Hi,

This seemed to be a fun project so I thought I'd give it a go. In the last little while, our cat has taken to watching the backyard skies and trees at night from the patio. We don't see or hear anything, but we see her head moving around as if she's tracking something.

After some thought and looking online, we decided that the cat is hearing some animal emitting sound above our hearing possibly bats.

Without looking too hard, I found lots of manufactured and homemade bat detector boxes. Never knew that bats were so popular especially in the UK. More fun to build my own.
Found about a dozen of posted bat detector designs online- most were either the frequency divider or heterodyne topologies.

Personally, I prefer the heterodyne designs. The basic blocks of this type of design follows:



I've got all the blocks figured out and simulated in LTSPICE. The dilemma comes here:

• The preamp gains in posted designs range from as low as 50 to over 2000 depending on the microphone
• Electret condenser mics specs rarely show response above 20kHz - sensitivity very variable > 20kHz
• Most posted designs using electret mics list microphone parts that are obsolete (older designs)
• I bought some Knowles mics which have response to 80kHz but are SMT - don't think there's enough area

My question to the Forum is:

Can anyone provide an actively available microphone (including PN) with either a preamp circuit (can just be a gain number, I'll do the design) that will boost my bat chirps from the mic up to a 1-2 Vp-p level for the mixer?

Any help, thoughts or comments are welcome and appreciated.

Thanks,

Nekojita

 

The essential parameter here is high frequency response.
You need to detect frequencies as high as 100kHz and maybe even up to 200kHz.

 

https://www.digikey.com/en/products/detail/knowles/SPH6611LR5H-1/9681721

 

Hi Guys,

Thanks for your responses. 200kHz may be a little high. The microphones I received today were these:

https://www.digikey.com/en/products...74?s=N4IgTCBcDaIMoAUCqAGALARhQGQEoFYAJEAXQF8g

I am concerned that the microphone aperture is just to small to gather sound from bats that are 10-30m away. The brute force method is to forgo the MEMS mics and use the electret mics with the gain jacked up. It would take some experimenting to find the optimal microphone/ gain combo.

If there were someone in the Forum who could just say use PN XXX-00034 with a gain of 250, I would be ready to finish the design and build up the prototype. Hopefully, somebody has that gold nugget.
BTW, just for ducks, I ordered two "unbranded" electret mics from eBay today one with a 10mm diameter, the other with a 6mm diameter. Both were spec'd to 20kHz. We'll see what I get next week.

Neko

 

I think your quest is more challenging than you may want to believe.
The bad news, bats emit sounds from 20k to 200k Hz but use higher range more often because they need the high frequency for small objects. The higher the frequency, the more directional so forget about picking up the high frequencies unless they fly towards your microphone.

Low frequency in bat world is 20k Hz. Still much more directional vs 60Hz but, at least there aren't a bunch of other noises in nature that you need to discriminate from.

the good news, some bats chirps can reach 100 dB or more.

 

Another way of increasing the gain might be to use a parabolic reflector, at the wavelengths of bat chirps, it would not need to be large to be effective.

 

A paper on the hearing range of cats suggests that 85KHz would be the upper end if it is something the cat is hearing. I was surprised to learn that cats have one of, if not the largest, ranges of hearing among mammals.

The behavioral audiograms of two cats were determined in order to establish the upper and lower hearing limits for the cat. The hearing range of the cat for sounds of 70 dB SPL extends from 48 Hz to 85 kHz, giving it one of the broadest hearing ranges among mammals. Analysis suggests that cats evolved extended high-frequency hearing without sacrifice of low-frequency hearing.

 

I think your quest is more challenging than you may want to believe.
The bad news, bats emit sounds from 20k to 200k Hz but use higher range more often because they need the high frequency for small objects. The higher the frequency, the more directional so forget about picking up the high frequencies unless they fly towards your microphone.
(some text removed for clarity)

Don't crack it up to be more difficult than it has to be. With the quantity of Bat Detector circuits on the internet, it looks as if there are plenty of simple circuits out there. It may not be necessary to capture or reproduce the entire spectrum of the bat's signals if the main purpose to to tell whether or not bats are present.

My favorite is a little more than a local oscillator, SA612, and a headphone amplifier.


But there are plenty of circuits already out there.

 

I would think about the preamp. Removing all noise below 20khz is important. Wind is going to be a problem. Probably removing above 100k is also important. In Dick's circuit C2 and C3 will help with the low frequencies. I might add one more stage to get a steep 20khz roll off.

At my house the Bats play near the IR night vision cameras. I set up cameras with motion detection to see wild life, but bugs swarm the camera and Bats come to eat. The motion detection goes off all night long in the summer.

 

Don't crack it up to be more difficult than it has to be. With the quantity of Bat Detector circuits on the internet, it looks as if there are plenty of simple circuits out there. It may not be necessary to capture or reproduce the entire spectrum of the bat's signals if the main purpose to to tell whether or not bats are present.

My favorite is a little more than a local oscillator, SA612, and a headphone amplifier.

View attachment 250605
But there are plenty of circuits already out there.

http://bertrik.sikken.nl/bat/
There are links to different circuits and a lot of info for example microphones etc.
This circuit http://bertrik.sikken.nl/bat/ne612het.htm
I built this about 10 years ago. 6mm no name electret microphone.
Worked well at least to about 45 kHz.

 

Hi Guys,

A lot of great collaboration, thanks! I don't know what my cat is hearing may be bats or other creatures. I also found the paper showing the hearing ranges of animals. 85 kHz for cats really surprised me.

Dick, I found that same circuit but initially didn't go with it because I couldn't find an LTSPICE model of the SA612 and I didn't have a good LO circuit at first go.

Below is what I cobbled together, very similar to Dick's post (included on the bertrik webpage):


I think that I'll the plan will be:

• Evaluate the 2 no name microphones that I ordered with my current preamp
• Determine if I need more high pass filtering (or low pass filtering)
• Initially build up the preamp on my solderless breadboard

Any more thoughts or comments?

Thanks,

Neko

 

Hello,

Here are some more pages with bat detectors:
http://integraonline.com/~wolfden/detector.html
http://users.telenet.be/airborne/Bats_in_the_garden.htm

Bertus

 

Hi Bertus,

I did find the integraonline site the other day but not the telenet site. Really cool! Thanks for your contribution to this thread.

Neko

 

Hello,

You will find many more with a google image search:
bat detector schematic
I have attached one with a simple 4069 chip.

Bertus

 

The detector at ultrasonic range there is another challenge of positioning together with the detection distance.
Because of the lower frequency components of the bat call are detected at longer range.
Since we can gain more information about surroungs by integrating the frequency domain.

https://www.wildcare.eu/media/wysiwyg/pdfs/UltrasonicMicrophones.pdf

An inventory collected onsite recording demonstrates ultrasonic analyzer how digital might be useful.
I thought I heard what sounded like cars driving by but not likely if slowed 32X, it could have been a flying insect about to be eaten.
The bat echo I think might appear shortly after the call in a lower frequency. possibly helpful in what is noise what is not.
The signatures are there interpreting each one would take a lot of experience. A modified digital recorder with USB might be helpful.

 

Hello,

You will find many more with a google image search:
bat detector schematic
I have attached one with a simple 4069 chip.

Bertus

Hi Bertus,

That's a crazy simple, something to consider.

Thanks,

Neko

 

The detector at ultrasonic range there is another challenge of positioning together with the detection distance.
Because of the lower frequency components of the bat call are detected at longer range.
Since we can gain more information about surroungs by integrating the frequency domain.

https://www.wildcare.eu/media/wysiwyg/pdfs/UltrasonicMicrophones.pdf

An inventory collected onsite recording demonstrates ultrasonic analyzer how digital might be useful.
I thought I heard what sounded like cars driving by but not likely if slowed 32X, it could have been a flying insect about to be eaten.
The bat echo I think might appear shortly after the call in a lower frequency. possibly helpful in what is noise what is not.
The signatures are there interpreting each one would take a lot of experience. A modified digital recorder with USB might be helpful.

Hi Sparky,

I looked at this sight briefly last week; will take a closer look.

Thanks for the post,

Neko

 

Hi Everybody,

I received the 10mm no name electrets from Amazon today. That was quick. Thinking that I would connect one up to the preamp circuit in the Bat Detector circuit I posted, I realized that I didn't know how to bias the mic from 9V, my battery voltage.

Looking at various circuits on the Google search page that Bertus, posted, biasing was all over the map. Doing my own search turned up this apps note from TI:

https://www.ti.com/lit/ug/tidu765/tidu765.pdf

I should be able to figure something out from that.

Neko

 

To be clear the circuit in post #8 is not my circuit, but it is very close to my favorite. It was "harvested" from the internet.

 

To be clear the circuit in post #8 is not my circuit, but it is very close to my favorite. It was "harvested" from the internet.

Hi Dick,

No worries! As we said when I was still a working engineer, I often heard the expression "Every poet is a thief." It is rare when I think back over a long career that it is not that common that engineers come up with a circuit design or topology that is completely novel that has never been seen at least in-part somewhere in some design, somewhere, for something.

I look at other's designs with experienced eyes to cull out what I think works best for what I have in mind. Someone once said, "Take the best and leave the rest." Works for me.

Thanks,

Neko