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Hi. Wondering how those cheap, 2 lead microphones work. Do they change their resistance or emit a changed voltage as the input changes?

Piezo-electric mics convert the mechanical energy carred by the sound waves into an electrical energy. Therefore, a volage is generated by the microphone, but the impedance is high (10ish of Meg-ohms I believe).
Piezo crystals are used to generate power for roadway flashers in Japan (the yellow lights on the barracades). They are also the element that generates the high voltage in a piezo igniter. In these devices, the microphone is struck with a spring-loaded metal device that causes a sharp deflection in the crystal's shape and generates a voltage.<p>Another common use for piezo crystals is in inertial sensors, such as solid-state gyroscopes and accelerometers.
These sensrs operate on the principle of an inertial mass being deflected, and therefore changing the shae of the supporting structure, and causing a voltage to be generated proportionally to the deflection.

Generally, you'll want to use a JFet pre-amp stage to accomodate the very high output impedance of the piezo crystals. There are numerous examples across the web.

Hey newguy good question. What is the range of this voltage....i am new to data sheets and all. Typical output range?

i ran across an unusual use 4 piezo speakers. i can't remember where. they make great pick ups 4 musical instuments. the xample was a rubber band banjo. the piezo is put under the bridge, connect it 2 a stereo amp. i made 1 and was amazed how well it worked. note: i used fishing line instead.
thank uuuuuuuuuuu.............................

Radio Shack has 2 lead condenser mics. They run off 2 to 10 volts. Hook a resister to lead 1 with the other end at V+. Use a 1 to 10 microfarad capacitor to feed your signal output. The other lead goes to ground. The output impedence of this is 1k ohm. This is a nice little mic, flat up to 5000 cycles where there is a little peak falling off to -10 db at 20000. They are omnidirectional.

1 to 10 uf is a little to heavy. Without considering filtering, the capacitor is only blocking out the DC, so a small ceremic capacitor will work fine. In fact, feeding the signal through a .01 uf capcitor and pulling the side of the capacitor not connected to the microphone to ground with a 100k resistor will give you a highpass filter that will attenuate 60 Hz noise and pass most of the audio range, from 159 Hz up.

Hmm. So far for two lead microphones we have piezo and electret condenser. Electrets are also (and mostly) available as 3 lead mics. All other kinds are 2 lead, at least their pickups are. Other 2 lead pickups include carbon mics, used in old telephones and dynamic mics which are electromagnetic, like a loudspeaker. Dynamic mics have a lower voltage output and a lower impedance than a piezo. <p>Have a fine Thanksgiving all! Well, Americans anyway. Canada has its Thanksgivings in October, just before the whole country freezes over.<p>Bob

You forgot, Speakers make great microphone also? That’s how the spy equipment listens in to your stereo when its running.

I tapped into the stereo speakers upstairs when I was a kid so that I could listen to it in the basement. It didn't take me long to hook one of the speakers up to an amplifier. They were 10-inchers that I'd put into a box. Talk about good bass response!<p>Of course, all of the intercoms, especially in the "olden" days, used the speaker for input and output.<p>Dean