The SRF02 is a single transducer ultrasonic rangefinder in a small footprint PCB. This Devantech ultrasonic sonar ranging module has I2C and Serial interfaces with a range of 16 cm to 6 m (about 6 inches to 19 feet)

The SRF05 is a single transducer ultrasonic rangefinder with either a single pin for both trigger and echo, or separate trigger and echo pins.The SRF05 can be a drop-in replacement for the SRF04. The SRF05 returns a pulse proportional to distance that can measure from approx 1cm (0.4in) to 400cm (13ft).