Build the remote control
Using a power (p1), two dimmers (i5), and one transmitter (w22), build the remote control. You can put it on a mounting board or a LEGO platform like we did.
Build the dumper
Using a power (p1), a receiver (w21), a servo (o11), and a buzzer (o6), build the dumper circuit. We added a bar graph on the buzzer just for flair. Make sure the receiver is on the same channel as the transmitter!
Attach the cup to the dumper
Attach the long servo arm to the servo using screws, then firmly tape a cup to the servo arm.
Set your trap!
Securely tape the dumper circuit somewhere above the door. Turn on both circuits, and use the sliders to set your trap (buzzer is off, cup is in the upright position). Fill the cup with pom-poms, feathers, confetti, or other safe and surprising things. Once the trap is set, keep doing whatever you were doing in your room - but keep the remote close!
Surprise your visitor!
When someone walks in, push the sliders to activate your prank! Dump the contents of the cup, and play with the buzzer to add to the surprise.