>  Now, to start building a prototype I have only an
>  electret microphone, a 8 Ohm loudspeaker, a 555 and a 567 pll (and, of
>  course,
>  plenty of PICs !).

I have started with some prior tests, with the 555 as a astable oscillator
feeding the speaker, and looking with a scope the signal driving the
loudspeaker and the signal given by the microphone. One of the few
things that  I can conclude with absolute certainty is that for frequencies
above 10 KHz I am absolutely deaf :-(
The other thing that I can conclude is that my wife does not like 10 KHz
sounds :-)

(Note : Both the speaker and the microphone work for frequencies well above
the audible band. I have not measured yet the cutoff frequency. I have not
tried neither to put the 567 tone decoder to work).

By the way, I have dismantled an ultrasound-based  distance measuring gadget
and I have taken away the emitter and the receiver.
How can I distinguish one from the other ? They look like the same.
The only difference is that one has a white spot painted and the other one a
blue one. Any clues ?

Any simple circuit for driving them ?

Thanks

Joan

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