This circuit can acts as a visitors alarm as well as a secret bell which can be used to identify family members. If you'e living in flats, it'll become necessary to verify the visitor before opening the door. This circuit senses single puff of breath and activates a musical bell automatically.
Working of the circuit
The core of this simple circuit has an NE555 timer and a well-known UM-66 musical IC. You need to fit the condenser mic inside the existing calling bell switch. Obviously, the mic will sense the air pressure changes following the breath. So it can trigger the alarm. On the other side, only you family members know about this system, hence they puff out before the hole, the door can be opened without any hesitation.
The BC 549 transistor act as an amplifier to boost the sinals received from the mic, it triggers the monostable wired NE555 IC. The power from this IC1 will trigger IC2 UM66 which generate music tone for a short period of time. The monostable timing period can be set by resistor R7 and capacitor C5.