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Infrared signal and an opamp?
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Infrared signal and an opamp?

by magician13134 on Fri Aug 06, 2010 11:15 am

Hi, I'm working on a project, I'm trying to monitor sleep patterns, and I've read that by using an IR emitter/detector pair aimed at your eyes, you can determine when you enter REM sleep. As far as I'm able to tell, there's a slight change in reflectivity based on whether your pupil is facing forward or to the side and it's enough to pick up with the detector, but I've read that you need to run it through an op-amp to amplify the signal.

Right now, I simply have an IR emitter constantly shining and a detector, then I connect the detector to the base of a 2N2222 transistor which controls a visible LED and this will light up the LED based on how close something gets to the IR LED. Now my question is, would the op-amp be able to amplify these signals, and how would I do it? I've got a 741 op-amp and I've read a lot of webpages on them, but I can't figure it out. I've read some pages that say you need both a positive and negative voltage in addition to ground, which I'm not entirely sure how to do. Also, I'm running the circuit between 5 and 6 volts, I'm not sure if the 741 needs more power or not. If anyone could point me in the right direction or would like to give me some feedback, it would be much appreciated.

Thanks.
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Re: Infrared signal and an opamp?

by oPossum on Fri Aug 06, 2010 11:24 am

- Use a better op-amp with rail-to-rail output and single supply operation (like a MCP6002)
- No need for a bipolar (+/-) supply if you use the appropriate op-amp
- Google transimpedance amp - that is good way to amplify photodiode output.
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Re: Infrared signal and an opamp?

by magician13134 on Sun Aug 08, 2010 12:20 am

Thank you, I will look into that Op-Amp and will let you know when I get stuck again :wink:
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Re: Infrared signal and an opamp?

by majolsurf on Wed Aug 25, 2010 7:07 pm

Sparkfun has these convenient little op-amp breakout boards with adjustable gain. http://www.sparkfun.com/commerce/product_info.php?products_id=9816 Only $5 and are nice if you don't feel like playing with op-amp designs.

They are simple non-inverting two-stage amplifiers. Somewhat limited in that you cannot use them for differential amplifers, inverting, etc... Also their output has a DC offset that is half the power rail.

But as a quick prototyping tool they are cheap and convenient.
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