Interfacing TSOP IR Receiver with Arduino
The IR receiver will output a high or low level depending on the presence/absence of the 38kHz carrier in the received IR signal. The scope screenshot shows what that looks like in a typical remote control signal. The top trace shows the signal as seen by the IR-photodiode, the first component in the receiver chain. You can see intermittent bursts of the 38kHz carrier. After this signal is demodulated the pulses of the RC command (in this example RC5) show up, see bottom trace.  



Now this is when you send data pulses like a remote control would. You will send the 38kHz signal continuously, so the receiver will see the carrier continuously, or not at all. In the former case the output of the receiver will be high, otherwise low. You just connect this signal to an input pin of the Arduino and test the level. A high level would mean an obstacle reflects the transmitted signal to the receiver, a low level means the way is clear.  

(The scope screenshot was taken from my webpage on RC5)