The circuit diagram of door bell circuit using UM 66 IC has been published here. This is the simplest door bell circuit in which the central part is the IC UM 66 as the name says. The difference of this circuit with the melody generator circuit is that the melody generator plays music till you release the switch after pressing whereas in this circuit, after one press full song will be played. In this circuit if once switch is pressed, the capacitor C1 (220 uF) gets charged and the transistor Q2 (BC 548) keeps the IC for playing music till its end. The charging time of the capacitor C1 determines the music playing time of IC UM 66. This can be adjusted by the potentiometer R1 (100K). The speaker K1 is driven by the transistor Q1 (BC 548).
- The voltage given to UM 66 IC should not be larger than 4.5 V.
- Adjust pot R1 to select the playing time for music, whether full tone or a part in one press.
- POT R2 is optional but if included, the loudness can be adjusted.