In terms of signals with frequencies over 20 kHz ultrasonic waves are ultrasonic waves or ultrasound call. The use of 36 kHz or 40 kHz frequency signals in circuits operating with ultrasound is common. Ultrasonic signals can not pass through physical obstructions (even if they are transparent).
For this reason,
there is no obstacle between the receiver and the transmitter working according to the ultrasonic system . The operating frequencies of the ultrasonic transducers used in the application are
around 35- 39 kHz. Using ultrasonic sound it is possible to control all kinds of receivers 25-30 meters away.
Simple ultrasonic transmitter with astable multivibrator: In the circuit shown in Figure 5.6, the transistors in turn form a square signal at point A as the transmission sector. Thanks to the variable voltage at point A, the transmitter emits a certain frequency of ultrasound. The frequency value of the ultrasound that the trans- mission is transmitting can be adjusted by changing the value of R2 or R3.