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?
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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.......