Want to hear the bat flying in the front yard

[Tomorokoshi]

Recorded from an HP 4918A Ultrasonic Translator.

[RoGeorge]

Nice!   

Thanks for mentioning the instrument.  By googling it I've learned that ultrasound are used industrially to detect gas leaks, debug HVAC, corona discharge, etc.  For example here it is used with a conduction wand to debug a Freon leak https://youtu.be/4oMYMYI17cM

That's a nice to have around tool, much more useful than hearing the bats.  First application that came to mind would be to detect which coils are singing in SMPS.   

Just curious how they made the HP Ultrasonic Translator, the service manual for another model HP 4917A Ultrasonic Translator Detector is available online, https://www.kennethkuhn.com/hpmuseum/scans/hp4917a_manual.pdf and has the block diagram at page 9 of 26, and the schematic at schematic at page 19 of 29.  There is a \$\pi\$ LC filter right after the probe to bandpass the 36-44kHz range, then the signal is mixed with a fixed LO of 40kHz.  The only setting is the audio volume knob.   



LATER EDIT:
Just for the docs, applications for HP Ultrasonic Translators featured in HP Journal - May 1967:
https://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1967-05.pdf

[Zero999]

I was playing with a Wein bridge oscillator recently, remembered this thread and thought it could be easily used as a modulator, by coupling a signal to the gate of the limiting J-FET. It could also be built with an op-amp. R1, R2 C1 & C2 form a filter which sets the frequency, which can be varied using a dual ganged potentiometer and fixed resistor. It will probably need some tweaks to be a practical circuit.

[RoGeorge]

That's very ingenious!   
Might be tricky to keep the Wien oscillating in practice, but definitely worth a try.

Speaking of unconventional ways to make a heterodyne, another one I was thinking the other days was to use the local oscillator output as a power source for the electret mic.  That will turn the electret mic into a multiplying mixer. 



Though, there are some inconvenients with any heterodyne when using for listening to bats:
- first is that the bats emit short pulses, so when hearing at that in real time it will sound more like burst of clicks, not much can be distinguish regarding the "voice/singing" of the bats
- second is that a heterodyne might reverse the spectrum when the LO is higher, like shown in the two examples.  First image is with the waveforms and the second with the spectrum corresponding to those waveforms.


Notice how in the last spectrum plotted appears reversed in audio (magenta spectrum near the origin of the axes - magenta is corresponding to the green signal mixed with the blue oscillator, where the blue LO is higher in frequency than the bats signal).

That will make, for example, an increasing pitch chirp to hear as decreasing in pitch.   

[RoGeorge]

Found this on the street   



and noticed it still has a small piece of PCB with the answering button on it, so it was from the bottom half of a phone, where a mic is expected to be, and guess what?  It does has a mic on that small piece of PCB leftover:



and it happened to be a MEMS mic, the analog kind of MEMS mic  F4-(S)MOE-N090R38-3P marked "ME V1.0".

Tested it with the ultrasounds coming from a piezo disc, and this MEMS mic seems to be working just find up to at least 135kHz if not more, outputting tens of mV (without any amplifier).   



The upper level of blue marks the mic out signal for a frequency sweep between 20kHz and 150kHz, while the lower level blue marks the response for the rest of the sweep, between 150kHz and 200kHz.

[RoGeorge]

The software is not yet ready, but couldn't resist the temptation, so last night took the handheld oscilloscope and the MEMS mic powered by two AAA batteries, and went outside curious if it can capture anything:





It took a while to find a good trigger level, and at first I thought the captures are coming from the rare passing by cars in the night.  Then I realized that the trigger captures are not from the cars noises, but the triggering happens more often in the presence of the car noises because the bat raises the Tx power when it is more noise outside! 

I don't think the captured signals were from the cars, because of two things:
- the signals look like a burst of only a few milliseconds long, thought the passing by cars were taking many seconds of sustained level of noise before fading away
- a few ultrasonic bursts were captured in the silence of the night background sound, with no strong noises to be hear

[Tomorokoshi]

For what it's worth, I couldn't hear crickets at all over the background noise with the ultrasonic translator. So apparently there isn't much energy from them between 35 kHz and 45 kHz.

[RoGeorge]

That was a surprise for me, too.  Even with the crickets and all other background noises of the night, the trace on the oscilloscope was staying mostly flat, except for the bat chirps, which chirps were comparable in amplitude with loud cars.  Bats are probably one of the loudest creatures, if not the most loud out there in the ultrasonic range.

I bet if it were for humans to hear ultrasounds, bats would probably sound very, very loud, like a dog barking only a few feet away, or rather like screaming seagulls above a net full of fish.

[Zero999]

Good find regarding the microphone, which is the most difficult part of a bat detector.

I recently attesteded a bat walk meet up with my 11 year old nephew and he loved it. We used heterodyne detectors, which worked from 17kHz ro 140kHz and I remembered this thread.

I'm now thinking of making or buying one.

I had another idea. Rather than heterodyne, use a sample and hold circuit, clocked with an adjustable squarewave oscillator. This has the advantage of not requiring a sine wave oscillator. It just requires a simple low pass filter to give a good output. The only thing is the oscillator needs to have a low duty cycle to work well, but even the TL072 should be able to cope with that up to 140kHz.

I've had a play with LTSpcie.

[RoGeorge]

Looks interesting. 

Ready made bat receivers can be quite expensive (for what they have inside).
Better build your own, so to implement the sampling idea in practice.

[Zero999]

Sorry, I forgot to ask: did you ever complete your project?

I've had a look for the microphone you used, but couldn't find it for sale anywhere.

[RoGeorge]

I've switched to digital, because it happened that I was already heaving a DSP devboard from TI ( https://www.ti.com/tool/TMDX5535EZDSP ), with a CODEC that can do 192kHz sampling, but it turned out the 192kHz was not really possible for frequencies above the audio.  There were some internal filters I couldn't bypass, got lost in programming and configuring the DSP+CODEC, and eventually never completed my bat receiver. 

For the microphone, I think most of the electret microphones should be good enough.  A MEMS microphone would be even better, though the bats ultrasounds are very loud when compared with any other sounds of the night.  Might need a preamplifier if the bats are not very close.

Look for any abandoned mobile phones, or search for MEMS microphone if you can not find any discarded mobile phone to scrap it's microphone.  The very new ones might have many microphones, but are often digital mics, and very, very small, really hard to reuse.  IIRC Adafruit was heaving some MEMS mics on a breakout board, or if you have one of those "17 sensors Arduino kit", there are one or two sensors with a small microphone.

If you don't have any ultrasound source to test which microphone works the best, shake a ring with (door) keys nearby, their metallic ringing sound will produce a lot of ultrasounds.

Beware with the electret microphones, they are very, very sensitive to heat.  Heat depolarizes the electret material inside, and makes them lose from their sensitivity.  That is why many mobile phones will use spring contacts to connect to their mic, and if not, usually the electret mic has some wires soldered from the factory.  Avoid soldering on electret microphones if possible.

The waveforms above were captured without any filter, with a handheld oscilloscope connected directly to the microphone.  The recording was made from a balcony, at about the same height as the bat was flying, so the distance was somewhere between a few meters and 20-50 meters maybe.  In the picture are loudest ones, when the bat was flying only a few meters (maybe as close as 3meters/10feet or so).

[Zero999]

It's a shame you didn't complete it. It's a bit silly having such a high sample rate, with all that filtering. It makes it pretty pointless. You could have added an external sample and hold circuit.

Yes, an electret microphone should work, but I've noticed some models have a built-in capacitor, which is designed to filter out RF GSM signals. It's typically 1nF, so would form a 72kHz low pass filter with the 2k2 bias resistor. I could use a lower value bias resistor, say 1k for a cut-off of 159kHz, along with a lower bias voltage, so the bias current remains within the permitted level, but I don't know how well it'll work. I might buy a few of each part and then open them up to check there isn't a capacitor.

I found this kit which uses sampling, but with an analogue switch. I would need to build a case and add a speaker amplifier, since his mum wouldn't want him wearing headphones, in case he sets the volume too loud.
https://whadda.uk/manuals/velleman/illustrated_assembly_manual_k8118.pdf
EDIT: Schematic added.

I'm not set on the sub-sample/aliasing/strobe idea. It's just I thought it might be easier to understand. I could go with heterodyne, perhaps with the incandescent lamp for the AGC, but with a J-FET in series to provide the amplitude modulation.

[RoGeorge]

Well, most of my projects remain unfinished, and I have no excuse for that.  Once there are some partial results, and only disciplined work remain for a project to be finished, something else pops into attention.  Learning new things or exploring new topics is more appealing then working to finish something.  Sometimes I return to older projects and finish them, but not for closure.  Usually I return to an old project when I need that device to do something with it.

Unfinished projects leave some sort of guilt behind them, so it is better to finish them if you have the discipline.

I've opened yesterday your simulation, and noticed you used ~150kHz.  Isn't that too high?  I remember bats were audible somewhere around 50kHz, maybe up to 80kHz or so.  150kHz osc - 50kHz bat = 100kHz at the output, which will need a second frequency shift to become audible.  I've read they might emit higher than 100kHz, though that doesn't propagate well enough through air, but I didn't try in practice how much of 100-150kHz can be received from a distance, outside.

Another inconvenience with frequency shifting, in general, is that bats might use a wider band than the audio width of human hearing, so not all the spectrum produced by the bat can be shifted into audio (that's why I've tried to move to digital, with the intention to use DSP for compressing a wider band of 50-100kHz into only 5 or 10kHz of audio bandwidth).  Though, was a little too pedantic from my side when I wanted to implement spectrum compression.  It is very interesting to hear the bats even when only a fraction of their spectrum can be made audible at a time.

[Zero999]

Why 150kHz? I noticed the bat detectors provided worked up to 140kHz, Wikipedia says >100kHz and wanted to account for the highst frequency. The idea is to make it tunable, with R4 being a 100k potentiometer with 10k in series. It's not the final design.

Is the wider spectrum a problem? The idea is just to detect bats. Adjusting the tuning would help.

Digitally recording and playing back at a much lower speed would give a more accurate representation, but it doesn't work in real time.

[Zero999]

I hope you don't mind if I continue to post my designs and questions in this thread, since you appear to have abandoned this project.

I've not stopped work on it.

I tested a few piezoelectric transducers designed for audio/alarm applications and noticed how they have various sharp peaks and troughs between 20kHz and 150kHz. I used two units for each test, one as a transmitter and the other as a receiver. I haven't recorded any detailed measurements, simply because I don't have a microphone with any response data over said frequency range. The intention is to use a transducer to test various electret and MEMS microphones to find a suitable candidate.

I've also had a failed attempt a designing a Wein bridge oscillator with AM, for the heterodyne circuit. The oscillator works, but the modulation index sharply declines, as the input frequency increases.

Here's the schematic.

I built it on a solderless breadboard. For simplicity I used the TL071, because it's single channel, so there's no unused op-amp to worry about.

R4 is a small incandescent lamp, (RS stock code 587-686) I found in a box of old bulbs. I chose this over the J-FET AGC for simplicity's sake. It's run off a +/-6V dual power supply, to avoid the additional complexity of single supply design. It is connected in series with the J113 to 0V. I measured the channel resistance of the F-FET: 54R. This is purely to test the concept, rather than a final design.

The oscillator works perfectly, giving a nice 51.5kHz (very close to the 48.2kHz calculated) sine wave of about 1.2Vpp.


Unfortunately, the amplitude modulation doesn't work properly. 1Vpp (500mVp) of different frequencies were applied to J1's gate. This was low enough to avoid the gate conducting significant current, when forward biased, causing the transistor to work in the enhancement region, yes depletion mode J-FETs can do that.

At very low frequencies, it was enough to saturate the circuit, resulting in a distorted modulation envelope.
128Hz input


This got much better as the input frequency was increased, but note the 45° phase shift and the modulation index decreases drastically, as the frequency is increased.
1.3kHz

5kHz


I tested with 56kHz in and there's hardly any modulation, rendering it useless for the intended application.


The question is: why the steep roll-off? I chose low impedances to avoid the parasitic capacitances of the breadboard being a problem. The output impedance of the signal generator is 50R, connected to 1m of co-axial cable, but I don't see how that's a problem.

[RoGeorge]

I've not stopped work on it.
...
The question is: why the steep roll-off?

Glad to hear you didn't drop the project. 

About the Wien bridge, IIRC it only oscillates when the amplification is precisely 3.  Anything slightly bigger than 3, and the amplitude will go to infinity, so it will generate distorted square waves.  Anything lower than 3 will extinguish the oscillations.  The desired number 3 comes from the attenuation of the RC resonant network.  The negative reaction has to match precisely the positive reaction.  The bulb is supposed to keep the amplification precisely 3.

My guess is that influencing the amplification (by the FET that interferes with the bulb resistance) means messing with the oscillation condition (and extinguish or goes to infinite amplitude) rather than just modulating the amplitude.

For amplitude modulation, I would let the Wien bridge and the light bulb do their thing at keeping the gain precisely 3 and outputting a constant amplitude, then add another circuit as an AM modulator after the Wien bridge.

[Zero999]

I've not stopped work on it.https://www.eevblog.com/forum/Smileys/default/phew.gif
...
The question is: why the steep roll-off?

Glad to hear you didn't drop the project. 

I'm glad you don't mind me using the thread you created.

About the Wien bridge, IIRC it only oscillates when the amplification is precisely 3.  Anything slightly bigger than 3 will, and the amplitude will go to infinity, so it will generate distorted square waves.  Anything lower than 3 will extinguish the oscillations.  The desired number 3 comes from the attenuation of the RC resonant network.  The negative reaction has to mach precisely the positive reaction.  The bulb is supposed to keep the amplification precisely 3.

My guess is that influencing the amplification (by the FET that interferes with the bulb resistance) means messing with the oscillation condition (and extinguish or goes to infinite amplitude) rather than just modulating the amplitude.

Yes that's true, but it doesn't happen immediately. It takes time to the oscillation do build when A_V > 3 and decay when A_V < 3. This is why my circuit doesn't work and is a bad idea.   At low frequencies, the Wein bridge oscillator has time for the oscillations to build and die away. Give it too much time (very low frequencies) and it'll die away completely and at build up to the point of causing distortion, over the cause of a cycle of the input signal. At higher frequencies, simply isn't time for the oscillations to build and decay, during the cycle. It's inherently limited in bandwidth. The only fix would be to put the input through a high-pass filter circuit with the complementary response, which would complicate things and there are simpler alternatives.

The circuit I posted a few years ago will have the same issue.

Thank you for helping me realising this is a bad design. It didn't click, until I responded to your post. I'm a bit annoyed it took me so long to realise what should have been obvious. 

For amplitude modulation, I would let the Wien bridge and the light bulb do their thing at keeping the gain precisely 3 and outputting a constant amplitude, then add another circuit as an AM modulator after the Wien bridge.

I agree.

I'm moving towards the stroboscopic, sample and hold idea.  I might use an analogue switch such as the 741GHC66 rather than a FET. The only downside is it's surface mount, which doesn't bother me, but I would like to go through hole, to make it so my 12 old nephew can work on it, although I plan on doing it myself, so maybe not.