> From: Werner Terreblanche <wterreb@plessey.co.za>
> I'm using a PIC in a battery operated device, and amongst other the
> PIC must output some sound on a speaker.  My question is... how do I
> increase the sound volume without draining too much extra power.   My
> current method is to use two I/O pins that would always reverse polarity
> and thus give you evectively 10Vpp swing across the leads of the
> speaker. And yes, I do have a capacitor in series to protect the I/O
> pins!  :)
>
> However I find that the sound is still not loud enough.  Would it
> help to rather use a transistor to drive the speaker?  Would internal
> impedance mismatch with speaker have a great influence on the volume?
> Is there any type of small amplifier that runs of the 5V supply that
> someone can recommend?

I had a similar problem for outputting DTMF tones from a small 8 ohm
speaker.  My solution was to use a small transistor radio transformer
(1K : 8 ohm).  I used one PIC (PWM out) pin coupled via a 1uF cap,
however using two pins in opposition would provide a better impedance
match, and twice the power.  Most of the loudness is obtained by
correct baffling of the speaker.  I cut a hole in the PCB the same
diameter as the speaker (about 40mm) and epoxied the speaker in place.

If this still isn't loud enough then NSC makes some 8-pin DIP amplifiers
that should do 200mW easily.  Check out their web page (which is one of
the better semiconductor sites).  Then again, if you are only interested
in square wave drive then a PNP/NPN combo with common base and common
emitter should give about 100mW into 8 ohms.

Regards,
SJH
Canberra, Australia