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 Devantech SRF04 sonar range finder is a single-transducer ranging module with a 2-pin serial interface. With a range from 3cm to 3m (or 1.2in to 9.8ft), it’s ideal for situations that require accurate ranging information.