The QRP HomeBuilder blog posts about listening in on bats. Bat detectors, or bat receivers, input bat audio in the range of ~15 -120 kHz and output audio frequencies into headphones, and/or give a Fourier transform of the received waveform on a small screen, or, in some cases, also provide the bats vocalized frequency range.

In summer months, around dusk, I venture into our garden to watch bats feast on flying insects. In my valley, bat research + conservation garners deserved attention.  Bats prove an important part of nocturnal insect control & also help cycle nutrients from wetlands into forests.

Bat receivers seem to fall into 3 basic categories:

• Direct conversion; also call heterodyne, or zero IF
• Direct digital sampling
• Down conversion using logic IC division to move the frequency into a spectrum humans can hear

I plan to design and build my own bat receiver and to be honest, this task seems fairly unremarkable for someone who on occasion, designs and builds radio equipment. What you might lack is test equipment.

The post then goes into the build, schematics of the receiver circuit and checking out waveforms with an oscilloscope.

See this excellent build here.