or u could try something else, i did something like this one day, to knok when people were coming in.
the thing i used was a cheychain laser a mirror, a LDR and a simple circuit vith a beeper.....
the idea is that u dont need any modulation signal and with a quality laser u can have a gread distance, a few tens of meters....
to solve the daylight problem wuold be to use a small tube, and fix the LRD at the of it, something like this...
----------------------------- ___
====LASER beam====|O___LDR
-----------------------------
if u get it. when the laser light falls on the LRD it shrinks its resistance to less than 1k.