is the bin going to be constantly filling up with these parts? or is the part just going to drop in and then immediately slide out before another one comes in?
in that case you could just put a simple switch on one corner of it such that when a part fell in it would trip it...
if not, you could use a piezo sensor, which would detect the shock of the part hitting the bin.
ajay here . you should probalbly use a photo detector (transistor) with a IR diode mounted the other side . you could probably use an array of these in OR'd connection . if any on of the detectors's are trigerred you would get 0 output at the emitter of the photo detector . invert the outputs and connect to an up / dn counter . the photo transistor costs about 30 rs and the ir diode about 5 rs. other components about 50 bucks . All the best.
how many parts per minute? you might want to use light curtains.
some are designed for counting applications and can detect fast
moving objects (multiple objects crossing detection zone at the same time).