how does this alarm speaker amplifier circuit to work?

[hero0765]

hi:
here is a alarm speaker amplifier circuit , there is two MCU IO pins to driver the circuit to make the alarm speaker sound different alarm voice. But I have two question here :
1. the circuit is made of four transisters. how do the four transisters work?
2. how do I change the MCU IO signal to make the circuit work?

thanks!

 

[IanP]

This circuit works like a bridge: Q3-Q2 or Q1-Q4, but not both pairs at the same time, so both inputs should not be “H” at the same time ..
To generate sound you need one of MCU pins to produce a square wave and vary the frequency if you want to change the tone ..

:wink:

 

[keith1200rs]

You need a square wave on one I/O pin and the inverse on the other.

Keith