Here it is:"A Cheap Ultrasonic Range Finder"
Do you need to add a distance sensor to your embedded project ? Build this simple ultrasonic range finder !
This quick & dirty PIC ultrasonic range finder will find a place in numerous projects : presence detector, robotics, car parking, distance measurement...
With a few cheap components and less than 200 bytes of code, this sensor will work from 30 to 200 cm, around 1 cm accuracy, with underflow and overflow indication.
More info here: A Cheap Ultrasonic Range Finder