You have to modulate the IR with bursts of 10-16 cycles at 56 kHz. You can get more information by looking at the IR receiver module datasheet. You could also use the IR beacon development kit, which ships without the sensors and so can be used for transmit only. Also, if you only need one-way detection, you might not need the beacon at all: you could just use the receiver module on one end and make a transmitter on the other.