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H1-41: ULTRASONIC MOTION DETECTOR 9 years 8 months ago #795

  • zzfixk23
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ID Code: H1-41
Purpose: Demonstrate how an ultrasonic motion detector works.
Description: This device senses motion by comparing the Doppler shifted wave reflected by a moving object with the original wave created by the device. Any frequency change is accompanied by a continuous phase change between the two waves, which is sensed and ultimately turns on a light bulb on top of the unit, corresponding to the alarm. (1) Moving some object toward or away from the device sets off the alarm light. (2) Moving an object across the beam does not cause a doppler shift, and therefore does not set off the alarm. (3) The frequency of the emitted wave is about 40 kHz. With a wave generator and supertweeter, one can shine waves of about 40 kHz on the receiver transducer, fooling the device into thinking that a moving object is in the vicinity (as seen in the photograph).
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