Em 26/4/2010 20:04, ivp escreveu:
>> do you think it would draw to much current to drive an old speaker ?
>
> Ohm's Law again I'm afraid ;-)
>
> If it's an 8 ohm speaker, that's pretty much shorting the output
>
> V =3D I * R
> 5 =3D I * 8
> I =3D 0.625A (625mA)
>
> Any port pin can supply only 25mA, the whole port perhaps 100mA
> but that's really pushing the specs. Work backwards to find the minimum
> speaker resistance which will (possibly) not munt the pin
>
> R =3D V / I
> R =3D 5 / 0.025
> R =3D 200 ohms (that is the DC resistance, not Z, the impedance)
>
> If you want to use a speaker you should use a transistor as a current
> amplifier or an audio amp IC. A piezo element will be OK as a sound
> producer too (see attached). They have been discussed here often
>
> Google for
>
> microcontroller speaker
> microcontroller piezo
>

Don't forget that for a magnetic speaker you will need to supply a
symmetric wave (in the sense that current must flow in both directions).
You could connect the speaker between two PIC pins (BTL - bridge tied
load) and drive them complementarly (when on is low the other is high
and vice-versa), or connect the speaker between a pin and a capacitor to
ground.


Regards,

Isaac
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