Ultrasonic Transmitter and Receiver

Introduction

Ultrasound is a form of cyclic sound pressure with a frequency greater than the upper limit of human hearing, this limit being approximately 20KHz. Some animals – such as dogs, dolphins, bats, and mice – have an upper frequency limit that is greater than that of the human ear and thus can hear ultrasound. Ultrasonic waves, however, being inaudible to humans, have little or no effect on the ear even at high intensities. Ultrasonics have found wide industrial use like nondestructive testing an object is irradiated with ultrasonic waves; variation in velocity or echo of the transmitted waves indicates a flaw. Fine machine parts, ball bearings, surgical instruments, and many other objects can be cleaned ultrasonically. Metals can be welded together by placing their surfaces in contact with each other and irradiating the contact with ultrasound. The molecules are stimulated into rearranged crystalline form, making a permanent bond. Ultrasonic whistles, which cannot be heard by human beings, are audible to dogs and are used to summon them.

Transmitter

Working

• The 555 IC is used here as an astable multivibrator
• It oscillates at a frequency of about 40kHz. An ultrasonic transmitter transducer is used here to transmit ultrasonic sound very effectively
• A 10k pot is provided to vary the frequency of the astable multivibrator to adjust for maximum performance
• During the positive half cycle npn transistor SL100 is turned on while during the negative half cycle pnp transistor SK100 is turned on effectively doubling the peak to peak voltage fed to the ultrasonic transducer

Design

f = 1 / (0.693 * ( Ra+ 2Rb)*C))
f = 40kHz which is the center frequency of most ultrasonic transducers
Let C  = 680pF     
      R_b = 10kOhm

Let R_a be made up of two resistors, fixed R_a1 and variable V_Ra so that
      Ra = R_a1  + V_Ra

Let R_a1 = 4.7kOhm
Then V_Ra = 16.47kOhm so that Ra ≈ 22kOhm
Practically we use V_Ra = 20KOhm variable resistor and tune it

Note : In our case, the closest capacitor to 680pF available in the market was 560pF. We replaced the same in the circuit and it worked with tuning of V_Ra

Receiver

• When there is an obstacle the ultrasonic sound is received by ultrasonic receiver transducer. It converts it to electrical variations of the same frequency
• Ultrasonic receiver also uses a two-stage amplifier, a rectifier and an operational amplifier in non-inverting mode. Output of op-amp is connected to a buzzer
• These signals are amplified. The amplified signals are then rectified and filtered
• The filtered DC voltage is given to non inverting pin of high frequency op-amp CA3140. The inverting pin of op-amp is connected to a variable resistor
• The 9v ouput voltage of comparator is made microcontroller compatible by using a 7805 voltage regulator

Circuit Diagram

Ultrasonic Transmitter and Receiver Circuit