Generally home made lucid dream masks do not detect rem sleep, instead you would set them with a timer, then the produce stimuli. Dream masks such as the nova dreamer detect rem then produce a stimuli which is more effective. If you were interested in timed stimuli you might be better off using somethign such as "sleep as an droid" app on a droid phone and recording your movement to detect your rem cycles intervals. once you have that information make an audio file that will play at said interval (birds chirping or something... somethign you can reality check.) in theory this would work just as well as the home made mask and not be as expensive or take the time to make just to now work that well. GOOD LUCK