This is the HC-SR04 ultrasonic ranging sensor. This economical sensor provides 2cm to 400cm of non-contact measurement functionality with a ranging accuracy that can reach up to 3mm. Each HC-SR04 module includes an ultrasonic transmitter, a receiver and a control circuit.
You can buy this Ultrasonic sensor with great ratings and cheapest price from ebay by clicking here.
You can get all the technical details for this sensor from it’s datasheet. Download datasheet from here.
Getting Started Video
Watch out this video to know the concept of Ultrasound and also how this particular module works.
Projects using this
2) Distance Measurement using Ultrasonic Sensor
3) Intelligent Breaking System using Ultrasonic Sensor