HC-SR04 based Distance Finder gives the distance from an obstacle in centimetres. It has a range of 2cms to 400cms. The project is build around AT89S52 which 8051 based microcontroller. The distance is displayed on Seven Segment Displays.The working of the ultrasonic sensors is quite simple and they are easy to interface with the microcontroller. The sensor module has 4-pins out of which Pin-1 and Pin-4 are +Vcc and Gnd respectively.Pin-2 is Trigger and Pin-3 is Echo pin.When a High pulse of 10us is applied at TRIG pin, the ultrasonic transmitter sends 8 consecutive pulses of 40kHz frequency. As the Eighth pulse is sent the ECHO pin of the sensor becomes HIGH. Now when the ultrasonic waves reflect from any surface and are received by the Receiver,  the ECHO pin becomes LOW.