As shown in the figure, the alarm consists of a multi-resonant sound circuit and a triangular wave generator. The multi-resonant sound circuit is composed of 555 and R1, R2, C2, piezoelectric ceramic horn LS1 and the like. The triangular wave generator is composed of chip IC2 (741) and R4~R7, C3, etc., which is actually a low frequency oscillator, and the oscillation frequency is about 1 Hz. The voltage waveform outputted on the capacitor C3 is approximately triangular wave. The triangular wave is applied to the control terminal 5 of IC1 (555) to change the reference control voltage of the time base circuit 555, thereby frequency modulating the multivibrator, and the multivibrator composed of 555 and R1, R2, and C2. In other words, the basic oscillation frequency is f=1.44/(R1+2R2)C2, and the frequency corresponding to the parameter shown in the figure is about 3000 Hz. After the triangular wave modulation with a period of about 1 second, the multivibrator generates a periodic change. The sweeping wave pushes the piezoelectric ceramic horn to emit a tone-changing alarm sound.
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