Art's Theremin Page: Ultrasonic Reflectance Detector

Ultrasonic Reflectance Detector

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 is adequate for many applications, such as proximity-operated commodity dispensers or tamper alarms for merchandise cases.

Circuit Description

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 signal is also provided to U3A's positive input, scaled slightly by R9 and R10. U3A performs the function of an AC comparator, where the reference is established by the averaged voltage at its negative input. The scaling provided at U3A's positive input prevents the stage from producing an erratic output when no motion is present.

When motion is present, U3A's output goes high, quickly charging C10 via CR2 and R12. When motion ceases, C10 is slowly discharged by R13. U4A is a hysteretic (Schmitt) 2-input NAND gate configured as an astable multivibrator. When the voltage at C10 becomes sufficiently high to reach the threshold value of U4A's pin 1, it begins to oscillate at approximately 10 Hertz. The 10 Hertz square wave subsequently keys U4C, a second astable multivibrator with an audible frequency of about 1kHz. U4B provides logic inversion so that U4C is activated with motion. U4C's output drives one side of audio sounder X2, while U4D drives the sounder's other side with a complimentary waveform. The combined functions of the four NAND gates produces a pulsating tone in X2 when motion occurs.

VR1 is a low-dropout, low ground-current regulator ideal for use with 9v batteries. At this writing, Panasonic unfortunately stopped manufacturing this part, and there is no known direct replacement. However, any means of providing regulated 7v to the circuit may be used. Current consumption is about 3mA, with about 750uA used by U1, and 200uA wasted in U3B, an unused op amp section. A reduction in current may be immediately realized by replacing dual op-amp U3 with an ultra-low power comparator such as Texas Instruments type TLV3701IP. Lower-current substitutes for U1's function may also be possible.

This circuit exhibits exceptionally good immunity to ambient acoustic interference, and may therefore be used in noisy environments without false-triggering. Overall power reduction by means of intermittent power application is possible, however, the circuit's various time constants require a settling interval of about 10 seconds after application of the supply voltage, before accurate detection can occur.

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