How to make an ultrasonic proximity sensor with the output as varying sound intensity based on the proximity - Quora
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High Precision M18 Ultrasonic Distance Sensor Analog Sensor 0-5v/0-10v/1-5ma/4-20ma Motion Detector Proximity Switch Sensor - Sensors - AliExpress
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Amazon.com: AOICRIE 5pcs HC-SR04 Ultrasonic Sensor Distance Module for Nano Robot XBee ZigBee by ElecRight for Ultrasonic Proximity Sensors Motion Detectors (5PCS) : Industrial & Scientific
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