Night Lights

11/02/2008 07:29

One of the nicest aspects of having a central home computer is that I can tie in the lighting system to the alarm system. This gives me an inexpensive way to create a feedback control loop using $10 motion sensors.  When I built the house, I made the decision to run extra wire to all of the rooms.  While I never thought I would use all of the wire, originally, it turns out that I wish I had run even more.  I hooked motion detectors into the 18/4 wire in most of the rooms.  Laszlo now knows if there is someone moving in the room.  A simple script allows him to check to see 1) if it is dark outside 2) if there is a light on in the room.  If it is dark and there is no light on and there is movement in the room, Laszlo will gently fade the light in the room up to 20% for the duration of the movement in the room plus 30 seconds.  The result is a roaming night light that follows you around the house.  I’ve found this particularly helpful if my hands are full.  The other benefit is a remarkable savings in electricity.  The other benefit is that Laszlo, if he doesn’t see movement in the room for a set amount of time (settable by room) will turn off the lights.  Now, this algorithm is select enough to relay here.  If, for example, I just walk into a room, Laszlo will bring the lights up and start a 30 second counter.  But if I turn the light on myself, he will note this and start a longer timer (usually 30 minutes to an hour).  Generally, I pick the time based on what one typically does in a room.  For example, one may stay motionless in the office longer than in the living room.  Rather than turning off the lights directly, I can have Laszlo dim the lights (or blink them for CFLs) about 10 seconds prior to shutoff to give anyone time to move around and let Laszlo know there is still someone in the room.  


Another unexpected benefit to the system is an unusual savings in light bulbs.  When I first moved into the house several years ago, I didn’t have enough money to purchase dimmable CFL bulbs (indeed, they had just hit the market and were $35 each).  So I decided to buy basic tungsten bulbs and, as they burned out, I would replace them with CFLs.  Trouble is, they haven’t burned out.  I’ve only replace two or three bulbs in the five years I’ve lived there.  My guess is that the slow ramp of voltage is easier on the tungsten filament than a step function of zero to 120 volts.  Plus the bulbs don’t get much on time as Laszlo keeps turning them off.  The result is low energy usage and few bulb replacements.