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Old 11-28-2008, 11:11 AM   #1
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Light strobing


Needing some help with a motion detecting switch in the bathroom. To make it short. All new wiring in conduit. When you turn off the exhaust fan the light will dim down and strobe. The fan is on a different circuit.Did it every time untill I swapped out the expensive fan unit for a cheapest one I could fine. It still does it sometimes but not every time now. HELP please.

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Old 11-28-2008, 11:43 AM   #2
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Light strobing


Heh, sounds like yet another loose neutral connection on a multi-wire branch circuit. (What is that, 7 in 2 months?)

Basically check all the connections. There's almost certainly a loose neutral somewhere.

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Old 11-28-2008, 11:45 AM   #3
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Light strobing


Is it a CFL bulb. Try a regular bulb. Many of the motion detectors won't work with CFL bulbs.
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Old 11-28-2008, 12:03 PM   #4
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Light strobing


Not a loose neutral. The light is a regular bulb. It is on it's own circuit which comes from the panel to the light. I thought loose neutral first and checked it first thing. It only does it when you turn off the fan which is 3 feet away on a different circuit.
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Old 11-28-2008, 12:29 PM   #5
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Light strobing


When you turn off the breaker for one of the circuits, do they both stop functioning?
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Old 11-28-2008, 01:29 PM   #6
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Light strobing


Seems strange to me. I don't know a fan on a completely different circuit could cause a problem with a light. Maybe some wiring got crossed somewhere.
Try a normal switch or another motion switch.
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Old 11-28-2008, 02:34 PM   #7
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Light strobing


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Originally Posted by naildriver View Post
the light will dim down and strobe.
Strobe meaning goes all the way off?
Flickers or you can see it flash on and off?
Can you make it just dim without flashing?
You have a DVM?
On an AM radio tuned between stations, see if you can pick up noise that is in sync with the flashing.

Motion detector electronics is maybe affected by bathroom moisture. . .?

This is about as strange as it gets.
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Old 12-01-2008, 08:57 PM   #8
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Light strobing


I think I have finally figured this out.I tried a new switch, a regular switch, shutting off breakers. The regular switch was the only thing that worked. It was fine with it installed but they wanted the motion one. I call it strobe because it was still pretty bright,and it was flashimg like a strobe light. Anyway Found out that my helper had also hooked up the little and I mean little 120v on demand water heater that hangs under the sink on the same circuit. unplug it and the light is fine. Somehow that switch is reading some sort of voltage through the element when it is off. When the unit is on it is fine. Pulling another circuit for the water heater by itself tomorrow.
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Old 12-02-2008, 09:04 AM   #9
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Light strobing


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my helper had also hooked up the little and I mean little 120v on demand water heater that hangs under the sink on the same circuit. unplug it and the light is fine. Somehow that switch is reading some sort of voltage through the element when it is off. When the unit is on it is fine.
That doesn't make sense to me; can you post what you believe to be the schematic of this whole setup?
It's always a good idea to find the root cause.
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Old 12-02-2008, 07:06 PM   #10
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Light strobing


I don't have a schematic right now and not for sure if I know how to scan one in. Just picture a circuit with 3 plugs, a fan/light combo unit,a light over the mirror, and a small on demand water heater under the pedestal sink. All 120v stuff. I know it does not make any sense. But if you had the light above the sink on (motion light) Then when you turned off the fan the motion light would strobe. Ran a circuit for the combo unit and it only did it after you shut off the hot water but not every time. seperated the w/h, Put the light on a circuit by itself and it works fine. Before seperating stuff the only thing that helped was switching out the copper wound fan motor to a elcheapo magnetic motor. All I had to do to switch them out was to unplug one then plug in the other. Been doing electrical for 20 years and never seen anything like it.
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Old 12-03-2008, 12:43 PM   #11
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Light strobing


Quote:
Originally Posted by naildriver View Post
I don't have a schematic right now and not for sure if I know how to scan one in. Just picture a circuit with 3 plugs, a fan/light combo unit,a light over the mirror, and a small on demand water heater under the pedestal sink. All 120v stuff. I know it does not make any sense. But if you had the light above the sink on (motion light) Then when you turned off the fan the motion light would strobe. Ran a circuit for the combo unit and it only did it after you shut off the hot water but not every time. seperated the w/h, Put the light on a circuit by itself and it works fine. Before seperating stuff the only thing that helped was switching out the copper wound fan motor to a elcheapo magnetic motor. All I had to do to switch them out was to unplug one then plug in the other. Been doing electrical for 20 years and never seen anything like it.
In other words, the motion detector oscillates.

By default I would load each circuit with 10A (a hair dryer) and see that the drop is less than 3v. I suppose a high upstream impedance, or some appliance in series with the line, could do this.
You could get some evidence for this upstream impedance idea by checking if the motion detector still misbehaves when you have only one very low wattage bulb screwed into it.

This is baffling because we are assuming something that isn't true. The hard part is finding out what it is.


Last edited by Yoyizit; 12-03-2008 at 12:50 PM.
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