At one time; ultrasonic sensors bonded to windows were popular on burglar alarms to detect breaking glass.

There was also a fad for automatic air freshener sprays that went off at intervals.

An enterprising burglar had the idea of planting one in the guttering above the window - the hiss from the aerosol periodically set off the alarm until the keyholder got fed up with the false alarms and turned it off.

If only those who work so hard to steal, where to put all that work into getting, keeping, and excelling in there jobs. There would be no security sales people, and with out security sales people the world would be a better place.
A.H.W.

 

Here's a blurb I found about the Zenith remote.

Worked on many of them while in high school. I could hear the first sub-harmonic, so I could tell if the problem was a chipped or broken tuning rod without any equipment. Later receivers were less complex. They had a resonant-blade relay. Think of a standard 2 or 3 pole industrial control relay, except smaller (no significant power to handle). The armature had five leafs, each a different length. The coil rattled the whole thing, but only the leaf at the correct resonant frequency moved enough to make contact.

The guy who invented the first remote control died a few years ago.

ak