Ideas for "down-converter" to listen to ultrasonics?

[Mr RB]

Inspired by the "bat repeller" thread and because I live in a tropical area where there are a lot of bats and insect life, I thought it might be cool to make a device that converts ultrasonic audio into audible sound.

Maybe "down converter" is not the right name but the idea would be to have a microphone that hears ultrasonic sounds (maybe 15kHz to 40kHz?) and reduces these sounds to 1kHz to 10kHz range and to be output from a speaker or headphones.

It would be fun listening to the wildlife and might also have some uses in electronics listening to ultrasonics generated by equipment and SMPS etc.

As a starting idea;
1. microphone
2. high pass filter/amp, only lets >15kHz sounds through
3. PIC ADC samples the sound at 15kHz sample rate
4. PIC PWM plays the sound back at 15kHz sample rate
5. capacitive coupled to audio amp, lets audio frequencies through

This should detect the freq differential between the ultrasonic sound and the 15kHz sample rate, so with a 16kHz input it would give a 1kHz audio output. Likewise a 20kHz input would produce a 5kHz audio output.

This system seems to be very simple, and a PIC that can sample ADC at 15kHz is very easy only requiring a small cheap PIC, with the same PIC PWM module to play the sound back.

Any suggestions for this system? Or maybe better ways to do it?

 

[ericgibbs]

hi Roman,
I guess you know that many bat detector kits/circuits already exist.

https://www.google.co.uk/search?q=b...s=org.mozilla:en-GB:official&client=firefox-a

https://www.youtube.com/watch?v=xsU6K7Fs-U4

 

[Nigel Goodwin]

Yes, it's a common project - Practical Electronics has done a number of designs over the years.

Here's one example as a kit:

https://www.magenta2000.co.uk/acatalog/Bat_Detector_Mk_2.html

The assembly instructions even give the circuit and PCB layout.

 

[hugo]

Have you seen this one :

PicoBAT, an ultrasonic bat detector with 3 components

https://www.micro-examples.com/public/microex-navig/doc/077-picobat.html

 

[Mr RB]

Wow. I hadn't thought it would be such a common project!

Thanks for the links guys, I checked some of them out and they seem to be using a digital divider and only 1bit ADC which seems a bit crude as they give a very distorted output with no dynamic information reproduced.

The Magenta kit Nigel linked to seems a bit better using a sine osc and double balanced mixer as a heterodyne so it would keep most of the dynamics of the sound intact.

I still like the idea of using a single PIC chip to do the heterodyning in digital and still reproduce the sound dynamics, but since there are so many other projects out there it's probably not worth doing just for that one small benefit in simplicity. Oh well, I'll put it on my rainy day boredom list with a hundred or so other gizmos.