I've done something like this before by making my own "infrared remote" and a 393 comparator with an infrared detector LED. Instead of using a remote with pulsed IR, just have an IR LED connected with a push-button switch. The comparator is connected to the IR detector, and a variable resistor is used to adjust the sensitivity. You can turn the VR until the output just goes off, and when the detector senses the IR light, the voltage increases and the 393 turns on the output (to the relay). I admit, this design is very crude, but it may still be able to do what you want it to do. I can try to post a schematic if you need one.
Good luck!
Der Strom