This is two tone alarm use op-amp IC, LM3900 (Quad Amplifiers). It produces the frequency of square wave in 2 level. It is high frequency and low then mix the sound in the end danger beep.
It is appropriate it has can to link up a loudspeaker with ,using power mosfet then have better use transistor which complicated and not durable. This circuit convenients for power supply 9V to 12V sizes and 8-ohm 20watt size loudspeakers will by large-sized have a voice loud small-sized better.
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