One of my favourite electronic designs (my own, that is) was a
bat detector, published in the UK as the (click here)
Bat Sonar. The circuit doubled as an ultrasonic remote control. On the right one sees what is called a block diagram, as I drew for all my major designs. I pushed the Bat Sonar to the very edge of internal noise, which means to the point where bat sounds would be swamped by the random motions of electrons in the circuit. This was the physical limit of sensitivity, given the quality of the components.
OBSERVATION: This circuit very effectively filtered out lower frequencies, so that it heard ultrasound in the same room where my son was playing the piano. A bat detector will hear a lot of things besides bats of course. In nature, all kinds of things. Shhh, the person who put up the design may be infringing copyright, but I love to see my designs out there.
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