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From: firstname.lastname@example.org (N. Thornton)
Subject: Re: Loudspeaker and microcontroller
Date: 23 Oct 2002 08:45:05 -0700
References: <email@example.com> <3DB65042.66FB6FCB@scazon.com>
NNTP-Posting-Date: 23 Oct 2002 15:45:05 GMT
firstname.lastname@example.org (carltons) wrote in message news:...
> In article
> email@example.com (carltons) wrote:
> > In article <3DB65042.66FB6FCB@scazon.com>, Paul Burke wrote:
> > > jools wrote:
> > > >
> > > > Hi
> > > >
> > > > Im trying to create a door sensor and door bell configuration using a
> > > > PIC and loudspeaker.
> > > >
> > > > The problem is making the sound loud enough to hear, a peizo is not
> > > > really loud enough to hear so a small speaker is going to be used. The
> > > > problem is can I drive is directly from the uC pin? Or does it require
> > > > more current than this can supply?
> > > >
> > >
> > > Just a FET buffer (see thread on output buffers - I used a BSS138) with
> > > a 100k pulldown on the gate (not strictly de rigeur), and a 50-75 ohm
> > > coil speaker up to the power supply (don't forget parallel diode). I did
> > > this recently for fun, quite enough noise for a doorbell.
> > >
> > > Paul Burke
> > Questions should be how much sound do you need? Speaker impedance is
> > meaningless when it comes to how "loud" a speaker is. Different speakers
> > have different efficiencies. If you look around you can probably find a
> > 16 ohm 40mm diameter speaker which can be driven with a bridge audio
> > amplifier using a power supply of 7.5vdc or so at some fairly low power
> > levels (~ 500 mW) which should give you about 90 dbSPL. If the signal is
> > a tone or a sort of tone, this will definitely get your attention. There
> > are three important parameters here, speaker efficiency, speaker
> > impedance, and supply voltage. You have to drive the speaker to the power
> > level necessary to get the sound level you need.
> > Steve WB4CZR
> Follow up to my above message. If all you want is a big buzzing noise to
> get your attention then many of the piezo's will give you this. I would
> caution, however, if this is going to be a door bell, that you should
> consider whether you want this to be buzz or something more pleasing to
> the ear. For example, you are working on your computer or doing some
> other task and all of a sudden you get this annoying sound in your ear.
> Yes, you know its the doorbell and you need to answer, but think about the
> mood it has now put you in, especially when you get to the door only to
> find out it's some religious group come to give you "the word of God".
> You could easily lose it and go off the deep end with very little nudging
> in this condition. I would suggest a simple kind of tone which will get
> your attention without annoying the hell out of you. Take it from a guy
> that has gone to the door in his underwear with a hangover only to find
> someone in a white starched shirt with a satchell and a pile of "Watch
> Towers". :-)
One other point, I would suggest taking a look at a smoke alarm's
sounder so you see how air cavity resonance is used to increase the
audio output, and perhaps you can apply the same trick to your
Also speakers vary in efficiency quite widely: for your app it would
be well worth looking at each speakers dB / 1 watt at 1 metre figures.
And finally you may find 1 transistor is all the amplifier you need.
If you need more than just on/off, you can use different signal output
pins, put each one thru an R, add an R to ground, and you have 4 bit
audio, or however many bits you want.
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