The schematic diagram alongside shows a straightforward design of an electronic musical doorbell basically comprising three important stages:
Timer: As the name suggests, the function of this stage is to produce a certain time delay for keeping the music or the output audible even after the call bell button is released and the power to the circuit is switched OFF. The configuration is basically a mono-stable multi vibrator which holds the output and the relay in an energized position for a time period determined by the value of C2 and the setting of VR1.
Audio/Music Generator: Here's where the popular chip UM 66, which contains an embedded piece of music information in it, becomes the primary part. This device looks pretty similar to an ordinary transistor, but is actually an Integrated Circuit performing the outstanding function of generating complex music codes at its output lead that just need to be amplified for listening.
Primarily the IC consists of three leads, which may be identified as follows:
Keeping the printed side towards us, the left lead is the ground or the negative; the center is the positive supply input while the right lead is used to receive the audio frequency. The IC operates specifically at 3 volts, which needs to be regulated in case the supply voltage used is higher. In the proposed circuit, since the voltage is 12 volts, a circuit consisting of R4 and Z1 is utilized for the required stabilization and provides a stable 3 volts to the IC. The output from the IC is fed to T3 via R5 for the necessary amplification directly over a small loudspeaker.
Power Switching: This configuration consists of a feed back loop connecting the output to the input mains supply via the relay contacts. The stage is used to hold the mains supply once the bell switch S1 is released.
The following section reveals the practical functioning of the above stages once built and installed.