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Topic: Can I connect my LM741 Op-Amp like this? (Read 8183 times) previous topic - next topic

2407wfp

Hey there,

I need to connect an op amp to my Arduino in order to measure the voltage from a photodetector. Because the changes in signal are on the order of mV, I have used an LM741 op-amp. I know the Arduino has a 5V limit so I was wondering how I should accomplish this. Is my diagram correct? If not, will connecting the photodetector to a 5V rail make it work?

Thanks for your help!


Grumpy_Mike

#1
May 05, 2010, 12:15 pm Last Edit: May 05, 2010, 12:17 pm by Grumpy_Mike Reason: 1
Yes that's a good way to blow up an arduino. This can put about +12V to -12V on the input. Use the protection circuit here only make the series resistor about 200R.
http://www.thebox.myzen.co.uk/Tutorial/Protection.html

No need to use R5 for DC offset trimming, it is not an issue here. The photo diode is the wrong way round. R2 & R3 are in parallel so only have one resistor. Put that to the positive instead of the photo diode.

pwillard

You could consider replacing that particular opamp with a single supply rail to rail solution.

An opamp that can work at 5volts single supply mode will not normally be able to swing it's output real close to ground or 5 volts.  So you select one specifically designed to get close (rail to rail)

One that comes to mind is the TLC272.


2407wfp

Thanks for the reply.

Grumpy_Mike, is this picture what you meant?



Alternatively, is it possible for me to do this?



I want to have a variable voltage reading depending on the amount of light on the photodetector.

pwillard, I don't quite understand what you mean and how this relates to my requirement above.

Mike Forst

2407wfp:

He means an opamp that can produce an output at or very close to the supply rails (i.e. Vdd and Gnd).  The opamp he suggested operates from a single ended supply so you don't have to have positive and negative supply voltages.  You can connect it to a +5V supply and gnd.

TI claims a typical Vout of 3.8V with a 5V supply so if you don't mind sacrificing a little bit of your 0-5V input range it might be easier to implement since you don't have to come up with a +/- supply.

good luck!

2407wfp

Sorry but can I ask why I don't need the offset trimmer variable resistors? Thanks!

erc

Can i just ask, what program did you use to make that circuit sheme?

Grumpy_Mike

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Grumpy_Mike, is this picture what you meant?


Yes but swap the photo diode and R2.

Second diagram doesn't get you anywhere.

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why I don't need the offset trimmer variable resistors

because that only helps null out a few millivolts of DC offset which in this application is totally swamped by the DC offset you get from the biasing of the photo diode.

As was mentioned before if you use a better op amp than a 741 you can use a single supply rail and even a 5V one so you don't have to have such a complex supply arrangement. I first used a 741 back in 1972 so they are a bit of an old design by now. There are much better ones. One problem with the 741 is that you can only get an output voltage to about 1.5V of the supply. So in your circuit the maximum output would be 13.5V, which is not a problem as such in this design as you are having to put clamping diodes and a series resistor ahead of the arduino anyway.


2407wfp

#8
May 05, 2010, 05:56 pm Last Edit: May 05, 2010, 06:08 pm by 2407wfp Reason: 1
Thanks, I'll try the circuit you recommended tomorrow.

I'm surprised that the offset doesn't need to be used because I've connected the first circuit to a voltmeter in place of the Arduino before and in order to zero it by a few volts (around 4 or more), I had to install the offset.

What are the specifications for the signal diodes that you mention of that I need so as not to exceed the Arduino's capabilities?

erc, I just used Microsoft Word to make the diagram.


MarkT

The 741 is such an old design I'm surprised you can get hold of them these days - no-one uses them for new designs since every aspect of its performance has been bettered many times over (and modern designs don't latch-up).  For Arduino circuits a single rail low voltage opamp is much more the way to go - think of the energy and complexity you save not having those archaic +/-15V rails.

I'm wondering what sort of gain you've got there - you seem to be relying on offset-null adjustment to do some sort of level shift - what range of currents do you need to measure from the photo diode?  Why have R2 and R3 in parallel in the original diagram?  What bandwidth do you want?

BTW if you're trying to get a huge gain (like 1000) from a single opamp, then you may have some significant issues - much better to chain two opamp stages together with more modest gain each.

Also your circuit is missing supply decoupling capacitors.
[ I won't respond to messages, use the forum please ]

2407wfp

MarkT,

I can imagine that the new opamps would work much better but due to time constraints and available resources I have to make do with the 741. Perhaps in the future I could use a newer opamp! :)

I'm actually measuring the voltage off the photodiode (photovoltaic mode) and I previously used this design to measure a range of about 15V. The gain is indeed on the scale of 1000 or so and I've done tests (with the 741) that show that the noise is much less on a single 741 than on two that are chained in series. I'm not really sure how to measure bandwidth so I can't answer your question.

What are supply decoupling capacitors? Thanks in advance.

Grumpy_Mike


2407wfp

Thanks for the link. After reading through the tutorial, I am unsure whether the IC refers to the LM741 or to the Arduino. Also, could you give more details on the signal diodes that you mentioned earlier?

Regards.

Grumpy_Mike

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the IC refers to the LM741 or to the Arduino

Both, every IC.

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could you give more details on the signal diodes that you mentioned earlier?

Any diode, it only needs to take a small amount of current. Try a 1N4444.

2407wfp

#14
May 05, 2010, 10:59 pm Last Edit: May 05, 2010, 11:51 pm by 2407wfp Reason: 1
I've just had a look around and it seems that the LM741 can be run using an input voltage of ±5V. If this were the case, would I need the additional components in order to restrict the voltage range? I am asking because although your help is appreciable, I don't fully understand all of it... This is primarily due to my lack of electronic engineering knowledge.  :-/

References for the LM741 input voltage:
http://www.matni.com/Arabic/Elec-Info/LM741%20DETAILS/741.html

http://web.ics.purdue.edu/~fwinkler/590E/Arduino_workshop_sensors.pdf


EDIT:
I have just found this rather interesting tutorial on measuring the amplified voltage using a 741 that actually uses ±15V for power
supply. I won't try it since you say otherwise, but surely ±5V power supply is alright? Thanks for the advice!

LINK: http://www.seas.upenn.edu/~ese205/Arduino%20Opamp_v7.doc

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