At 14:18 1998-01-24 +0200, you wrote:
>Hi,
>
>Can someone tell me how to connect piezo buzzer from wrist watch to PIC16F84.
>
>Evgeni
>
>

Actually the "buzzer" is more like some kind of capacitive loudspeaker, and
a little as a resonant crystal.

So,
1)      you have to drive it with a frequency
2)      There is one ore more frequencies that hits a self-resonance, so select
frequency to get the most of it.

DRIVE
Easiest way is to connect the plate to Vdd or Vss, and the piezo
metallization to an output pin. As the piezo has some nF of capacitance
this will cause nuise in the pic chip supply, so I recommend also using a
series resistor.

DOUBLE DRIVE
To get the double amplitude: connect as above, but the metal plate to
another pin instead of supply and drive it to opposite voltage. You the
have the program option of slecting drive amplitude. Or use an external
74HC04 to buffer and invert. Or another buffer chip that can drive higher
amplitude.

BOOST
Piezos of this kind normally kan be driven to 30VAC. So it is possible to
build a boost stage, easiest by using an transistor, inductor (creates high
flyback voltage) and a two resistors. Some watches use this technology;
maybe cut it out from the watch?

BEWARE
The piezo may degrade of DC voltage, so when resting keep the voltage zero,
(or put a cap in series)

Beware when soldering to the piezo chip; the metal plating may melt as easy
as solder. Maybe re-use the contact spring fopm the watch? If possible, use
low temperature solder. Make the soldering point thin to not make cracking
of metal plating in the edges of it suspectible. Make it quick so you cause
as little damage as possible to the glue between the chip and metal plate.

The metal plate is usually of stainless steel. Hard to solder to. Maybe use
a small spring or metal clip around the edge? Or conductive tape.

OPTIMIZATION
As in the watch, it helps to have some kind of air resonance system. Like
Helmholz resonator = Built as a small volume air captured between the
"buzzer" and a cover (approx 1-3mm air), and a small hole in center of
cover (approx 4mm) where most sound will be generated.

Or as in the watch without the hole in the back cover, so the air is just
compressed and the most sound is generatde from the "buzzer" plate.

Maybe keep the "buzzer" in the back cover of the watch?

Hope this helps
/Morgan

Morgan Olsson, MORGANS REGLERTEKNIK, Sweden, ph: +46 (0)414 70741; fax 70331
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