05 am Last Edit: Nov 14, 2015, 09:10 am by raschemmel
Quote
It's like an echo ...   
Yeah, it did seem like that but my point was that the OP could have found that with Google, if he knew what to look for but I don't think that's what he needs anyway since the sensor is a current source and I believe someone already told him to use a current to voltage converter (TomGeorge ?)




Actually, this post is still not on track. The OP's posted a link to the  datasheetfor the sensor that gives it's output as current "40 uA/100 klux ", so it is a current source and should be used with an op amp
op amp current to voltage converter  (which I believe someone else has already pointed out)
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

CrossRoads

So it needs a resistor from the + input to Gnd. Not a big deal.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

raschemmel

#38
Nov 14, 2015, 09:14 am Last Edit: Nov 14, 2015, 05:17 pm by raschemmel
see Reply#29

Somewhere in this thread I read that the sensor max output voltage is 10 mV
The feedback resistor should be 500 k ohm (or 499 k)
and the sensor should be applied between V+ and GND , not between V+ and V-.
This is not a Differential Amplifier application, this is a current to voltage converter application.
See op amp circuit link in Reply#36
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

mrsummitville

#39
Nov 14, 2015, 05:22 pm Last Edit: Nov 14, 2015, 05:40 pm by mrsummitville
@mrsummitville, thank you!
I connected GND of the amp. to arduino's GND directly, i connected pin 3 and 4 directly, and connected pin 1 directly to arduino A0. The output is string of zeros, when i turn on a table lamp, it outputs 8,9,etc. So i think these readings are responsive to the amount of light that falls on the sensor.

Right now i only have one resistor in this circuit which is the one between input and output. Other op-amp circuits have two resistors, is that normal or do i need a second resistor?

Thanks again,
Belal


OK, so you are getting a reading of 8 and 9 from the Arduino Analog A0 input.
That sounds like "Good News" but those values seem very low because ...
( 9 / 1023 ) x 5 Volts = 43 millivolts


Now, please let's go back to STEP #1 ...
Getting the OpAmp circuit functioning properly BEFORE connecting Arduino Analog input

You never did tell us what Voltage your OpAmp produced when fully illuminated.

Using your Voltmeter ...
What is the voltage between Pin #1 and #4 of the OpAmp when fully illuminated?

That will be the same voltage the Arduino Analog A0 input will measure
When the Licor is fully illuminated, the Output Voltage should be about 4.999 Volts.

30 microamps ( at 100 klux, full sunlight ) x 180K Ohms = about 5 Volts.
Yes, this circuit is that simple !
Are you reading only 43 millivolts on your Voltmeter?
Do see a problem here or not?

Also, do you understand the POLARITY of the Licor sensor?

I understand that you are very anxious to get the Op-Amp connected to the Arduino.
But it is very important to get STEP #1, the Op-Amp circuit,
working properly before advancing to STEP #2, the Arduino.

Then you asked, "Other op-amp circuits have two resistors?"
I am going to be blunt here ...
I do not care about OTHER OpAmp circuits
In message #13 of this thread I posted a link for a working Licor Current-To-Voltage OpAmp Circuit.
See page #2 here ...

Licor_Schematic

That Licor OpAmp circuit will work.
It is a very, very simple circuit, with very few components.

We are now on Page #3 and message #39 of this thread and
you have not yet mastered this simple 3 component Current-To-Voltage OpAmp circuit.
You need to focus on making Step #1 successful first.
You need to ignore the Arduino Analog input, until later.


raschemmel

@mrsummitville,
Bravo !

I admire your patience. I was tearing my hair out at msg #29...
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

mrsummitville

So it needs a resistor from the + input to Gnd. Not a big deal.
But, it is a big deal.
The OP cannot get a simple 3 component circuit consisting of:
1) Licor Sensor
2) OpAmp
3) A resistor
functioning properly after 39 messages.
And you think making a more complex circuit will be easier?
Please, do make this any harder than it already is.
What is "no big deal" for you, is "way too complex" for the OP.



raschemmel

Quote
So it needs a resistor from the + input to Gnd. Not a big deal.
Actually, for a current to voltage converter, the feedback resistor is from -V to Output and the sensor input is applied from +V to GND. In addition, there is a small cap (0.1 uF) across the feedback resistor.

There is NO input resistor and no resistor from +V (+ input) to GND.
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

mrsummitville

#43
Nov 14, 2015, 11:00 pm Last Edit: Nov 14, 2015, 11:00 pm by mrsummitville
Actually, for a current to voltage converter, the feedback resistor is from -V to Output and the sensor input is applied from +V to GND. In addition, there is a small cap (0.1 uF) across the feedback resistor.

There is NO input resistor and no resistor from +V (+ input) to GND.

RE: "Input Resistor"
I think what others are suggesting is ...
Attach a resistor across the Photodiode ( Current Source ) in attempt to create Voltage Source,
then design a High-Gain Voltage Op-Amp Circuit. It is a possible solution with its own set of problems.


Digikey Photodiode Amplifier Tutorial:
DigiKey

Colorado EDU Photodiode Amplifier Tutorial:
Colorado_EDU

Hamamatsu Photodiode Handbook
Hamamastu


mrsummitville

#44
Nov 15, 2015, 04:38 am Last Edit: Nov 15, 2015, 05:22 am by mrsummitville
Did you notice the outputt connection to analog in is not shown.
Also, it is drawn showing the 10 k resistor in series with the op am - voltage (gnd) which is wrong. I don't know where the 10 k resistor belongs but I know where it doesn't belong, namely, in series with the op amp return (gnd).
10K resistor?
When I zoom in on the FILE "1.JPG" that was attached to Message #24.
I see a ...
a) Black-Brown-Red resistor = 1K not 10K, in the "Ground Connection" ?
b) Purple-Green-Black-Red Resistor = 75K not 180K, for the GAIN ?

Am I misreading the values of these two resistors?

Of course, the "ground" resistor should not even be there.

And the GAIN resistor is too low by a factor of 2.5X
But that still does not explain 40 millivolts per the Arduino A0 Analog values of 8 and 9
instead of about 4 volts, which is a factor of 100X.


Then in Message #4, I re-read this ...
"1) The Licor sensor I'm using is similar to this: http://www.licor.com/env/products/light/photometric.html"
Note: The word SIMILAR, not the word EXACTLY.
So, do we really know anything about this project?

Maybe, the Feedback Resistor really needs to be 1.8 megohms or even 18 megohms?
I have seen large values in other Transconductance Amplifiers + Photodiodes.

If 1.8 megohms changes the 40 millivolt ouptput to 400 millivolts
then 18 megohm should change the output to 4 volts, at full illumination.

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