4kHz piezo transmitter in 5 minutes

Tried to find the circuit diagram of my old Doppler ultrasonic radar experiments, not found. Had to have the parts somewhere, BINGO. No idea how to connect that old coil from a Philips acoustic remote (transducers are from remotes too), Measured inductances, see circuit diagram on paper in background, build a simple 1 transistor oscillator with most likely connections, worked first time:

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Starts up at 1.2 V (remote battery?), at 12V in it produces 200Vpp 44.X kHz across the piezzo. Just 10 mA current or so.

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Note his coil has an ADJUSTABLE core!

The other piezzo picks it up all right, but I could only find one coil, need to wind one to tune the other, know the inductance now :-)

Maybe the old remotes had a 9V battery?, that would make sense considering the voltage levels.

On a sunny day (Mon, 25 Nov 2013 15:28:42 GMT) it happened Jan Panteltje wrote in :

This is a close up of the inside of that coil:

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Note the adjustabe core tube, the core is in the center somewhere. Looks like Philips melted everything together? Or was it me?

The circuit diagram of my test circuit, it fell out of the picture above:

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Given the inductance ratio or winding 1-4 to 2-4 of about 9000 / 50 = 180, expect turns ratio of sqrt(180) = 13.42. So if the circuit runs at 12V DC, then Vce will be 24 Vpp, and the voltage over the transducer 13.42 x 12 about 160 Vpp. I measure 200 Vpp, so somewhere in that ball park OK.

Note to J.L: the extra turn allows me to use a simple BC548-B low volt transistor to drive the piezo in resonance. Fc can be set precisely with the core (cannot find my trimmer set).