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Thread: Ultrasonic Sensors
www.piclist.com/techref/io/sensors.htm?key=sensor
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face BY : Dan Michaels email (remove spam text)



Rich Ottosen wrote:
>Dan:
>I think you are making a wrong assumption about the resistor that you
>labeled "probably too large".
>
>What controls the size of this resistor is not the energy of the
>transmitted ultrasound! The transmitted power is determined by how fast
>you discharge the capacitance of the transducer.
>
..........
>If I remember correctly, the capacitance is about 0.01uf for an
>ultrasonic transducer. 22k ohm times 0.01uf is a 220us time constant.
>


Hi Rich, this is precisely why I thought the R was too large.
Since it's a 40 khz device, I figure you need to recharge in
less time than 1/40khz = 25 usec. So for 0.01uF, R should
be maybe 1-2K.

I am assuming it doesn't so much "resonate" at 40 Khz, but that
you need to "stimulate" at 40 khz. OTOH, it's an assumption.
This weekend I'll try stimulating my transducer, and see what
really happens. [Jinx, no wisecracks, mister].

- danM

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