The sensors used are silicon phototransistors and Cadmium Sulfide (CdS) photocells. Both of these sensors allow less current to flow when they are dark than when lighted. Phototransistors change their conductance while photocells change their resistance depending on the intensity of the light falling on them.
http://home.cogeco.ca/~rpaisley4/PhotoDetectors.html
detects the move of the human being or the animal using the pyroelectric infrared sensor
http://www.hobby-elec.org/e_pyro.htm
This alarm transmits a ultrasonic and receives it after it is reflected at the obstacle and gives the alarm.
http://www.hobby-elec.org/e_sonic.htm
it gives the alarm when the object which is in the set distance passes away.
http://www.hobby-elec.org/e_soniarm.htm