CD4017 simple doorbell

This article introduces a music doorbell circuit that can be composed by yourself. It can play 9 notes in sequence and is suitable for electronic enthusiasts who like to pursue individuality.

The circuit is shown in the figure. The 220V AC is stepped down by the transformer, rectified by D1~D4, and filtered by C1 to obtain a +6V DC voltage. IC is a decimal counting divider CD4017, Y0~Y9 are ten counting output terminals, and R is a high-level reset terminal. After the power is turned on, the voltage at the C3 terminal cannot change suddenly, so the CD4017 completes the power-on reset. Only the Y0 terminal outputs a high level, and the other output terminals are low levels. CP is the counting pulse input terminal of CD4017, and the counting is triggered by the rising edge of the pulse. Transistors VT3, VT4 and C4 and RP1~RP9 form a note oscillator, and the operating frequency of the oscillator is controlled by the high and low levels output by the CD4017 counting output terminals Y0~Y8. VT1, VT2 and surrounding components form a counting pulse generator circuit, which sends a counting trigger pulse to CD4017. The CE terminal of CD4017 is the enable terminal of the counting input. When CE is connected to a low level, the CP terminal of CD4017 receives the trigger of the rising edge of the pulse to count; when CE is connected to a high level, the CP terminal will be locked and will not receive the trigger of the pulse. At this time, the state of the output terminal of CD4017 will remain unchanged at the previous moment. The working principle of the circuit is: after power-on, CD4017 is reset, Y0 outputs a high level, Y1~Y9 outputs a low level, and the low level of Y9 is connected to the CE terminal, so that the CP terminal can receive the trigger of the counting pulse. The high level output by Y0 turns on the diode D5, and RP1 is connected to the note oscillator circuit. The operating frequency of the oscillator is determined by RP1 and C4. Adjusting RP1 can make the speaker emit the first note. At the same time, the oscillator composed of VT1, VT2 and R1, R2, and C2 begins to send counting pulses to the CP terminal, and CD4017 starts counting. The first counting pulse makes Y1 output high level (other output terminals are all low level), diode D5 is cut off, D6 is turned on, RP2 is connected to the note oscillator circuit, the working frequency of the oscillator is determined by RP2 and C4, and adjusting RP2 can make the speaker emit the second note. Similarly, the second counting pulse makes Y2 output high level, diode D6 is cut off, D7 is turned on, RP3 will be connected to the note oscillator circuit, the working frequency of the oscillator is determined by RP3 and C4, and adjusting RP3 can make the speaker emit the third note. And so on, then the fourth, fifth... until diode D13 is turned on, and the note oscillator emits the ninth note. Finally, Y9 outputs high level, and this high level is connected to the CE terminal to lock the CP terminal of CD4017 and no longer accept the trigger of the counting pulse. The output terminals Y0~Y9 will remain at the high level of Y9, while the output state of Y0~Y8 at the low level remains unchanged, D5~D13 are all cut off, the note oscillator stops vibrating, and the speaker does not make any sound. K is the doorbell button. When a guest arrives, press K, the R terminal becomes high level, CD4017 resets in an instant, the Y0 terminal outputs a high level, and the other output terminals become low level. At this time, the low level output by Y9 is connected to the CE terminal, so that the CP terminal is unlocked and resumes receiving the trigger of the counting pulse. Under the triggering of the counting pulse, the diodes D5~D13 are turned on in turn, and RP1~RP9 are connected to the note oscillator in turn. The speaker plays nine notes in turn to complete a working cycle. Increasing or decreasing the value of capacitor C2 can change the period of the trigger pulse, thereby changing the length of each note. Carefully adjust the values ​​of RP1~RP9 to make the note oscillator emit nine different musical notes as needed. The nine musical notes emitted in a row form a short piece of music. If you want to make one of the notes sound longer, for example, the last note is usually pronounced longer. Just connect the two output terminals Y7 and Y8 in parallel and use a diode to isolate them.