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I suspect that my NIR emitter and Photodiode are not working.

Emitter http://www.thorlabs.com/thorproduct.cfm?partnumber=LED1450E Photodiode http://www.thorlabs.com/thorproduct.cfm?partnumber=FGA20

For emitter circuit, I connect it to 5V and a 200 Ohm resistor, to try to achieve a voltage drop of 1.2V and current rating of 20mA (according to datasheet)

For Photodiode, I tried to bias it as the circuit in the datasheet also.

Would you please showing me how to test if they are still in good condition.(For the NIR emitter, my phone camera cant capture anything).

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    \$\begingroup\$ Is it correct that you once had a working circuit, and now it is not working, so you expect some of the components are damaged? If you never had a working system, it would be better to show us your complete circuit and ask if there is anything in the design that would make it not work. \$\endgroup\$
    – The Photon
    Commented Mar 11, 2013 at 17:04
  • \$\begingroup\$ Thank you The Photon. To be frank, I am playing around with the idea of a glucose monitor. At this stage, I just tried to replicate this experiment: digitalcommons.mcmaster.ca/cgi/… I am following the circuit there. If things are correct, according to the circuit in page 30, then when there is no NIR light, the circuit will give a reading of around 1.5V, while when there is maximum light, around 4.9V. However, even when I tried to vary the position of the diode, the reading are always 1.56V. What do you think ? Thanks! \$\endgroup\$
    – Josh Vo
    Commented Mar 11, 2013 at 17:23
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    \$\begingroup\$ If you haven't gotten it to work yet, you could open a new question to ask why (but do a search for old questions first, we've had several exactly about building IR blood-glucose monitors). Either way, I do recommend getting the IR viewer card like the ones in my answer --- it's very helpful to know for sure when your LED is actually working. \$\endgroup\$
    – The Photon
    Commented Mar 11, 2013 at 17:32
  • \$\begingroup\$ Photon, I can only found some previous threads asking about glucose meter, not building IR blood-glucose monitors. Is it ok if I start a new thread about that? \$\endgroup\$
    – Josh Vo
    Commented Mar 12, 2013 at 1:05
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    \$\begingroup\$ No problem starting a new thread...but we do want you to check out previous questions first, so your new question can add something new to the site. \$\endgroup\$
    – The Photon
    Commented Mar 12, 2013 at 4:45

1 Answer 1

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1450 nm is far enough into the IR that most cell phone cameras won't see it. That's because the sensors are silicon CMOS or CCD sensors, and silicon sensors only respond to wavelengths below about 1.0 - 1.1 um.

You can check if your emitter is working using an IR viewer card like one of these:

enter image description here

The VRC2 and VRC4 models cover your wavelength. These have a flourescent coating that absorbs IR light and re-emits visible light so that you see a bright spot on the card when there is IR present. 2 mW, like your LED is rated for, should be plenty to create a bright spot on one of these cards.

If the emitter is working, but your overall system is not, then suspect the receiver.

The easiest test is simply to replace your photodiode with a new one of the same type and see if that fixes the problem. If it doesn't then you'll need to look at your whole receiver circuit, node by node, to see where the behavior is not what you expect.

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  • \$\begingroup\$ Hi Photon. It has been a while. Now I can be back to the project for hobbies. I bought the card, and use Thorlabs LED1450E to test out. Some how I can't get the bright spot appear. Even when I have charged the card with visible light, and did things in the dark room. Did I miss anything? \$\endgroup\$
    – Josh Vo
    Commented Sep 20, 2013 at 5:34

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