Devantech SRF04 Sonar Rangefinder

This sonar ranging module is a good option for projects requiring accurate ranging information. 

There are a couple of requirements for the input trigger and output pulse generated by the ranger. The input line should be held low (logic 0) and then brought high for a minimum of 10 microseconds. The pulse is generated on the falling edge of the input trigger. The ranger's receive circuitry is held in a short blanking interval of 100 microseconds to avoid noise from the initial ping and then it is enabled to listen for the echo. The echo line is low until the receive circuitry is enabled. Once the receive circuitry is enabled, the falling edge of the echo line signals either an echo detection or the timeout (if no object echo is detected).