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This low-cost circuit uses the piezoelectric element of a piezoelectric vibrator as an input sensor. The piezoelectric element sheet is attached to the middle of the foil using cello tape. Apply a small amount of glue to the edges between the panel and the door. Extend the wire about 1 to 1.5 meters from the piezoelectric to the circuit. IC NE555 (IC1) is configured in monostable mode. When it receives an input pulse, its output will stay high for the time determined by VR1, resistor R5, and capacitor C3.
IC UM66 (IC2) is used as the tone generator. When knocking on the door, the piezoelectric plate generates an input pulse that is amplified by transistor T1. The amplified signal triggers the NE555 timer IC and its output pin 3 goes high to activate the tone generator. Music is heard from the LS1 speaker. After the set time, the melody will stop.
Assemble the circuit on a general-purpose PCB and place it in a suitable enclosure. Attach the piezoelectric element to the door and place the speaker in the central room of the house using a long wire. The circuit operates with a voltage of 5-12 V DC. The music duration can be adjusted through VR1 by changing the timer's R-C time constant.