The heat shrink tubing you over the IR how much does it concentrate the beam to the receiving IR?
Hi Kris,
Perhaps semantic gymnastics, but the heat shrink tubing does nothing to concentrate the beam. The purpose is to ignore extraneous or ambient IR energy coming from the sides of the detector...such as from outside sunlight, overhead room lighting, or incandescent engine headlight or passenger car lighting. Think of it as looking through a cardboard tube, you don't magnify (concentrate) what you're seeing but you ignore the peripheral activity. Electrically speaking, this makes a more reliable digital detector since it provides more separation in photo-transistor current between the illuminated on-state and the blocked off-state. In other words with digital on-off circuitry you want to maximize "black-and-white" and minimize in-between "gray-areas".
I see you're catching on to this stuff because that's the correct Columbo question! That is, in addition to messing with the LED output, another option for increasing operating beam distance is to fiddle with the detector. In terms of "concentration" the simplest is to capture a greater cross-section of the beam. A 3mm LED obviously only "sees" a 3mm diameter cross-section of the beam. You could put a lens that is, say, 1cm in front that focuses a 1cm diameter cross-section down to 3mm but I'd suggest that's impractical for a train layout.
As you might suspect you could use a 5mm diameter IR photo-transistor which captures almost 3 times (i.e., 5/3 squared = 2.8 times) the cross-section of a 3mm photo-transistor. Same price on eBay...about 20 cents each in small quantity. But even with a 5mm IR photo-transistor you still should use a heat-shrink sleeve to keep the photo-transistor looking just at the LED beam.
