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We have a lamp post in front of our house, that uses a 50-watts HPS (high-pressure sodium) lamp, and opens by itself at night using a photo-cell.

For a while now, it's been acting out: when it opens at night, it brightens gradually for about 50 seconds, then goes out, "sleeps" for about 80 seconds, and that cycle repeats all night.

Here's a video of this in action: https://video.nest.com/clip/a3cf509f134546acad14117a4fcb58dd.mp4

And a 4x time-lapse that shows the change in brightness during that same period: https://video.nest.com/clip/436e5f3e215748268b8d9582ebef8c46.mp4

Last time we had an electrician look at it, he thought it might have been the photo-cell that was acting out, and he replaced it. It was OK for a while (many months) but now it started doing exactly the same thing as before!

What could be the problem?
Can I swap that HPS lamp with something else, maybe a LED lamp..? Not sure what the connector is...

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    I used to have a night-sensing light out front; when we parked our car in just the right location, it "saw" its own light in the window reflection, and turned off because it thought it was daytime. Sounds like that's not your issue, but maybe others searching for this will benefit from moving reflective objects! – j6m8 Sep 14 '16 at 14:21
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This looks suspiciously like the documented cycling behavior of a HPS lamp that is near its end of life. The HPS lamp is similar to a fluorescent, but instead of using a surface-coated phosphor to convert the UV to visible light, it uses sodium mixed in with the mercury. As a result, the lamp has an internal discharge that is struck upon starting and gives it a negative resistance characteristic, controlled by a ballast (typically an inductor).

When it nears end of life, as in the case of your lamp, reactions with the lamp's alumina (synthetic sapphire, really) shell have depleted much of the sodium from the lamp, raising the voltage required to maintain the discharge to a point where when the lamp is fully hot, the ballast can no longer provide that voltage, causing the lamp to extinguish and re-strike when it cools down. Newer HPS ballasts have a counter in them that keeps them from sitting there and wearing out the lamp trying to strike it repeatedly -- instead, they will "lock out" the lamp until power is removed/reapplied.

Replacing the lamp should get you another 20,000 hours of light -- unlike with incandescent or compact fluorescent lamps, LED retrofits for HPS aren't nearly as advantageous lifespan-wise, although they exist if you wish to go that route for color rendering or disposal considerations, as HPS lamps have mercury in them and should be disposed of along with their fluorescent counterparts.

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    I didn't know that! That lamp was changed some 3.3 years ago, which seems to indicate it's nearing 15,000 hours of light. – Guillaume Boudreau Sep 14 '16 at 1:53
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    There are several types of discharge light. Cycling on and off is the specific way HPS lights fail. (by contrast, mercury vapor lights turn really green.) Change bulb. If still cycles, bad fixture, get on Craigslist and find a used fixture for $5 as everyone is going LED. – Harper Sep 16 '16 at 16:54

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