Ultrasonic movement detector

The ultrasonic movement detector is used to detect any movement within the range of the instrument. The ultrasonic waves are longitudinal waves traveling through a medium. The ultrasonic waves are not audible to human ears. Ultrasonic are sound waves which have frequency greater than 20 kHz they have shorter wavelength so they are diffracted negligibly and so they can travel over long distances. They travel as highly directional beam, and without appreciable loss of energy.

The ultrasonic waves are transmitted to the surroundings by the transmitter is received by the receiver transducer, the received ultrasonic waves result a respective dc voltage level from the receiver preamplifier. When a motion is occurred in the path of the ultrasonic wave, an irregular voltage is got from the pre-amplifier and an alarm is activated indicate that a movement is happened. This is the working principle of the ultrasonic movement detector. The ultrasonic movement detector is very sensitive to small movements also.




The basic organization of the ultrasonic movement detector can be shown as below:
Transmitter:
40 kHz oscillator – ultrasonic transmitter driver – 40khz ultrasonic transmitter crystal
Receiver:
40 kHz receiver crystal – preamplifier – negative peak detector – amplifier – window detector – monostable multivibrator – alarm
System description

The Ultrasonic movement detector have basically four blocks namely, ultrasonic transmitter, ultrasonic receiver, decision circuit and alarm. All these blocks are supposed to be on the same PCB (printed circuit board).
Ultrasonic transmitter:

The 40 kHz crystal oscillator connected to a 40 kHz ultrasonic sound producing component constitute the transmitter block. The ultrasonic transmitter driver circuit can be a 40 kHz crystal controlled relaxation oscillator built around a 4049 hex inverter.

The Ultrasonic transmitter block provides sustained 40 kHz ultrasonic sound waves into the surroundings.
Ultrasonic receiver:

The matched pair ultrasonic receiver transducer and the pre-amplifier together forms the ultrasonic receiver section of the movement detector.

The 40 kHz ultrasonic receiver transducer pick up the ultrasonic waves produced by the transmitter of the system and the pre-amplifier outputs a voltage signal modulated by the emf (electromotive force) generated at the terminals of the ultrasonic receiver transducer.
Decision circuit:

The decision circuit is the section of the ultrasonic movement detector which takes the decision on the received signal whether a movement is occurred or not. A negative peak detector, an amplifier, a window detector and a monostable multivibrator together forms the decision circuit.

The negative peak detector (envelope detector) tracks the negative peak voltage levels from the pre-amplifier (The output of the negative peak detector will be a constant voltage level when there is no movement within the range of the system.). The output of the negative peak detector is amplified and fed to the window detector to check if any movement occurred is sufficient to produce an alarm. The monostable multivibrator holds the trigger signal to the buzzer driver up to a preset time if the window detector output is positive.
Alarm:

A buzzer driver amplifier and the buzzer constitute the alarm section. The buzzer will sound as long as the output of the monostable multivibrator is “HIGH”.

The reader is supposed to realize the circuit by self.
Extra notes:
» The ultrasonic crystals must be matched type. (The crystals are available in the market as Tx-Rx pairs)
» Select the crystal with frequency as 40 kHz.
» The 40 kHz oscillator can be a crystal oscillator realized using IC4049 hex inverter.
» The ultrasonic transmitter crystal and the ultrasonic receiver crystal are supposed to be on the same PCB and both the crystals must be installed on the same side of the PCB and must be close to each other. The crystals should face towards the same side also.