IR receiver+IR LED as approaching sensor problem

Hey! I'm trying to make a approaching sensor, using exactly the description of "Simple IR Distance Sensor Tutorial"( You can search for this title in arduino.cc, it should be the second result)
except that I'm using a different IR sensor, probably GP18A38AC (according to the shape :P) . Its pin-sequence should be GND,5V,"gap",OUT.

As long as I have the LED connected, it will show 0 in serial monitor(means it detects something), no matter how I block them from one another. The funny thing is, if I change the LED to a wire, it will also show 0. If I disconnect the LED or connect its input to an unused pin or a 5v pin, the serial monitor will show 1. I'm wondering what's the problem, as the sensor and LED only have their GND in common. And, of course, my approaching sensor is not working :frowning: