belt with an on/off through circuitry & a belt or item that counts or reads an item on the belt

I would like too have the ability to turn on/off the conveyor belt. My thought for the other one is for the conveyor belt to read or count the items on the belt. When I say reading I mean close to sorting. Now when I say counting this is self explanatory but it would be like a tally of items on it to send a signal depending on the amount say 100 wood then it will send them on there way.