You can make a simple Knock Alarm using a Piezoelement. The Piezo element used in buzzers has a film of Piezoelectric material called Lead Zirconate Titanate. The crystals of this material generates around 1 volt when it is mechanically or thermally stressed. This principle is used to make this vibration sensor.
CD 4011 is a CMOS IC with 4 gates. The first gate is used as a signal amplifier and the remaining 3 gates form a monostable multivibrator. Gate 1 is biased by a presettable voltage through R1 and VR1. When the Piezoelement vibrates, the voltage from it super imposes the preset voltage at the input of IC1 and it triggers. The monostable then gives a high output for a brief period and the buzzer sounds.
Connect the Piezoelement with the circuit using one core shield wire. The piezomaterial coated on the element is heat sensitive. So take utmost care while soldering. Apply solder gently only once. Otherwise, the piezomaterial will detach. White part of the element is positive and the rim is negative. After soldering, paste a cello tape over it or apply some glue to prevent detachment. Glue the fine side of the Piezoelement on the door that is to be protected.