This video shows the excavator which is used in my automated Lego coal terminal. The excavator is controlled by an Arduino Uno with an NXShield from Mindsensors. With the shield, the Arduino controls three motors:
Move left/right
Full beam up/down
Half beam up/down

The bucket is moved by a pneumatic piston. In the first design, the bucket was moved while the upper half of the beam was moved. It turned out that the bucket didn’t get enough movement so I needed to install a separate activator. Not being able to add another linear activator+axle, I choose for a pneumatic piston, since there will be other pneumatic components in the automated Lego coal terminal.

The pneumatic switch is moved by a small servo motor, also from Mindsensors. The excavator fills a silo under which trains will stop to load coal in the hopper wagons.