It will be a challenge over that distance in daylight. Here are a few ideas; I know you don’t want to do some but they may be needed…
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Optics - You need to shield the sensor from extraneous light, and try to increase the amount of light you are collecting. Try a paper towel tube with the inside painted flat black. Add a lens and focus it on the detector on the other end. Even without the lens, it will help. If nothing else, put the detector on the rear of the enclosure with the light shining in thru a small hole on the front.
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Modulate the light source and look for that modulation in the detector; that will help decide whether the signal received is from the light source or just sunlight swamping the detector. I’d look into 38kHz stuff used for IR remotes. Keep in mind that this may be temperature sensitive (both oscillator and discriminator) which could be an issue if used outside. If you can, use the same oscillator for the modulator and demodulator so they will track regardless of the actual frequency
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Positioning: Try to position both the detector and retroreflector so they don’t get swamped by sunlight; there’s not much you can do to overpower the sun…
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Try a laser; you don’t need an “eye burning” one; a nice collimated beam from a 1mW laser will probably work much better than a single LED. IR may be better but I would start with red since it will be easier to aim,
/mike