Transistor with Buzzer

[metRo_]

Hi,
I'm trying to drive a buzzer using a BC547 and it is driven by microcontroller. The microcontroller is an Atmega328p.

Connecting the buzzer directly to the IO make a loud beep but using the BC547 I almost can't hear it.

The buzzer is http://www.pololu.com/catalog/product/1260, what do you think is happening?



Thanks

[Andy Watson]

The buzzer appears to a be a piezo (nothing more) which behaves as a capacitor. To make it operate you have to drive current into it and then draw the charge back out, that is you have to sink and source current - a single transistor will only do one. 

Edit: Ooo! a circuit has appeared! But it doesn't change my previous explanation. When you close the switch the "speaker" will charge up. Then, when you open the switch, .... nothing happens, the piezo remains charged.  The datasheet specifies that the speaker is designed to be driven with a 9V p-p square wave, i.e. you have to drive it with a square ware, it's not a buzzer. You could try a resistor across the speaker terminals to remove the charge.

[metRo_]

Thanks for your explanation

The resistor make effect but it still doesn't make louder noise than direct driven by the IO pin.

How can I do that with two transistors? I think I'll need a H-bridge.

[Andy Watson]

Do you need the extra transistor(s) ? I'm not an ATmega expert, but I believe it has several outputs which are capable of driving tens of mA (both sink and source). I would connect the piezo direct to the processor - possibly with a 100-200R resistor in series to limit the switching current. If you want to bridge to increase the volume - use two outputs.

[metRo_]

after you tell me it is a piezo and it is diferent from normal speakers I searched on internet about it and I to get it louder I need to increase the voltage.

[Psi]

Try a 470 ohm resistor across the piezo.  That will allow current to flow through the transistor.