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IR LED filters design for noise rejection

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vimalkhanna

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I need to design a filter for rejection of average red LED,light levels ,other noise from AC switchings and accept only IR signals from my IR transmitter .
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However ,I tried putting a light filter ,,smoky and deep red acrylic sheets but sensitivity reduced to 2-3mtrs .
The normal supply suppression changes the pulse transition responses ......I'm stuck for a wizard to guide me ...
 

is your receiver screened? (to the extent that you can, minus the hole for the photodiode). An IR optical filter appears black usually, not red. You can probably
use a piece of plastic from an old remote control if this is a hobby project, otherwise the filter material is not cheap. Also, are you sure the AC supply signal that the
receiver is picking up is via optical means, or could it be from the supply rails, or do you have long wires off the photodiode?
Btw, there are different wavelengths for IR devices, so for optimum range you'll want to ensure the photodiode is most sensitive to the IR transmitter wavelength.
 

Sir ,The recvr diode optical filter is deep maroon ,almost black no pinholes that I can see...The diode is mounted directly on the PCB,no wires .There is an outer case cover made of mu metal body 35mmx 25mm x 8mm size surrounding the amplifier .The power to the unit is /Lm7805 ---5VDC thru a 470ufd and a ceramic 0.22u across it .When roughly 2.5mtrs ,,the data picked up is ok. but after this it just fades ....
 

Hm, hard to say what it could be. Are you sure that there is AC supply noise, or do you think it may just be the limit due to the level of IR being transmitted being low? If there is AC noise, and if it is coming from the supply rails, then you could check that by powering the circuit from a battery temporarily. If that solves the issue, then perhaps an LC filter is needed on the supply. If it is not AC supply noise, what is the gain stage? Might need a lower noise amp, and make sure the resistances are low to keep noise down. I don't really know much about photodiode interfacing, there may be some good designs on the internet.
 

I think the high single stage gain needs to be changed to a differential mode with with very low bandwidth and lo pass active filter to 5Hz ..
Let me try more work on this and will revert for help................
 

ohhh.. thought you might have been modulating at (say) 20-30kHz. I didn't realise it is for just constant IR light level. That would be
hard to pick up amongst normal ambient light level.
 

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