I have a small device that's picking up small rocks from a pile and moving them to another place. Its a kind of crude way of trying to push the whole pile onto a bigger gear and hoping one of them is pushed to one of the spaces between gears and taken around and falls off on the other side of the spinning gear. Here I want to know if the machine successfully got a rock here, if not it should spin the gear until it turns up a single rock on the other side of it. If a rock is present at the spot, the gear should stop spinning until the rock is taken care of by the rest of the machine.
What kind of device can I use to sense if I successfully succeeded in getting a rock on the other side of the gear?
This is just a part of a bigger system. To sum up, I need the sensor to signal when a rock is signaled out and separated from the rest so it can continue work on that single rock.
I am building this using an Arduino to move the gear around, so the sensor needs to be something that can be controlled by an Arduino.