How to tame the screech

HollyMinkowski

I have a really simple project that uses a pretty limited
uC. I don't have any code space left and I don't want to
rewrite the thing but I am driving a small speaker from
a single pin to create a simple audio alarm signal with
different beeping patterns as a telltale. I'm using a 3904
as a cheap amplifier of the square wave pulse signal.

The screeching square wave tone sounds just awful! It really
sets your teeth on edge :-(

Is there some simple way I can at least smooth out that
square wave signal a bit to make it less painful? If it could
sound just a bit more like a nice pure sine wave that would be
sweet.

Phil Pilgrim (PhiPi)

You can add a lowpass filter either before, or as part of, the transistor amplifier. For examples see:

http://www.qrp.pops.net/filter1.asp

-Phil

Bob Lawrence (VE1RLL)

Add a filter. With a passive one you would loose some gain. You could add an op-amp and design an active filter with this TI Active Filter design software: http://focus.ti.com/docs/toolsw/folders/print/filterpro.html

HollyMinkowski

Thanks guys, I will try to construct and add a filter and see what happens :-)

Hmmm, I was just thinking about how simply adding a capacitor across the
output of a power supply smooths out the ripple... I wonder if that would take
a bit of the edge off this screeching tone generator of mine. Seems like it
might also lower the volume too though, when I get back to where my workbench
is I will try these things out.

I'm stuck here in a hot room arguing with fools for a day or two, someone rescue me ;-)
The drone of my hot air station blowing would be a welcome relief from these guys...and
it's breath would be sweeter too..LoL

Mike Green

A power supply filter capacitor is just a low pass filter with the internal resistance of the transformer and rectifier providing the resistance. Try a variety of values just for fun and see what audible results you get. Remember that a small speaker will have poor low frequency response and will emphasize the undesirable higher frequencies, so you'll need a lower cut-off and faster roll-off than you would need with a larger speaker with a broader response.

HollyMinkowski

Can't wait to try the simple filters out.
The speaker I'm using is a real small one that was probably
designed for a kids toy of some sort. It has a white plastic
cone and a really tinny high pitch sound. (they were cheap, what can I say :-)

Maybe changing the frequency of the pulses and the addition
of a simple filter can combine to find audio goodness.

No idea why I didn't think of just trying a freq change to see what that
would do. I'm at 700/sec now...perhaps 300-500 would be kinder to the
ears.

Spiral_72

If you use a resistor and cap, it'll round the corners of the square wave nicely.... then another cap to filter DC and you should have a crude sine wave with big fat peaks and a high slope on the EDIT[ zero crossing] transition.

It'll make a more pleasant tone and it don't look too bad on an o-scope either.

HollyMinkowski

Spiral, thanks..I will try that and see what it looks like on a scope too.

BTW, whenever I look at your avatar I always blink when the mean little stick man
pokes the unlucky little stick man ..LoL
:-)

Spiral_72

Also, Jameco and the like has beepers with a built in oscillator at varying frequencies. I think you're stuck with one frequency though. I might recall beepers for dual-tone, but you'll have to check. That's too easy though