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Topic: Can I connect my LM741 Op-Amp like this? (Read 8209 times) previous topic - next topic

pwillard

You will get it to work with just +/- 5V... but as I was trying to point out, you will have a limited operating range at the output pin of the amp.

2407wfp

#16
May 06, 2010, 02:27 am Last Edit: May 06, 2010, 02:43 am by 2407wfp Reason: 1
I see... I think I'm slowly understanding things a bit better! I will see if I can get the op amp you suggested and if so then hopefully my circuit design will be much improved! Thanks for the replies, they are very very helpful! :)

Also, if I use the TLC272, how do I set the gain of the amplifier and do I need to wire it so it is inverting? Finally, should I use the +5V rail for the other devices too (photodetector and LED)?

Grumpy_Mike

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how do I set the gain of the amplifier


Using the feedback resistors exactly the same way as you do with the 741.

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Finally, should I use the +5V rail for the other devices too

Yes you can use it for other devices, if there is cross talk just add some more decoupling on the supplies.
http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html

2407wfp

#18
May 07, 2010, 09:37 pm Last Edit: May 07, 2010, 09:49 pm by 2407wfp Reason: 1
Hi guys,

I read up the datasheet for the TLC272 and I have designed the following circuit which is meant to be a gain-of-100 inverting amplifier.



I have a few questions:

1.) EDIT: Am I supposed to supply pin 3 with 1/2 VDD (2.5V)?
2.) If I want the lowest reading from the photodetector to be 0V even though it receives light, how do I do this? Do I make the TLC272 output the differential voltage, and if so, how?
3.) Will I still need to use signal diodes in this design to limit the voltage?

Thanks in advance.

2407wfp

#19
May 08, 2010, 05:12 pm Last Edit: May 08, 2010, 05:35 pm by 2407wfp Reason: 1
Update:

I've tried the simple photodetector circuit below and I manage a reading between 0 and 10.



I can't seem to replicate this with the TLC272 op amp installed, however. What should I do?

2407wfp

Richard Crowley,

Thanks for the reply. The following diagram is from the TLC272 datasheet which specifies the pins functions. Like I said, I'm unsure what to do with pin 3 but the output pin in this op amp is pin 1, not pin 6.


2407wfp

Thanks, I didn't know that. I'll update any following schematic diagrams to show the proper symbol.

Like I said, I need to use the TLC272 as an inverting amplifier. According to the datasheet, I'm meant to connect it up like so. So far I've done this by connecting a 10k resistor between pin 3 and 5V, and another 10k resistor between pin 3 and GND.



Any ideas?

2407wfp

Without the TLC272 amplifier, I am getting a range of 0.05V on the photodiode.

I'm trying to maximise the range on the Arduino, so I would like as close to 5V as realistically possible.

I chose to invert it because I don't have details for an alternative amplifer circuit. Can I get some clarification as to why I shouldn't?

Thanks

2407wfp

I'm using it to detect the change in light intensity due to displacement of the light source from the photodiode. I'm detecting changes on the scale of millimetres and frequency of movement is less than 20Hz.

I'm using the photodiode in photovoltaic mode and not photoconductive mode. I have used the sensor before in a similar setup without an Arduino where it has worked flawlessly, and am trying to adjust it for the Arduino's 5V requirements.

Thanks for the resources, I will take a look at them.

Grumpy_Mike


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1.) EDIT: Am I supposed to supply pin 3 with 1/2 VDD (2.5V)?

Yes two equal resistors (say 10K) from the input to each rail should do it.

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2.) If I want the lowest reading from the photodetector to be 0V even though it receives light, how do I do this? Do I make the TLC272 output the differential voltage, and if so, how?

Replace the two 10K resistors with a pot. then you can trim the input to what ever level output you like. You will need to do this because you have a high gain and a DC input.

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3.) Will I still need to use signal diodes in this design to limit the voltage?

No because this can't give any more than 5V.

2407wfp

Thanks Grumpy Mike. Been waiting for a proper response to my question since Friday :D

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