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#214763 - 01/22/15 12:41 PM Why is a Photocell porch light flickering
Todd11 Offline
New Member

Registered: 01/22/15
Posts: 5
Loc: Washington, US
Hello.

I manage an apartment building and I have a photocell front door light that is flickering on an off every quarter of a second.

It has a photocell that points down from the fixture. It is happening when it is night time and there is no natural light.

It is using a CFL bulb.

I have the same light in front of 20+ other apartments in the same building configuration and reflective light situation and none of the others have ever had this problem.

I have swapped the light fixture out with another identical fixture and get the same flickering problem.

I have tested the voltage coming to the light and it is a constant 120v (no flickering of the voltage).

If I put black electrical tape over the photocell, the flickering stops and it stays on.

If I take the tape off and shield the photocell with my hands (to block any light reflection) the flickering starts again.

By the way, when I first install the new fixture, it does not flicker right away but starts after a few minutes.

Any suggestions on how to fix the problem other than keeping the tape on there and therefore having the light on 24/7?

Thanks for your help.

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#214768 - 01/22/15 07:10 PM Re: Why is a Photocell porch light flickering [Re: Todd11]
JoeTestingEngr Offline
Member

Registered: 11/04/05
Posts: 786
Loc: Chicago, Il.
I suspect the photocell ckt. had a cap open up. Check it in daylight. Block it with something opaque, not your hand, and time how long it takes for the light to come on. Remove the cover and observe how long it takes for the light to turn back off. They should both be substantial time delays. But you say you swapped with an identical fixture (known good???) and got the same results? You should still do the timing test on a non-flickering fixture as a basis for comparison.
Secondly, the CdS might be reacting to a non-visible light source. I don't know the spectral response off hand, just that the resistance varies from hundreds of ohms in strong light, to millions of ohms in darkness. A filter film over the window might fix that.
Photocell ckts are designed with hysteresis, also called positive feedback, snap action, differential, deadband, schmitt trigger action, and some I'm forgetting. If some of the light from the fixture that you're controlling, can reflect back to the photocell, you might be exceeding the switching threshold. Or could this light be interacting with another fixture controlled by a photocell??? I had to troubleshoot a complaint about the lights going off at a bus vaulting island a few years ago. I climbed the ladder and eyeballed the viewing window for the photocell. I could see the pretty new street light that the city installed across the street. The fix involved an empty pop can, tin snips and a couple cable ties. I blocked the sight line between the photocell and street light, while still allowing ambient light to enter the window.
Joe

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#214769 - 01/22/15 07:15 PM Re: Why is a Photocell porch light flickering [Re: Todd11]
JoeTestingEngr Offline
Member

Registered: 11/04/05
Posts: 786
Loc: Chicago, Il.
Any new tenants with car alarms that flash super bright LEDs when the system is armed?
Joe

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#214770 - 01/22/15 07:39 PM Re: Why is a Photocell porch light flickering [Re: Todd11]
Todd11 Offline
New Member

Registered: 01/22/15
Posts: 5
Loc: Washington, US
Here is some additional information after my initial post.

When I said it flickers every quarter of a second… To better describe the flicker, it is a constant and consistent on/off/on/off with it pausing just a fraction of a second before it changes (like someone is moving the light switch up and down as fast as they can).

It is hard wired (I bypassed the light switch).

It is actually the third, identical fixture (all from different orders) to do the same flickering.

It is mounted in a very dark place (the other lights don’t shine into the area it is at).

Thanks


Edited by Todd11 (01/22/15 07:40 PM)

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#214771 - 01/22/15 08:28 PM Re: Why is a Photocell porch light flickering [Re: Todd11]
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
A CFL does not have enough load when off to turn off the PC. You can prove that to yourself by putting in a regular lamp.

I had the same problem with an occupancy sensor.
_________________________
Greg Fretwell

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#214773 - 01/23/15 12:19 AM Re: Why is a Photocell porch light flickering [Re: Todd11]
Todd11 Offline
New Member

Registered: 01/22/15
Posts: 5
Loc: Washington, US
This Photocell fixture is designed for a CFL (it has a GU24 base) and all of the other identical fixtures on the property are working correctly with CFL’s.

Thanks.

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#214774 - 01/23/15 01:06 AM Re: Why is a Photocell porch light flickering [Re: Todd11]
JoeTestingEngr Offline
Member

Registered: 11/04/05
Posts: 786
Loc: Chicago, Il.
Are you saying that the fixture used to work in this location, and now it doesn't, or has this location always had a problem?
Joe

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#214781 - 01/23/15 10:07 AM Re: Why is a Photocell porch light flickering [Re: Todd11]
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
Is there a white or light colored wall/floor across from this light that is reflecting back at the PC?
_________________________
Greg Fretwell

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#214782 - 01/23/15 10:07 AM Re: Why is a Photocell porch light flickering [Re: Todd11]
Todd11 Offline
New Member

Registered: 01/22/15
Posts: 5
Loc: Washington, US
Three identical fixtures over the past year have never worked at this location.

I think years ago the same model fixture used to work there but I am not sure on that.

Thanks.

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#214783 - 01/23/15 12:47 PM Re: Why is a Photocell porch light flickering [Re: Todd11]
geoff in UK Offline
Member

Registered: 12/30/02
Posts: 172
Loc: UK
Whatever the exact reason for the problem is you have proved that the photocell is incompatible with that particular application or location. I would suggest a different type or make of photocell, or moving it to a different location, depending on what's most practical.

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