by RichD » Wed Jan 31, 2018 9:34 am
Those are actually IR phototransistors. You must use one that matches your emitters or your counter will not work properly. Both the emitters and detectors operate over a very narrow frequency range. The common values are 850 and 950 nanometers. If the values are not matched the sensors may not be triggered unless they are very close to the emitters. Since you probably don't know which emitters you have you should order some of each. Install one and if it works OK you are good to go, otherwise try the other frequency.
Here is a Digikey 880nm sensor part number: QSD123-ND