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I'm designing a small heart rate sensor using green smd LEDs and a photodiode. The photodiode needs to measure the reflected light, and not direct light from the LEDs. The LEDs are near the perimeter of the photodiode, so I'm trying to figure out some sort of thin wall to block their light from hitting the photodiode. The LEDs have a relatively wide viewing angle (110˚) and are ~1mm tall, so ideally something that is about 2mm tall.

I'm wondering if anyone has suggestions for some type of wall or component I could place between each LED and photodiode to block light?

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  • \$\begingroup\$ Get some thin brass shim stock. Cut into strips and wrap around the diode. A drop of 5-minute epoxy will hold it in place. If the brass doesn't appeal, get some heavyweight paper and use that. 20 pound bond should do pretty well, and if you need more absorption, wind a couple of layers. I expect handling these small pieces will try your patience. Or get some thin-wall tubing of the correct size, cut off a small length and slip over the diode. Again, you won't have fun will the small sizes, so you'll have to be patient. \$\endgroup\$ – WhatRoughBeast Jun 26 at 14:44
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    \$\begingroup\$ Which answers are useful probably depends on whether you're building single unit or small number for personal use, or trying to do this at scale. Question should perhaps mention that. \$\endgroup\$ – R.. Jun 27 at 15:40
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My advice is to not bother doing what you're doing, and spend the $3.50US to get the part designed for the task, like https://www.digikey.com/product-detail/en/osram-opto-semiconductors-inc/SFH-7060/475-3174-2-ND/6137022

enter image description here

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I've had luck with drilling holes in lids to microcentrifuge tubes (or other small bottles), cut pieces of copper tubing from the hardware store, or even a piece of black electrical tape in a pinch (my least favorite option). I have also made little barriers out of thin cardboard stock cut from packaging...easy to cut and form...that worked well.

Hotglue, epoxy, and silicone all make decent adhesives. Be careful with silicone, as the RTV stuff can sometimes corrode electronics.

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  • \$\begingroup\$ The acetoxy type of RTV releases acetic acid when it cures - you want to avoid this type with electronics. \$\endgroup\$ – D Duck Jun 27 at 20:27
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Sometimes reworking your PCB a bit allows you to place an existing part between the LED and the sensor which will block the light. It's not even unheard of to place otherwise useless cheap parts to act as walls: it may be much cheaper than special light-blocking components, especially if you already have a suitable part on your BOM, and ordering it in higher quantities will make it even cheaper.

Light pipes can also work wonders, if your PCB / case can support them. They are made of transparent acrylic plastic, but thanks to the internal reflections they can be near-100% opaque from the inside.

I'd also take a look at LEDs with smaller view angle and photo-diodes with a smaller field of view.

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Make some slots in the PCB and solder a small piece of metal there. I've seen it work remarkably well in real-life applications.

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mylar? The thinnest thing i know of that blocks 100% of light, developed by NASA to block heat and light with minimal addition to the weight of a craft, fuel ain't cheap. Mylar reflects light so if you do not want that then black electrical tape or something black, with a rougher surface than electrical tape, will reflect less. Charcoal finish would be perfect for that.

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