Originally Posted by Fit Charlie
...a wireless door knocker.
I'm trying to picture a wireless door knocker.
I guess you could use the same troublesome wireless door button which transmits a radio signal inside the house... at a receiver which trips a solenoid connected to a large steam valve... which lets a big blast of hot steam (you DO have a large steam boiler going full blast 24/7 don't you?) The steam passes through a flexible hose that leads to the front door, and then through a small hole on the inside of the door just opposite of where the heavy brass door knocker is on the outside. The big puff of steam uses its venturi effect to LIFT the heavy brass door knocker... and holds it up... until the visitor releases the wireless button... at which time the door knocker suddenly falls... and makes that customary door knocking sound.
It's really quite simple: