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LM386 Bat Detector Stability

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rogs
I have built the detector. The first part of the circuit is exactly as you describe, apart from using a 4024 instead of your 4017. The amplification is great and the circuit is stable.
To get a simple idea of whether you are in the 'ball park', sensitivity wise, you can gently rub your thumb and forefinger together, directly in front of the transducer
It passes your test at a distance of about 1/2 metre.

It wouldn't drive the buzzer so I have added an LM386. This works fine provided it is off the stripboard on a breadboard. If I put the LM386 and the buzzer on the stripboard I get feedback.

It may be my crude circuit (as usual) can you comment on that? Maybe another low pass filter somewhere would stop it?
View attachment 69102
 
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A buzzer produces many high frequency harmonics.
If the microphone can hear the buzzer then it is amplified and produced in the speaker then is picked up again by the microphone and is amplified more, over and over which is feedback howling. The sound goes around and around.

EDIT:
Your circuit needs an explanation:
 
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Why did you use 33 ohms when the datasheet for the LM386 shows 10 ohms??
It might oscillate with 33 ohms.
 
Ignorance! The lowest value resistor I had was 33R and I guessed that the values of the components weren't critical. Wrongly obviously!
 
rogs
I have built the detector. The first part of the circuit is exactly as you describe, apart from using a 4024 instead of your 4017. The amplification is great and the circuit is stable. It passes your test at a distance of about 1/2 metre.

It wouldn't drive the buzzer so I have added an LM386. This works fine provided it is off the stripboard on a breadboard. If I put the LM386 and the buzzer on the stripboard I get feedback.

It may be my crude circuit (as usual) can you comment on that? Maybe another low pass filter somewhere would stop it?
View attachment 69102

The reason the original circuit used a 4017 was to provide a divide by 10 output of the amplifier 40KHz signal, thus producing a 4KHz pulse stream from any one of the 4017 outputs. This was the resonant frequency of the AC piezo sounder I had selected, and thus provided a simple way of producing an audible output.
By using a different counter, and probably a non resonant 'buzzer' there will not be enough energy to make any sound -as you have discovered.
Certainly using a power amplifier will solve that problem. But it brings others.

As Audioguru has explained, your selection of components for the Zobel network on the amplifier output has not been close enough to the specified items, and the amplifier is 'taking off' or oscillating. You might like to google 'Zobel network' to read more on why that needs to be included.
There probably are different component combinations that would work, but you'd need to try them out to see which combinations did function, and which didn't .....probably simpler to use the recommended component values from the data sheet...or at least fairly close?

The other problem is that the use of a power amp will seriously increase the current drawn by the circuit (it will use more than all the rest put together!), and reduce battery life.

Add to that the fact that this simple version of a bat detector only provides a series of 'clicks' as it's output, and doesn't really need a linear power amplifier at all. You would achieve the same results by using a simple transistor connected to the counter output, and use that to drive the buzzer. We are not about talking 'quality' audio here... it really is a very simple series of 'on/off' pulses we are creating here.......
 
Spring is here again (sort-of) and so is my bat, so I have dug out the detector again.

Because of the poor design of the first one I made I think I became fixated on feedback on the circuit board and didn't take as much notice as I should of peoples' comments about audio feedback. I have done as you suggested and removed the amplifier. Using an earphone there is no more feedback. All I need to do now is to add an "AC piezo sounder". Should I use a transducer or a buzzer? I think I need a transducer since the digital chip already produces a DC square wave, but as you may have noticed I am no expert...

https://www.endrich.com/en/55515/piezoelectric+sound+generators+(transducers+and+buzzer says:
"If the disc is driven by an external oscillating circuit, the piezo sound generator is called piezoelectric transducer. If the disc is driven by a built-in oscillating circuit it‘s called piezoelectric buzzer."
 
Should I use a transducer or a buzzer? I think I need a transducer since the digital chip already produces a DC square wave

Yes you'll need a square wave powered transducer, but as we have already discussed, the resonant frequency will have to match the output from your counter. In my original design I used one of the output pins of a 4017 decade counter (any one will do) to derive a 4KHz square wave that will produce an audible output from a sitable transducer - like this one for example:

https://www.rapidonline.com/Audio-Visual/Miniature-Piezo-Transducer-12-5mm-35-0045

If you are continuing to use a 4024 counter instead, then you will have to locate a piezo that has aresonant frequency close to your chosen output frequency. If it's not anywhere near the resonant frequency, you probably won't get anything audible out.
As we've discussed before, there isn't much 'energy' to work with, unless you use a power amp of some sort. You need the 'resonance' of the transducer to help you get the most out, when using such a low powered signal...
 
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