variable capacitors - piezo transducer?

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.

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..

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

Don

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.

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