Musical Doorbell Circuit

Posted on Feb 7, 2014

This featured circuit is a musical doorbell. After the button S1 is pressed, a short melody is played. When the button is pressed many times in rapid succession or pressed longer, a different melody is generated and the melody plays longer. The circuit works this way: when the button S1 is activated the inputs of U3 and one input of U1 switches to logic 0 ³. The data input (pin 7

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of IC 4015) becomes logic 1 ³. The 4015 is a static 4-bit shift register. Each clock impulse coming from U4 shifts this logic 1 ³ further in the register. The clock frequency is around 5 Hz. The number of shifted logic 1 ³ is directly dependent on the length of time the switch S1 is closed. Once at least one shift register is logic 1 ³, a current flows to the base of T1 through a corresponding resistor. The transistor T1 functions as a current controlled oscillator. The tone pitch is dependent on the logic state of the different flip-flop outputs of the shift register. Each clock pulse shifts the logic 1 ³ in the register. One output of the register (pin 2) is coupled back to U2 and U3 so that all the logic 1 ³ in the register always run in a loop.

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