as industrial process get more and more complex.there comes some means of monitoring industrial activities. these activities include. detecting moving of an object. the presence of object etc. the plc need this position and increment movement of an object through some special device known as Sensors. the plc act on the information gather for it activities. there are four different type of limit switch. this include. Roller lever limit switch, Roller limit switch,m lever limit switch and plunger limit switch.