555 two-tone electronic doorbell circuit (2)

Published:2011/6/7 5:16:00 Author:TaoXi | Keyword: 555, two-tone, electronic doorbell | From:SeekIC

As the figure 15-28 shows, the two-tone electronic doorbell uses the 555 as the core to form the multivibrator to drive the speaker. In fact the 555 is a voltage controllable oscillator, when you press AN, the C1 and C2 are charged respectively, the 555 starts working, the oscillation frequency f1=1.44/(R3+2R4)C1, when you loosen the AN, the oscillation frequency f2=1.44/(R2+R3+2R4)C1, the figure parameters's corresponding frequencies: the former is about 750Hz and the latter is about 500Hz. At the same time, the C2 is index discharged through R1 and adds to the 555's control port pin-5 to change the standard comparison voltage of the internal substrate, so after you loosen the AN, the doorbell circuit outputs the tonal modification sound until the voltage of C2 is empty.



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