divxhacker

A bell-ringer telephone can also do the trick, too. There was a "remote control" lamp switch system that was sold from the '70s to the late '80s which also operated on ultrasonics. My brother had one for his T.V. It was activated by the bell-ringer telephone or by anyone "shush"-ing at the switch.

You can find those in the "antique appliances" on eBay. Hard to find one with a working whistle-remote, though.

I tried using the Clapper, but my dog made that thing worthless. She times her "ow-wow" and "ow-wow-wow" barks so expertly, that she can activate the switches, for the room fan and the lamp...