And now something completely different...

The basic principles of radio receivers can be and are used for listening to the sounds of bats. Instead of an antenna, one uses a suitable ultrasonic microphone. The heterodyning principle is often used to downconvert the ultrasounds to the human audio range, which is roughly from 20Hz to 20kHz. However, the bats use not only higher frequencies but also a wider range of frequencies compared to what we humans do. So, some of the content is lost if one just uses an AM direct conversion receiver tuned to the local bat station. The more advanced signal processing methods use clever frequency compression to squeeze several tens of kilohertz's worth of content into the human audio bandwidth. These more complex techniques are actually used in hearing aids to shift audio around frequencies that a person does not hear properly.

In any case, there are bats also in the Tromsø region. So, if you live somewhere in the south such as in Tromsø, a bat detector would make a fun electronics project.

My home town Longyearbyen is almost 1000km north of Tromsø and there are no bats here. If there were, they would probably be big, white and furry. Bats already have fangs and claws, so no need for further upgrades there. In fact, I am sure that the Svalbard Bats would hunt for bearded seals, reindeer and other small insects during the polar night using echolocation to their advantage. The sound pressure alone would probably be sufficient to stun humans. So, maybe it's better we don't have bats up here.

But we do have whales in the fjord. Belugas, in particular, are known to be "chatty" and they, too, use high frequencies for echolocation and communication.

The ultrasonic microphones I have in my drawer have a reasonable response from about 10kHz to roughly 100kHz, but they are not waterproof. So, I built something less exciting that does not need any frequency tricks. My underwater microphone, or hydrophone, is a ceramic piezo element sandwiched between two polycarbonate plates. Its audio response probably stops somewhere south of 10kHz. Because I am not sure what size will work best, I have a selection of small and big hydrophones to try out in the field. The most time-consuming task was to build the CNC mill to machine the plastic parts (note the whale shape on the smaller one). But that's another story.

In the web, there are also plans for more complex DIY hydrophones. I think the one from NOAA looks like a pretty cool project:

Of course, I have not seen a single whale since I built the device...

A pod of belugas hanging out a few hundred meters from the shoreline
A quick evening boat ride: no whales, only engine rumble....
A small and a big hydrophone and the electronics.

The circuit is very simple comprising a dual op-amp and an audio amplifier. I had bought some op-amps with JFET inputs for the bat listening device, so that's what I used. One op-amp creates a "virtual ground", or midpoint between ground and the 9V battery voltage. TL072 is not what I would use for this purpose if this were a precision amplifier or filter circuit: one needs something like AD8031 with a low source impedance (at a wide frequency range). Works just fine here.

The signal from the hydrophone is preamplified by another op-amp and the output is fed to an audio amplifier driving the loudspeaker. The non-inverting amplifier circuit has a high input impedance and the highpass filter (C1 and R2) hopefully filters out the worst engine rumbles from other boats. Depending on the sensitivity of the hydrophone (and loudness of underwater sounds) the gain of the preamplifier may need to be increased by picking a larger value for R6.

While I had pretty much everything already in the drawer, I used Elfa Distrelec and Mouser for the missing parts.