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I am working on a project that requires a night vision camera. I have a small camera placed on the PCB and 4 IR LEDs placed around it to illuminate the dark room. The rooms are about 20-30qm.

I have selected the OSRAM SFH 4250 IR SMD LED. It has forward voltage of 1.5V, 100mA forward current.

I have two questions:

  • Are there any security issues with this kind of IR radiance? I want to use the system in my home with 2 small children. Is this kind of IR radiance dangerous to the eyes?

  • Do I need to use a LED driver? Or is it enough to run with a MOSFET (N-Channel) is shown in the schematic below.

If I need a LED-driver. What is a good choice. There are so many devices to choose from and I actually find it difficult to find an appropriate one.

Thanks for your help. Phillip

enter image description here

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IR can be dangerous. But you would need much more energy to be harmful. So this is fully OK.

You do not need a driver. The schematic proposed seams to be OK. It may not be the most power efficient solution but it's a working one.

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This document on page 2 (page 44 if you download the full document) states: -

The American Conference of Governmental Industrial Hygienists (ACGIH) guideline for IR-A exposure of the anterior of the eye is a time-weighted total irradiance of 100 watts per square meter for exposure durations exceeding 1,000 seconds (ACGIH 1992 and 1995).

Per square centimetre (approximately the front surface of the eye) this is 10mW

The IR LEDs you have emit 60mW and the half power angle is 60 degrees. At say 100mm from the device the power/sq metre is thinned quite a bit but close up the whole eye could take the full 60mW and this would be unsafe for long durations.

The mosfet you have shown (BSH103) has an on resistance of 0.5 ohms and with the circuit you have drawn will only dissipate 10 or 20 mW.

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  • \$\begingroup\$ Thank you very much for this info. Children don't stand still. Not a minute ;-). As I understand it would be a problem if someone looks quite some time directely into the IR leds? Moving people that sometimes move their head to the IR leds but then looking back again, looking back a few seconds later, etc. Is that a problem, too? I have bought webcams with night vision. They have 15 IR leds around the sensor? Are these LEDs just way less powerful than the one I selected? \$\endgroup\$ – Phillip Schuster Apr 13 '15 at 14:47
  • \$\begingroup\$ I can only quote sources found on the web and I don't know what LEDs are used in your webcam. \$\endgroup\$ – Andy aka Apr 13 '15 at 14:54
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I recommend doubling your current-limit resistors. 100 mA is the absolute maximum for DC (rather than pulsed) operation, and you should never run parts at their absolute max.

Try this for eye-safety: http://www.laserpointersafety.com/safetyinfo/safetycalcs/

For worst-case safety, assume the eye has an iris of 1 cm diameter, and a close-focus distance of 10 cm. With an (again worst-case) assumption of 10 mW/sr, that's a power level of 0.08 mW into the eye, and a beam divergence of 50 mr. This gives a hazard distance of 1.5 inches. Since the eye cannot focus at this distance, you're good. At closer distances, of course, you get more power into the eye, but it gets spread out even more at the retina, so it's not a problem.

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