Ultrasonic waves are defined as the sound waves whose frequency is greater than 20Khz and it is not audible to the normal human ears. This ultrasonic possess wide variety of applications from industries (to detect flaws in machineries ) to medicine(for treating and detecting ailments). In fact some animals like Bats, dolphin etc uses these waves to interact with the external environment. This circuit demonstrate constructing a simple 40KHz ultrasonic transmitter built around timer ic 555.
WORKING OF ULTRASONIC TRANSMITTER CIRCUIT:
The IC 555 was wired to work as an astable multivibrator which forms the oscillator part of the above circuit. This IC U1 was configured to produce a continuous serial square wave pulses of 40KHZ. And the output frequency was governed by the equation F= 1..44/((R2+2R1)C2) . In the above circuit the components R1,R2 and C2 was selected to give 40KHZ square wave as output.
This is the main component of the above circuit which is responsible for converting the input pulse to Ultrasonic waves of equivalent frequency. Usually it was made up of Piezoelectric crystals which have the property of changing size when voltage is applied. Apply input signal will cause them to oscillate and in turn produces high frequency sound waves. Also some components adapt other methods to produce ultrasonic waves. You can also buy one of these in ebay.
The transistor 2N2222 was used to drive the transducer used. You can also replace the 555 oscillator with oscillators built around crystals for long sustained oscillations.