I'm currently trying to promote energy saving in my home.
Principle stay simple, each time a light is switched on, a X10 command is triggered starting (or reseting) a timer.
If in the lighted up room, motion is detected by a standard X10 motion detector (hawkeyes and so on .. MS13, MS14, MS16), the timer is reseted to the starting value.
When 15 minutes are elapsed (without motion), main light is dimmed at half power (or switched off, a backup light (fluorescent bulb) being lighted up) to avoid "blackout" effect and therefore blast the WAF factor (Woman Acceptance Factor).
At 20 minutes (without motion), all the room lights are switched off.
Seems obvious, but main problematic stay the reliability of Motion detector. First, with 16 installed in a house of 4 people, amount of transmitted information is too high for the X10 PLC brandwith (each motion sensor could trigger a command each 4 seconds). To solve this issue I used the principle of the MR26 (receive Motion RF signal, interpret it without sending it on the PLC).
The second problem was the Motion detector sensibility (especially when people are seated in from of the TV without making any movements).
Sure, it's always possible to interface a "professional" detector providing dual detection (IR and micro waves). But stay always the problem of wiring.
To address this issue, I discovered a PIR replacement part which is electonically compatible with the original one (you can substitute it straightforward without any additional modification) and offers a higher sensibility (4 quadrans in place of 2, giving the ability to detect people approaching in the axe of the detector (original one, detect mainly prople crossing the axe).
The reference is RE46B.
Doesn't seems to be highly referenced on the web, found only following "explaning" URL
Hope it helps
Thierry