Ultrasonic motion detectors emit and receive ultrasonic sound energy in a region of interest.Object motion within the region of interest and in the range of the ultrasonic motion sensor is detected and an alarm signal is produced.The range of these detectors differs from the designed range whenever the actual atmospheric sound propogation conditions vary from the atmospheric conditions assumed in the design .Ultrasonic motion detectors are commonly used for automatic door openers and security alarms. ultrasonic beams are easily blocked by thin materials, including paper and as a result False triggering is easly caused by reflections from blowing curtains, pets, and flying insects.

its is a popular device for detecting motion in a free space. it is not as versatile as the passive infra-red type mainly due to its restricted range, usually only about 9 meters max. It can be used in situations and environments which mould be unsuitable for the infra-red type as it is not affected by temperature chances.The human ear can hear sounds of frequencies between 20Hz and 20KHz (20,000Hz). Sound waves do exist above 20KHz but me cannot hear them, these very high sounds are called ultrasounds or ultrasonics. Imagine you are standing in a street and a police car or some other emergency service vehicle is speeding towards you. As it passes the sound of the siren seems to change. The reason tor this is because sound waves travel at a finite speed (700mph approx) regardless of the speed of the sound source. In effect as the source moves towards you the sound waves are compressed and sound higher, on the other hand, as the source moves away the sound waves are expanded and sound lower. This phenomenon is known as the Doppler Effect after the person who first discovered it in 1845.
The wiring of motion detectors is a relatively easy task as you can see below. The diagram shows a unit with the cover removed to reveal a terminal block for the connections to the control panel. These consist of:
1. A Normally Closed Alarm Relay. When the detector is stable (no movement) then this relay is closed. This is connected to the appropriate alarm zone at the panel and hence the alarm loop will be closed. If however the detector senses movement then it will open this relay for approximately 5 seconds after which it will reset itself.
2. A Tamper Switch Output. This is closed until the cover is removed and is connected to the tamper circuit at the control panel.
3. 12VDC Supply Terminals. These detectors require a 12Volt supply from the panel.


Circuit description
The circuit uses a matched pair of 40 kHz transducer elements to detect movement up to 22 feet away. Crystal locked circuit for maximum stability. An LED is switched on for movement indication. The transmitter sends out a steady ultrasonic signal at 40kHz( wavelength ~= 6 mm).
The reflected sound is detected by ultrasonic receiver. The
signal is then amplified by amplifiers.An amplifier is selfbiasing. The time constant of the first amplifier is set to let the 40kHz signal through. Between
the first & second amplifier there is a negative peak
detector consisting two diodes which follows the envelope of
the 40kHz signal. If there is no movement the envelope is
just a straight line. The time constant ofsecond amplifier is much slower so that it will follow this envelope. When
there is no movement neither diodes can conduct. If the signal falls or rises the diodes conducts which the output to go high. A low pass filter screens out unwanted spurious signals. Specific details about the circuit are available in this link:
http://electronickitskit/complete/surv/k49.pdf


you can do modifications
1.can use non-polarized capacitors rather than capacitors which have polarity to avoid the mistakes in polarity
2.you can use a 39.270 KHZ crystal rather than a 40 KHZ crystal because exact 40 KHZ crystal is may not available in the market