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Just moved into a house. External sconce light wasn't working. I swapped it out for another bulb rated 110/120V. The light flashes on and off repeatedly.

I removed the bulb and put my multimeter leads into the screw base at the + and - terminals and read voltages hopping around between 90 and 100V.

I went upstream to the easiest access point (not the immediate next one) and measured 120V.

So I removed the sconce and measured the wires there and get 120V.

From that point, the cable runs directly into the sconce and to the bulb base.

I've repeated this multiple times and get the same thing. There's nothing else between the wall/sconce 120V and sconce/bulb 100V.

Does this just mean there's something wrong inside the sconce?

Edit Update: there might be a photocell in the fixture I was not aware of. We have two of these sconces, and the other one I turned on at night but wasn't on this morning. There's no apparent photocell anywhere, so maybe it's in the fixture itself somewhere. My understanding is a photocell changes resistance depending on lighting, so that would mean the voltage drops during the day, explaining the 100/120V discrepancy. Will update.

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    Would say so. Rust/corrosion or a loose connection.
    – crip659
    Commented Dec 4, 2021 at 21:49
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    As @crip659 states, likely corrosion. In particular, a lamp socket shell might be aluminum, which, outdoors, oxidizes and causes bad contact at the brass rivets holding it. If you remove the sconce, you might try cleaning the socket and mashing the rivets tighter.
    – DrMoishe Pippik
    Commented Dec 5, 2021 at 2:45
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    You could measure the resistance between the disconnected wires and the bulb socket. It should be close to zero. That would confirm the issue.
    – Steve Wellens
    Commented Dec 5, 2021 at 5:34
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    Thank you @SteveWellens a new discovery this morning (other, same model sconce not working but was working last night) suggests there is a photocell in these sconces I didn't know about. I can use your suggestion now and tonight to check if there is a photocell if the resistance changes.
    – user1713450
    Commented Dec 5, 2021 at 16:41
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    a photocell would burn up if it dropped the mains voltage via resistance. it controls a triac's much smaller clamp current, not the light's current. Not saying you're not measuring downstream of the triac, just that you're understanding the photocell operation slightly wrong.
    – dandavis
    Commented Dec 5, 2021 at 18:42

1 Answer 1

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The light flashes on and off repeatedly.

If your new light bulb is LED and the previous was incandescent, this is an indication that some part of the path from panel to bulb is not LED compatible. This can include:

  • Smart Switches
  • Dimmers
  • Timers
  • Motion Sensors
  • Photocells

Since you indicated that you may have photocells, that fits the nature of the problem. Depending on how it works, a photocell could vary power flow more than just "on" vs. "off". With an incandescent bulb, mostly on = on but a little dim, which is fine. With an LED bulb, that will often (not always) result in flashing, as described.

In addition, LED bulbs typically have a wide range of power supply specifications. For example, they might work 100V - 250V. That allows the same bulb to be sold in both 120V and 240V locations. However, drop below 100V and instead of either turning totally off or getting dimmer, they will flash and also likely die prematurely.

A "dimmer compatible" LED might do better, depending on the nature of the problem.

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    should point out that the problem can indeed be addressed after the "path from panel to bulb" by using a LED bulb that can be dimmed from triac dimmers, as many bulbs can these days, ecosmart for example.
    – dandavis
    Commented Dec 5, 2021 at 18:44
  • Yes, there are some sort of electronics in the fixture that OP doesn't know about.
    – Harper - Reinstate Monica
    Commented Dec 6, 2021 at 6:38
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    Thank you. Yes it was a photocell. I removed the photocell from the circuit to keep the lights always powered since these were smart bulbs, and they're on a timer now + motion sensors.
    – user1713450
    Commented Jan 3, 2022 at 5:00

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