Piezoelectric Transducer
Piezoelectric transducer are one that can generate a voltage proportional to the stress applied upon it. These transducers work according to piezoelectric effect.
When a mechanical force or pressure is applied across piezoelectric crystal, an electric potential appears across the surfaces of crystal. This generation of electric potential is due to displacement of electric charges. On the other hand, if a varying voltage is applied across the crystal, there will be a change in the dimensions of the crystal. This phenomena is known as piezoelectric effect. Piezoelectric effect is reversible as discussed above.
source: nec-tokin.com

Applications of Piezoelectric Material(Transducer)

Piezoelectric materials found many applications for industrial, environmental, medical, musical and personal use.
1. The most common application is in microphones where sound pressure is converted into electric signal and this electric signal is amplified to produce louder sound.
2. It is normally used as an accelerometer due to its excellent frequency response.
3. Automobile seat belts lock in response to a rapid deceleration is also done by piezoelectric material.
4. It is used to measure force, pressure and displacement in terms of voltage.
5. It is used in Inkjet printers, buzzers, piezoelectric humidifiers and electronic toothbrushes.
6. It is used in medical diagnostics.
7. One of the application of piezoelectric material is in electric lighter which is used in kitchens. When pressure is applied on piezoelectric transducer, it generates an electric signal which causes flash to fire up.
8. It is used is automatically open/close doors that you might have seen in Hotels, airports, etc.

Advantages of Piezoelectric Transducer

1. Since they are Self generating transducer; so no need of external source.
2. It is easy to handle and use because of its small dimensions  and large measuring range.
3. It has very high frequency response. It means that the parameters change very rapidly.
4. Natural quartz and Barium titanate can be made in any desired shape and form.

Disadvantages of Piezoelectric Transducer

1. Piezoelectric transducer need high impedance cable for electrical interface because the device operates with the small electric charge, 
2. It is not suitable for measurement in static condition.
3. This Transducer output is low so some external circuit is attached to it.
4. Its output is affected by temperatures variation.
5. Its output is affected if relative humidity rises above 85% or falls below 35%.
Excercise
Q. What are the common materials used for piezoelectric transducers?

Ans. Material used for piezoelectric transducers are of 3 types
1. Natural crystals. example, quartz and Rochelle salt.
2. Synthetic crystals. example, lithium sulphate, ammonium di-hydrogen phosphate.
3. Polarized ceramics. example, barium titanate.
Q. Why is natural quartz considered most suitable material for piezoelectric transducer?
Ans. Natural quartz is considered most suitable material for piezoelectric transducer because
a. It has lower temperature sensitivity.
b. It has a higher resistivity.
c. It exhibits linearity over a wide range of stress levels with very low hysteresis.
Lower temperature sensitivity and higher resistivity provides an inherently long time constant which allows static calibrations.
Piezoelectric Transducer Working Principle & Applications
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