The LEGO Rotunda
This LEGO NXT Robot takes balls from the mixer drum and by using a series of cunning devices puts them back into the mixer.
It uses a Light sensor to count the balls as they go into the skip. When five balls have been counted, sometimes more if the lighting conditions change, the skip is rotated and the balls are dumped onto the conveyor belt. This takes them to the lift which takes them up to a drop off area. The balls then fall back into the drum.
The conveyor belt is driven with a PF motor controlled by an IR Link. The lift starts as a timed event triggered by the rotating skip. So fresh batteries are a must for the conveyor belt. Both the rotating skip and lift use the motors rotation sensor to set their positions. The drop off area at the top of the lift is controlled mechanically.