The Devantech SRF05 is a solid, 2-element ultrasonic ranger featuring great range, and selectable 1 or 2 I/O interface
The Devantech SRF05 is a dual-element ultrasonic distance ranger designed for excellent all-around performance. It's not as "smart" as other ultrasonic sensors, but it has a very good operational range, and being dual-elements, you can read much closer distances than with a single-element ultrasonic module.
The SRF05 can run with 3 or 4 connection lines, depending how you want to use it. Two lines are power and ground.
In the 3-line mode, trigger and echo-detect pins are combined in function. Starting low, hold the trigger/echo line high for at least 10uS, then drop it back low again to initiate the pulse ping (sent on the falling edge of the signal). This line then toggles to an output, which will pull low when the echo is received back. Then it's simple math to convert the distance using the approximate speed of sound in air of 0.9 feet per millisecond measured.
In 4-line mode, the trigger and echo-detect lines are separated, each needing a I/O line. Yes, it takes up another I/O line, but sometimes it's easier to work your code that way, especially if you have I/O to spare!