Description-
HC-SR04 Ultrasonic Sensor Module sends 8 square wave pulses (40Khz) and automatically detects whether it receives the returning signal. If it detects a signal,  a high-level pulse is sent on the echo pin. The length of this pulse is the time it took the signal from first triggering to the return echo.
 
Specifications-
Operating voltage: 5 Volt DC
Operating current: 15mA
Measure angle: 15°
Ranging distance: 2cm - 400cm
Working frequency : 40Hz
Trigger input signal : 10uS TTL pulse
Echo output signal : Input TTL lever signal and the range in proportion
Working voltage : 5 Volt DC
Static current: Less than 2mA.
Output signal: Electric frequency signal, high-level 5V, low-level 0V.
 
Applications / Suggested Uses-
To avoid and detect obstacles with robots like biped robots, obstacle avoider robots, path finding robots, etc.
To measure distance within the range - 2cm to 400cm
To map the objects surrounding the sensor by rotating it
Depth of wells, pits, etc. can be measured since the waves can penetrate through the water.