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De-tuning Ultrasound transducers

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Hazmat

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I make bat detectors.
I detune the center resonance peak of the transducer by adding a bit of inductance i.e. 40khz rx transducer with 6mH coil across it increases the band width from 24 - 47 khz (-3db level) thus I can hear a wider range of "batty squeaks".

This does not work with a 25 khz transducer though.
It just seems to stop the transducer dead.
Any thoughts why this should be??
 
Using a 25 KHz transducer rather than a 40 KHz is sort of going in the wrong direction, frequency wise, I would have thought?
Congratulations on only losing 3dB of gain though. I lost a lot more than that when I tried to 'detune' a 40KHz device. It was a while ago though.....maybe they're different now?

Think I'll try something like this device for my next experiment though.

Bit more expensive, but should give better results over the 'useful' bat ultrasonic range....
 
You appear to be de-Q'g the transducer, not tuning it.

You can put a shunt resistor and increase bandwidth but you will lose some sensitiity. This is what medical transducers do. They run at a de-Q'd number of 1 to 3, compared to their unloaded Q of about 20 to 30. Medical transducers de-Q mechanically by putting deadening material on the backside of the resonator.
 
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Do you have a data sheet on the two transducers? Part number?

Sort of:-

40khz one is unmarked open cap 25mm so manufacturer is unknown but
Receiving sensitivity: -65 dB (min)
Capacitance @ 1kHz: 2400 pF says the japenglish 2" square data sheet.

My parallel 6mH (ish) self wound coil (by calculation) within a ferrite core (not tunable) strapped directly on the legs of the transducer.


25khz one stamped ZRX25/3 16mm open cap but I can't find a manufacturer name to match the stamp though.
Receiving sensitivity: -52 dB (min)
Capacitance @ 1kHz: 2500 pF says the japenglish 2" square data sheet as above.

16mH (ish) self wound coil ( by calculation) with a slug tunable core.

Not much to go on really.

If I was clever (and owned a digital camera that worked) I'd try for a screen shot of the response of the 40 khz setup BUT I dropped my wifes one so that's a work/repair in progress.

The 25 khz one won't respond to ANY ultrasound with the coil attached else it works a treat @25khz (ish) without the coil.

Surprisingly bats squeal/k from 14khz to an unbelievable 100khz.
In the UK, 18-50khz I've found is a useful range.
I don't sweep frequency though, I prefer direct frequency division to VCO mixing / hetrodyne.
Bit too complicated for me is hetrodyning.
 
Surprisingly bats squeal/k from 14khz to an unbelievable 100khz.
In the UK, 18-50khz I've found is a useful range.
I don't sweep frequency though, I prefer direct frequency division to VCO mixing / hetrodyne.
Bit too complicated for me is hetrodyning.

Horseshoes go even higher than 100KHz -- but most common bats have at least some content within the 40KHz range, so you'll hear something with your 40KHZ freq division detector!

We had a recent thread on this subject over here: https://www.electro-tech-online.com/threads/lm386-bat-detector-stability.127619/

You might find some of the comments interesting.

I found the frequency division type of detector a bit frustrating. Difficult to tell one species from another. Heterodyne is the way to go, IMHO. Design doesn't have to be complex - I used a few opamps and a CMOS 4052 as the mixer!!

Biggest problem is finding a good (cheap!) transducer. I used a Knowles electret capsule type EK3132 (superseded by now, I think - try EK23132), but if I was going to do it again, I'd try the device I linked to in my earlier post.

Heterodyne detectors can give you a lot more detail --- I was able to identify that the pipistrelles in our local river valley were sopranos, as opposed to common 'pips'. And the Noctules nearly blow the headphones off your head!!

If you really get 'bitten' by the bat detecting bug, and go for 'real time' recorder type devices, then it really gets expensive.......:)
 
And there is the problem word- - - - EXPENSIVE!
It only costs me £20 to make a box with interchangeable transducers and a lot of FUN.

My only attempt at hetrodyning used a simple jfet mixer. Worked sort of ok.
I tried to get a bit sophisticated by sweeping the little VCO I built with "stop scan on detect" latch.
Then I discovered that my keys, zips, virtually anything metal on metal gave out all sorts of Ultrasound chirps and by the time I reset the scan, no bat.
Typical me, grew a brain, got complicated, and lost out.

That's why I use frequency division. Simple 4 position switch to provide various divisions. No twiddly bits.
Besides, I've now added a simple "batty detected" LED.
Being fairly directional, it's tie wrapped onto the night scope and I get to spend a few seconds to a minute chasing the little furry things so usually get a fairly good view most of the time.

Regarding the LM386, I find it a lovely little chip. Ideally suited for battery use and as I use a dicast box, pretty safe regarding RFI instability. I tried to build in a little plastic box. Disaster. Every so often it would burst into oscilllation. Took a long time to trace that down to my mobile phone polling the network. Then, once I found that little titbit, turning on the night scope went and did the same thing.
Got to love the pratical aspects of circuit design haven't you?
 
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That's the beauty of hobby electronics --- just go for the way that suits you! :)
I liked the extra 'chirps' that the FM content of the ultrasound gave with the heterodyne version, rather than the simple 'clicks' of the freq div version. And the ability to use the fine tuning for species identification. ( I found I could leave the detector at around 45 KHz to find something from most species, and then 'tweak' a bit to help identify which!)
But, as a hobby, it's not compulsory! If you're happy with freq div --go for it!
As you say, it's certainly cheaper!......:)
 
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