Last night's DIY

[James R. Coplin]

>You'd probably detect more than just bats, like certain insects and the 
>neighbours television whine. We have a lot of bats that feed just 
>outside the back window near a light.

The background noise is easily sorted out from the bat signals.  It does
pick up engine knock, june bugs, etc. but less than one might think. It's
quiet most of the time but if you move, kiss the wife, tinkle your ice cubes
in the glass, it goes off. 

>I don't know about everyone else, but I can hear bats quite clearly. 
>Maybe most and especially the larger species of bats have a significant 
>part of their sonar squeaks reach into easy hearing range.

We don't have the larger species of bats where I live, just small brown bats
and a couple others.  I have had them in the attic where I used to live and
you can hear squeeks etc.

>> About $20 in parts.
>You must have built the boutique version ;)

The ultrasonic transducer was like $13 of the cost.  It is hard to find one
with enough sensitivity and frequency range to really pick up the signals.
Some are so narrow banded you only get part of the signal and some species
not at all.

James R. Coplin