In many cases, a doorbell that sounds off a musical tone is preferable over the common buzz sound. 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”. The data input (pin 7 of IC 4015) becomes logic “1”. The 4015 is a static 4-bit shift register. Each clock impulse coming from U4 shifts this logic “1” further in the register. The clock frequency is around 5 Hz.
The number of shifted logic “1” is directly dependent on the length of time the switch S1 is closed. Once at least one shift register is logic “1”, 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” in the register. One output of the register (pin 2) is coupled back to U2 and U3 so that all the logic “1” in the register always run in a loop.
When S1 is released (opened), the register runs until the capacitor C1 gets discharged through R2. When S1 is again pressed (closed), the capacitor C1 stays charged causing the musical bell to play continously.
The difference between two ways of activating the switch S1 is that different combinations of logic “1” are inputted into the shift register. These different combinations produce the different melodies plyed by the circuit. The musical doorbell must be connected to an audio amplifier. The supply voltage is not critical. It can be between 5 and 15 volts. The circuit consumes around 15mA.