We’re now going to look at how a doorbell circuit works, so let’s start by looking at the schematic.
We have a single 555 timer IC that’s configured in the astable mode. C4 is the filtering capacitor for the IC control voltage while C1 is used to remove the DC component of the output signal before it is fed to the speaker.
A typical doorbell sound has two distinct tones or frequencies and we need to reproduce these frequencies to create the effect. Switch S1 is used to trigger the doorbell and notice that it is also connected to the reset pin of the timer IC which means that the timer IC is held in the reset mode until the switch is pressed.
When S1 is pressed, there is a voltage drop across R4, which switches ON the timer IC. At the same time, capacitor C2 charges to a voltage that’s close to the supply voltage. Diode D1 is forward biased and capacitor C3 charges through R2 & R3 generating one of the frequencies.
When S1 is released, D1 is no longer forward biased and the time constant for C3 is now controlled by R1, R2 and R3. C2 was previously charged and the voltage across it keeps the timer IC out of reset mode. With time, C2 will discharge through R4 and this will cause the timer IC to enter the reset mode where it waits for the user to press S1 again. D2 prevents the voltage across C2 from reaching the RC circuit. Similarly, D1 ensures that C2 is only charged when the switch is pressed, and not through R1.
This is what the assembled PCB looks like and pressing S1 will cause the doorbell to sound.