Ultrasonic Reflectance Detector

Posted on Feb 7, 2014

This circuit detects motion within approximately 5 inches of a piezo-ceramic element ultrasonic transducer. The detection distance is much smaller than obtainable with other ultrasonic techniques, however, it only requires a single transducer, as opposed to the two-transducer arrangement typically found in other designs. The short-range detection

Ultrasonic Reflectance Detector
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is adequate for many applications, such as proximity-operated commodity dispensers or tamper alarms for merchandise cases. X1 is a directional, piezo-ceramic element, 40kHz ultrasonic transducer driven via R2 by U1, a square-wave astable multivibrator. U1 is tuned to X1`s resonance with RV1. X1 emits ultrasonic energy from its aperture. When X1`s aperture is unobstructed, the AC signal voltage at U2A pin 3 remains constant. Introducing an obstruction near the aperture causes some of the ultrasonic radiation to reflect back into X1`s element, affecting the element`s vibration. This causes a disturbance in the amplitude of the signal voltage, as the reflected energy either cancels or reinforces X1`s emission. U2A is configured as a unity-gain buffer, providing the signal voltage to a half-wave rectifier, CR1, via R3. The anode of CR1 is biased via R4 and R5 so that the lower portion of the signal voltage waveform is clamped. C4 provides a low impedance for the bias. The upper part of the waveform is averaged by R6 and C5. The voltage at the junction of R6 and C5, therefore, is the detected level corresponding to the voltage at X1. U2B, in association with R7, R8, C6 and C7, multiplies the detected level by an AC gain of about 100. The AC-amplified signal appears at U2B`s output, and is further averaged by the long-time constant of R11 and C8. The averaged level is provided to the negative input of op amp U3A. The AC-amplified...

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