Bhavin_cp, The simple way to detect a person sitting (in front of a computer or not) would be to install a pressure switch in the padding of the chair. Alternately, you could aim a lazer (with companion receiver)at the location which would then have it’s beam broken when someone interrupted the beam. You could also get a very cheap keychain camera and wire it as a monitor of the specific location for both recording and/or motion detection downstream via software but that is usually for accomplishing a lot more complicated goals. If you just wanted to detect if someone has used the computer, again, a keychain camera could be wired to turn on when someone turns on the power switch of the targeted computer. You could even wire a throw away cell phone to predial a number when activity of your choice triggers a event. The solutions are many!!!