HC-SR04

hcsr04front
hcsr04back
Description ultrasonic ranging module
I have paid $0.72 / piece (free shipping)
Dimensions board size: 45 x 20 mm, 4 x mounting hole M1.4
Datasheet hc-sr04.pdf
Library hc-sr04.ino (test code)

HC-SR04 vs US-015

You may find more similar models on the Internet. I have compared HC-SR04 with US-015 using the same code. Both the units were measuring fixed distance for certain amount of time. As you can see, both of them are quite accurate, however there are few bad samples.HCxUS1Detailed view shows that the US-015 performs better. You can get more stable output using this module.HCxUS2

References

Lesson 18: Measure Distance with Arduino and Ultrasonic Sensor (youtube.com)