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.
You can demonstrate the Piezoelectricity using this two component circuit. Just connect a high bright transparent LED to the Piezo element and gently tap on it. You can see the LED flashing without a battery power. Try and make a Fun.
Ferroelectricity is the phenomenon in which a spontaneous electric polarization occurs in some materials by the application of an external electric field. Ferroelectricity was observed in Rochelle salt by Valasek in 1920 and the term was introduced in analogy to Ferromagnetism which is the magnetic property of some materials. A very good ferroelectric material is Lead Zirconate Titanate crystals used in piezoelements. The combined properties like piezoelectricity, Pyroelectricity and memory make the ferroelectric materials ideal for making sensors.
Piezoelectricity is the accumulation of electric charge in some solid materials such as crystals, ceramics, biological materials like bones, proteins DNA etc. The electrical charge accumulates when these materials are subjected to mechanical stress or heat. The word Piezo electricity was derived from Greek terms Piezein meaning squeeze or press and electric. The piezoelectricity is generated as a result of piezoelectric effect. The Piezoelectric effect is the linear electromechanical interaction between the electrical and mechanical states in the crystalline structure of the material.
Pyroelectricity is the capacity of some materials to generate a voltage when they are subjected to heat or cold. Due to the variation in the temperature, slight changes occur in the position of atoms with in the crystals as a result, the polarization of the crystal changes. This develops a voltage across the crystal. The voltage that develops across the crystal is not stable and when the temperature change remains as such, the voltage ceases due to leakage of current. This may be due to the movement of electrons in the crystals. The term Pyroelectricity was originated from Greek word Pyr meaning fire and the term electricity.
This Pyroelecrtic Fire Alarm is one of my popular circuit published in Electronics For You Magazine December 2009 issue.The circuit exploits the direct piezo electric property of an ordinary buzzer element to sense fire. This is an unusual application of the Piezo element. This device is used in buzzer to produce sound and in transducers to generate electric signals.When the piezo crystals in the element are electrically stressed, it generates sound but if it is mechanically stressed, it generates electricity. In the circuit, it is stressed by heat and thus is act as a very good fire sensor.Full circuit in PDF.
Piezoelectricity is the ability of some materials such as crystals and certain ceramics, to generate an electric potential in response to applied mechanical stress or heat. If the piezo crystals are not short-circuited, the applied charge induces a voltage across the material.