Jun 202015
 

Electronic Doorbell Circuit using a 555 timer and a CD4017 decade counter

This electronic doorbell circuit gives us all the possibility to change the tone of each of the 10 musical notes that the circuit has. This means we can create our own sound sequence.

The circuit consists of a 555 timer configured as an astable multivibrator, a decade counter, a relaxation oscillator implemented with an UJT (Unijuntion Transistor) and a power amplifier implemented with a transistor that drives the speaker.

The relaxation oscillator having a frequency which depends on the values of the C2 capacitor and the R14 resistor, plus the value of the corresponding potentiometer connected to each of the outputs of the CD4017. The electronic doorbell circuit generates ten notes, which can be modified with 10 potenciometers, varying the frequency at which the UJT oscillates.

Electronic Doorbell Circuit using a 555 and CD4017

How the Electronic Doorbell Circuit works?

The astable multivibrator output (pin 3) sends a square waveform signal to the clock input of the decade counter (pin 14).

When we press the button PB, the pin 15 of the CD4017 decade counter changes from 9 volts to 0 volts, so the clock input (pin 14) of the CD4017 is enabled. As long the doorbell button is pressed, the CD4017 delivers at its outputs, in sequence, a high voltage pulse. (see diagram).

Each output of the CD4017, enables the UJT relaxation oscillator with a different value of “R” (R = R14 + potentiometer). As the potentiometers are modifiable, you can change the oscillation frequency for each output the CD4017. The relaxation oscillator output, taken from the junction of the resistor R13 and the UJT, is applied to the base of the bipolar transistor that drives the speaker.

To change the cadence of the sound of the 10 notes, we can change the output frequency of the 555 timer. This is achieved modifying the values of the R2 resistor and the C1 capacitor.

List of components for Electronic Doorbell Circuit

  • 1  555 timer IC (IC1)
  • 1  CD4017 decade counter IC (IC2)
  • 1  470K resistor (R1)
  • 1  220K resistor (R2)
  • 1  15K (R3)
  • 1  10K (R14)
  • 1  150 ohms (R13)
  • 1  220 ohms (R15)
  • 10  100K potentiometer
  • 10  1N4148 or 1N914 diodes
  • 1  1uF / 250V capacitor (C1)
  • 1  47nF / 250V capacitor (C2)
  • 1  100uF / 12V capacitor (C3)
  • 1  2N2646 Unijunction Transistor (UJT) (T1)
  • 1  NPN 2N2222 Bipolar transistor (T2)
  • 1  normaly Open (NO) contact button (PB)
  • 1  8 ohms miniature speaker

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