Problem with 555 Door Bell Circuit

Hi Klaus and thanks for the reply.
You're right,now i give DC to the speaker, still i have to do this compromise,since the speaker is used like 1 second a day,it won't blow up. Sound from my speaker isn't distorted,so i think is ok even if i let it like this. I could have used a better amplifier,but it's more convenient and less usage of components. If you have a suggestion,like using 2 or 3 components to amplify the AC signal please tell me.
Silviu

mtn_silviu said:

I also used a BD140 for amplification, a larger speaker and had the 100uF capacitor removed. Used a 500mA wall adapter with a 15000uF capacitor.

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Unless you use a push-pull arrangement, the current in the speaker will always be positive. The speaker cone will be pulled half-way and there will be severe distortion. Such a configuration should always have the speaker capacitor coupled. See the comment #20. Using 15000 uF filter capacitor serves little purpose in the present case.

Of course there is distortion, the signal from the 555 is a square wave and the BD140 simply switches on and off making the square wave. A square wave produces a distorted BUZZ sound.
The output power from the BD140 is not much. If the little 9V battery is brand new it might drop to 7V when the BD140 conducts. The BD140 will have a voltage loss of about 1V then the peak signal in the speaker is only 6V so the output power is only 1W. More output power or a better amplifier would need a higher voltage and higher current battery.

Radio Shack had a wireless doorbell that sounded awful when it produced a square wave BIZZ BUZZ (like the one at the entrance of their store) so I filtered the square waves into sinewaves then it produced a pleasant DING DONG. It used an audio amplifier IC instead of a transistor.