Clever mechanical design enables energy-saving bulb control

5/21/2010 10:19 AM EDT

I recently purchased a set of three candelabra (small base) light-control socket adapters at Home Depot ($15), made by AmerTac , model SLC2. These light-sensitive adapters turn your bulb on as it gets dark. In my case, they are going into an outside fixture with three such bulbs.

And there's the challenge. As any bulb comes on, its glow–both direct and reflected from the glass of the fixture enclosure–may "fool " the other two adapters into thinking that it is still daylight, so they will not turn on their respective bulbs. (We'd call it co-channel interference or optical crosstalk.)

The solution is easy, but not as easy as you think: rotate each control adapter to minimize both direct and reflected light. But doing this may make the control adapter be only loosely screwed into the original socket, leading to poor contact and intermittent operation, which would be seen as annoying flickering.

What the designers of this product did was simple yet clever: they spring-loaded the center contact, so it can touch the corresponding contact of the socket even as you rotate the control adapter. It took only a minute of trial and error rotating of the adapters to find positions for all three where all the lights came on in the darkness, without crosstalk or internal reflection problems. At the same time, the adapters made good contact in the sockets and the bulbs did not flicker, even when I tested by shaking and striking the fixture.

By spring-loading the base's center contact, the designers anticipated a problem and solved it nicely, without requiring any special steps on my part to ensure proper contact. That's a good design approach. ♦