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Posted: Mon Nov 11, 2002 4:59 pm
Hi. Wondering how those cheap, 2 lead microphones work. Do they change their resistance or emit a changed voltage as the input changes?
Posted: Tue Nov 12, 2002 10:46 am
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.
Posted: Tue Nov 12, 2002 10:52 am
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.
Posted: Tue Nov 12, 2002 7:38 pm
Hey newguy good question. What is the range of this voltage....i am new to data sheets and all. Typical output range?
Posted: Thu Nov 14, 2002 9:48 am
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.
Posted: Thu Nov 14, 2002 12:11 pm
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.
Posted: Tue Nov 26, 2002 12:42 pm
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.
Posted: Tue Nov 26, 2002 3:13 pm
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
Posted: Tue Nov 26, 2002 4:16 pm
You forgot, Speakers make great microphone also? That’s how the spy equipment listens in to your stereo when its running.
Posted: Wed Nov 27, 2002 9:38 am
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