This SRF04 is the most popular sonar sensor for use in hobby robots. It transmits an ultrasonic "ping" when instructed by your program and returns a signal when it receives an echo. Your program determines distance by timing the interval between signals. Its overall range is approximately 1" to 9'. It can detect a 1" diameter broom handle 6.5' or further away. This device works great for most "target avoidance" applications including most combat competitions. For simpler programming and greater control, consider the SFR08 sensor.