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@allObjects
Adding an IR-filter helps to increase selectivity, but in the strange situation of the black plastic tab I made sure that no other light entered the sensor...
Since the analogRead() was parallel only for investigative purpose and was parallel to the planned digitalRead() on a different pin set with "input_pullup", I got a stable line in disconnect state. For final runtime I do not need the "pullup" since the circuitry is clearly
pulled down by the 5K6 emitter resistor at the sensor - called the load - in off/obstruction state, and pulled up by the transistor's emitter in on/no-obstruction state.
Just the 30..50K pulldown - as you suggest - seemed not to be enough of a load to the sensor under 3.3V operations conditions for robust operation (do not exactly understand why, but may be it IS the low 3.3V over the CE-R path and the LED driving with 10[mA] only - measurements for data sheet are taken with 5..10[V CE]/100[Ohm] load and 20[mA] light current). I can give it another try to just use a GPIO pin in pull-down input for the load.
By thinking about noise resistance, I'm now not sure where to place the load resistor: at the sensor 6 feet away, or at the pin... I guess it should be at the pin, then it is system-comparable with a 20...40[mA] loop, just with 0.4..0.5[mA]... on the other hand, voltage difference puts the low with 0.00###[V] and high with 2.###[V] safely away from the digitalRead() switching point, which is about 0.44V, correct?
Pulldown left 'in parallel' to the 5K6 doesn't make functional difference. For the out-of-regular-tolerance sensor though - which needs a 18K load - 30..50K become significant. But I trashed this sensor anyway because of its Gauss-bell-rim lifestyle... ;-)