Electronic doorbell produces a chic sound design

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This circuit is a two-tone doorbell composed of a 555 time base circuit, and the sound of the sound is beautiful and pleasant. Its circuit is shown in the figure.
555 and R1~R3, Dl, D2, C2, etc. form a multivibrator. SA is the button switch on the door, which is normally disconnected. In the case of SA shutdown, the 4 pin of 555 is low, which causes 555 to be in the forced reset state and the output of pin 3 to be low. When someone presses SA, the power supply Vdd quickly charges C1 to Vdd through SA and D1, and the 4 pin of 555 is high, and the 555 oscillator starts to vibrate. The seedling frequency at this time is fc1=1.44/(Rd+2R3)C2
In the formula, Rd is a DC resistance of D1 and D2, and is about 500 Ω. At this time, the charging circuit of the oscillator is: Vdd→Dl→D2→R3→C2; the discharge circuit is: C2→R3→chip internal discharge tube. At this time, the oscillation frequency is about 1230 Hz. After the doorbell button SA is pressed, since C1 is full of electric charge, that is, the 4th leg of 555 is at a high level, 555 will continue to oscillate, but the charging circuit at this time is: Vdd→R2 →R3→c2, the charging time constant is increased, and the discharge time constant is still R3C2. At this time, there is fc2=1.44/(R2+2R3)C2
The oscillation frequency of the illustrated parameter is about 680 Hz, which is lower than the oscillation frequency when SA is pressed. As C1 discharges through R1, the voltage on C1 gradually becomes lower. When it falls below 0.4V, 555 is in the forced reset state, and then stops. In this way, the doorbell is at. The initial treble "叮" sound, followed by the "咚" sound, the so-called "叮咚" sound.
The 555's box is restricted by R4, and the VT1 power tube is amplified to drive the speaker to make a beautiful and pleasant humming sound.