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The screen of mobile is made up of light emmiting diode and photo diode.which sens the darkness but now a days it only sens human touch it depends on human electric field.led are work on different freqency they are in make one picsle from red,green,blue led and gives every type of color bye there particular combination at received frequency.
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There  are  two  major  type  of  screen  type ;LED,LCD. They  are  embedded  widh  sensors  which  opperate  at   diffrent  principles  according  to  the  manufacture's  choice  .Some  opperate  in  such  away  that  when  you  tourch  the  screen  with  your  finger  there  is  a voltage  drop  at  that  point  hence  a  signal  is  sent  to  the  processing  unit  . Some  also  opperate  when  the  light  emmited  by your  screen  is  blocked  by  your  finger.

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