I just recently made a circuit using an op-amp as a comparator. One input i had a steady +5V and on the other input the voltage would change based on a photo resistor. When the photo resistor went below +5V the output of the op-amp would go high. This was then used to activate a transistor. I just made it turn on an LED but you can connect the transistor to whatever you would like. Ill try to put up a schematic tomorrow.
I suppose you must have studied transistors.You can go making a TV remote jammer.You need an astable multivibrator,a transistor and an ir(of 37Khz frequency).You can make astable multivibrator using op-amp.First you make this & i will tell rest.