My latest robots communicate bot-to-bot using 38 khz IR receiver modules, yada yada, for very reliable communication. Strong IR signals reflect and bounce off walls, ceilings & everywhere, and the signal can go through many visibly opaque objects and/or leak through little openings. Besides the digital signals, I wanted to get a "proximity alert" based on IR signal intensity when two bots get within a foot or two of each other (I'm saving my laser Pings for another project!). I built & tested a few different analog phototransistor/op-amp circuits which mostly work as intended, but the 5mm T-1 3/4 phototransistors have molded-in lenses and are pretty finnicky about optical alignment so detection range isn't very consistent. Then I tried a simpler approach, just adding various filters in front of a regular 38 kHz receiver. Most things are an all or nothing proposition, blocking-wise. Black plastic notebook covers, black posterboard, black heat shrink tubing and good quality black electrical tape block 100%. Most everything else lets strong IR through at close range. I tried several different colors and multiple layers of posterboard, construction paper & duct tape, and everything else at hand. What finally worked best? Dollar Tree electrical tape! Two rolls for a buck in a blister package. Looks & feels just like my regular black UL electrical tape (from Walmart, IIRC) but it definitely it lets more IR through, they must have skimped on colorant, yippy Skippy! The only visible difference I see is that the DT tape has a shiny finish and the WM tape has a matte finish. I just wrapped one layer tightly around the IR sensor and I'm in business, detection range is 1-2 feet, just perfect. Simple, no circuitry to build, easily duplicated and much less sensitive to alignment. The KISS principle wins again!
Maybe I'll stock up on that DT tape, repackage it as IR signal attenuator tape and make a million bucks selling it on Ebay.
"When you make a thing, a thing that is new, it is so complicated making it that it is bound to be ugly. But those that make it after you, they don’t have to worry about making it. And they can make it pretty, and so everybody can like it when others make it after you."
- Pablo Picasso