Ultrasonic Sensor Module HC-SR04 Distance Measuring Sensor for arduino SR04http://www.ebay.com/itm/Ultrasonic-Sensor-Module-HC-SR04-Distance-Measuring-Sensor-for-arduino-SR04-/400489624461?Price: US $1.571.This module performance is stable, measure the distance accurately. performance nearly the same as SRF05, SRF02 SRF05, SRF02 ultrasonic distance measuring module and other comparable. Module High precision, blind spots (2cm) super close.2.Main technical parameters:Voltage: DC 5VStatic current: less than 2mAElectrical level output: 5V highElectrical level output: at the end of 0VInduction angle: not more than 15 degreesDetection distance: 2cm-450cmHigh precision: 2mmWiring: VCC, trig (control side), echo (receiving end), GND3.Module works:using IO trigger distance, the high signal to at least 10usmodule automatically sends eight 40khz square wave, automatically detects whether a signal return;a signal back, through the IO output a high electrical level, high electrical level duration is: the ultrasound time from launch to return. Test distance = (time of the high electrical level * speed of sound (340M / S)) / 2;Package Included:    1 x Ultrasonic Sensor Module HC-SR04 Distance Measuring Sensor for arduino SR04