Hi, Roger. Seriously, they sound like they're a little too big to be a
microphone. If the cylindrical cans are black thermoplastic, your
parts may be piezoelectric beepers. There are three basic kinds of
beepers:
1) Those that have all the drive circuitry enclosed, and just require
that vThey sound like they're too small to have enclosed drive
circuitry, so you'll have to drive them yourself.
2) Those that have an internal transistor and a couple of resistors --
you have to add a transistor and one or two parts to make them work,
and
3) the bare piezoelectric element.
The best way to tell if you've got a 1) is to just apply a low DC
voltage, and see if it beeps (option 1) or just clicks (probably option
3). Judging from size, I'd guess you've got #3. The piezo concept
means that a change in applied voltage causes the piezo element to
flex, and that flexing the element produces a change in voltage. You
can use piezo discs as sensors (flex causes voltage), and you can also
use them as beepers (voltage causes flexing like a tiny speaker) by
applying an AC voltage to them of the recommended mechanically resonant
frequency (usually 3KHz or so for the smaller units).
For most applications, a digital drive signal is applied to both sides
of the device to produce the "AC". For the really small ones (like
yours), you might be able to do something like this if you've got a
74HC14 and a 5V power supply (view in fixed font or M$ Notepad):
| ___
| .--|___|--. Piezo
| | R | Beeper
| | |
| | | |.-.|
| | |\ | |\ A || || B
| o---| >O--o--| >O-----|| ||-------.
| | |/ | |/ || || |
| | | |'-'| |
| --- | |
| C --- | |
| | | |
| | | |\ |\ |
| === '-----| >O--| >O-----------'
| GND |/ |/
|
|
(created by AACircuit v1.28.6 beta 04/19/05
www.tech-chat.de)
The 74HC14 is an inverer IC with schmitt trigger inputs. You can use
that with an R and a C to produce an audio frequency square wave as
shown, with R and C chosen for the appropriate frequency (again,
usually around 3KHz for the smaller piezo elements). This is fed to
the other gates, so that when A is high, B will be low, and vice-versa.
This means that, for a 5V supply, you're applying a 10V peak-to-peak
square wave to the piezo transducer. If you've chosen a frequency
close to mechanical resonance, and the capacitance of the piezo element
is small enough to be effectively driven by the HC14, you'll get an
audio tone.
This drive method is only recommended for the smallest piezo speakers.
Larger ones can be driven by an LM556 or even a capacitive bridge
driver for the larger piezo elements. This bridge-type switched DC is
much more efficient than sine wave AC, because the fast rate of change
of the square wave produces a much faster flex of the piezo element
(and hence, more sound).
I hope this has been helpful. As to your specific part, I believe
there are about 4 or 5 manufacturers of these little piezo buzzers in
Taiwan, and everyone puts their own numbers on and sells them under
their own trade name. Can't help you there.
If you're interested in designing one into a project, you just might
want to buy one or two. The smaller ones are very inexpensive even in
small quantities.
