variable capacitors - piezo transducer?

Discussion in 'Electronic Basics' started by Mad Scientist Jr, May 17, 2007.

According to this wiki page

http://en.wikipedia.org/wiki/Variable_capacitor

a transducer is a type of variable capacitor.

Does this include the Radio Shack 273-073A Piezo Transducer?

If so how can you measure the capacitance (uF) it puts out?

What are some other devices that function as variable capacitors,
using either sliders, knobs, or other funky (ie touch-sensitive, light-
sensitive, sound-sensitive, etc) inputs?

Thanks...

2. Stephen J. RushGuest

You've got it backward. A transducer converts some physical signal
(temperature, force, motion...) to an electrical signal. Using a moving
part to vary the value of a capacitor is one approach, but there are also
resistive, magnetic, photoelectric and piezoelectric transducers.

If you have a variable capacitor, you can put it in series with a resistor
across a constant voltage, then measure the voltage across the capacitor.
If the capacitance changes, the voltage will change. The first capacitor
microphones worked this way. You could use the capacitor to control an
oscillator and measure its frequency, or apply a known current to the
capacitor and measure the time required to charge it to a given voltage,
which is how the capacitance function in some digital multimeters works.

3. defaultGuest

A transducer is a device that converts a physical variable to an
electrical signal AND visa versa The trans in TRANSducer. Voltage to
sound and sound to voltage.

A magnetic loudspeaker does the same thing without significant
capacitance.

Piezo devices happen to also be capacitive - as frequency goes up
impedance (capacitive reactance) goes down - but that doesn't make all
variable capacitors transducers or all transducers variable
capacitors.

They will frequently specify the capacitance of the device or you can
measure it - with some caveats . . . they will exhibit different
characteristics at different frequencies (all caps do - but piezo
devices have strong mechanical resonance's as well as electrical
ones).

Capacitive Reactance is used in a variety of devices - touch sensitive
screens, keyboards, level sensing, liquid sensing, dielectric sensing,
pressure sensing, etc.. What did you have in mind?

The piezo electric devices can be used as voltage multipliers like
step up/down transformers - or used as spark ignition devices for gas
BBQ grills. They can be used for a lot more than just making noise.
Ultrasonic cleaners . . . ceramic phono cartridges . . . microphones .
.. . micro positioning devices for mirrors and lenses etc..

4. Don BoweyGuest

Many transducers are bi-directional. Some are intended to only transmit or
only receive, but will do both.

Don

5. defaultGuest

Interesting. I had to look it up. In the "old days" a transducer had
to talk both ways, that was the definition of "transducer." Like the
sonar transducer used for depth/object sensing. Transmits out to
bounce an echo off an object in water or air then senses the return
signal.

Today transducer is applied to any device that converts from one
signal form to another form. Technically, by that definition, a
mechanical wind vane, to sense air flow, is a "transducer."

This is probably definition changing over time.

6. Don BoweyGuest

My first exposure to transducers was with depth sounders in mid-50s. They
were bi-directional, and were base on a Nickel core or a chuck of crystal.

My how times change.... I love it. I don't use speakers in my sound system,
I use transducers.

Don