I'm trying to learn to design digital circuits so as a project I wanted to build a counter that would work as follows: When a reset button is pushed, it will set the counter at 0 When a + button is pushed, it will add one to the counter when a - button is pushed, it will subtract one from the counter and the complicated condition: When a IR sensor/IR led pair gets interrupted it will add 1 to the counter So basically I want to make a counter that goes up by one when something blocks the beam and then the +/- buttons are for manual adjusting it in case of an error or something. I know I can build the sensor with a phototransistor as a switch, but the thing is I'm not sure how to limit the circuit incrementing the counter 1 and only 1 time when the beam is interrupted? Also, I'd like some advice on whether an IR led / phototransistor is the best motion detection combination, if there is an easier alternative I would gladly hear it. At work they have something similar, but I'm not sure it's IR. They have a beam that shoots and reflects off some kind of kalediscope looking mirror. I don't think it's IR, because when looking at mirror from angle is beam you can see red light being reflected. If I knew exactly what it was, I would make use of their beam and mirror, and just use my own detector. Thanks for any help.