Piezo sensor as audio input using darlington pair

The generation of electric potential by the application of pressure, strain or any force is known as the piezoelectric effect. This effect can also be implemented to sense any audio signal. This can be achieved by using a piezoelectric diaphragm as audio sensor as shown in this project. Apart from the piezo sensor, other parts used in the circuit are as follows:

1. Transistor BC548

2. Resistances

3. Speaker, and

4. Capacitances

In this project, a piezo diaphragm, when subjected to pressure variation due to sound in its vicinity, generates an electric signal. This output is then supplied to a darlington pair which basically consists of two transistors. Transistors T1 and T2 (BC548) have been used in the darlington pair which has a very high current gain. When the diaphragm receives any audio signal, it produces a voltage difference across its opposite faces. This signal is filtered for any DC component by a capacitor C1 (0.1uF). Transistor T1 of the darlington pair amplifies the input signal and the output appears across resistor R2 (3.3k). Resistor R1 (470k) provides base-collector bias to T1. The output of T1 is fed to the base of transistor T2 after it is filtered by another capacitor C2 (0.1uF). The output of T1 is further amplified by T2 and the amplified signal appears across R4 (1.2k). The resistor R3 (220) is used to provide necessary bias to T2. The output of T2 is filtered by capacitor C3 (10uF) and given to the speaker to play it. Resistor R5 (2.2k) checks the current fed to the piezo diaphragm.