Recently musical doorbells have become very common. Musical and continuous power doorbells serve as an attempt to bridge the gap between newer digital circuitry and older doorbell wiring schemes. A major difference between the standard setup of a wired doorbell and a musical doorbell is that the musical doorbell must maintain power after the doorbell button is released to continue playing the doorbell song. Here we design a touch sensitive musical bell with timer that can be implemented very easily.
This touch sensitive musical bell circuit produces a musical tone whenever someone touches the touch point in the circuit. Here we use music generator IC UM66 unlike the commonly used UM3561 or UM3481. The circuit is powered by a 5V regulated supply. The touch interface consists of two plates separated by a small distance (a kind of resistive touch).
Touch Sensitive Musical Bell Circuit
This circuit is built around CMOS IC CD4011 and melody generator IC UM66. When touch plates are bridged by hand for a moment. the circuit starts to generate music. After a few seconds the music automatically stops.
Maximum supply voltage for this circuit is +5 volts. The IC UM66 can not operate beyond 3.3V voltage. IC 7805 regulator based power supply can be used to power this circuit.
Time delay can be changed by changing values of capacitor C1 and resistor R2. Three silicon diodes connected in series between pin 2 of UM66 IC and positive 5-volt rail keep voltage applied to pin2 of UM66 below 3.2 volts because of the drop of approximately 1.8 volts across them.
The article was first published in January 2007 and has recently been updated.