magnetic tuning of 44kHz piezo receiver demo AVI

Nov 26, 2013 4 Replies
J

Here you see a 44 kHz piezo transmitter (top right) and receiver (next to it under the black rubber handle of the pliers),

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The receiving side is tuned by a 330 mH inductor tuned down (permeability) by a very strong bar magnet at some distance from it, to about 80 mH I'd guess,.



Watch the scope and movement of the magnet relative to the coil.



So we need no caps, no variable inductors.... just glue the magnet in place ;-)



Inductor, and the magnet, in an IC tube, bottom left...



Circuit is very simple



[ ] ////// -|[]|- 44kHz piezo transducer from old acoustic remote parallel. bar magnet inductor in ferrite housing

Q is Ferry Hi , and the Rx side needs precise tuning to the Tx side.



Now the Rx signal at this distance (few cm, maybe an inch) is about .5 V with 200 Vpp on the transmitting transducer. These high level signals could make an acoustic speedometer more reliable, much better that 5 V PIC output pulses....



This is also the basic Doppler setup, then the Tx and Rx signals are mixed in a dual gate MOSFET, and the difference frequency says something about the speed of the receiver transducer relative to the transmitter transducer, or any reflected object in the beam. Could that be air? :-)



And the test goes on...


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G

Fun, Of course air is terrible mismacth (impedance wise) to the peizo. (If you stuck a piece of plastic between the two you might get big signals... well you'd need some goop too. to take up the little air gaps)

So now you can try tuning with an L and some Y5U ceramic caps.

George H.

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J

On a sunny day (Tue, 26 Nov 2013 06:53:33 -0800 (PST)) it happened George Herold wrote in :

These piezos are made to transmit i nair, are from old acoustic remote controls, have a little cone shaped tweeter like thing in it:

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I just tried it with some 1 uF ceramic, and then 2 x 1 uF ceramics in series on the LC meter (so I could add DC via resistor), I must have good caps, or used a too low voltage (few volts), not much difference in C.

I will wind the exact coil to tune this thing, the next experiment is also fun: | | reflective object transmitter [] -> | | receiver [] direction of motion

44kHz

While moving away you see null after null in the amplitude of the reflected signal :-)

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G

(If you stuck a piece of plastic between the two you

ittle air gaps)

ntrols,

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Neat.

ies on the LC meter (so I could add DC via resistor),

fference in C.

ed signal :-) Hmm, why a null? I guess if you have multiple paths back to the Rx you mig ht see some beating. But I think it'll have to be moving pretty fast. .. doppler shift goes like velocity/ velocity of sound. (VOS ~300m/s) So 1m/ s is a part in 300... so 50kHz-> 20us period... beats every 6ms or so? (th ere might be some factors of two in there.)

George H.

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J

On a sunny day (Tue, 26 Nov 2013 09:10:58 -0800 (PST)) it happened George Herold wrote in :

Yes, it behaves very much like when playing with microwaves (GHz range), all sorts of reflections interference etc. The wavelength in air for 330 m/s velocity (IIRC) and 44000 Hz makes labda = 330 / 44000 = 0.0075 meter, 7.5 mm.. So any 360 degrees something seems to cancel, probably direct wave from reflected?

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Exactly 7.5 mm. :-)

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