Dawn/Dusk Sensors

Hi All
Had a job today, lady recently moved in, hall light had packed up.
Turned out to be a faulty dawn to dusk sensor. White plastic thingy about 80mm long, 30mm dia with a male bayonet one & a female the other. Removed it, Robert is your fathers brother. Got me thinking though!
How do these things work? I can see the logic of 'it's dark, switch on' but how do they stay on?
I assume they have a photo cell of some kind? Why doesn't the light coming from the bulb trigger the sensor and turn the bulb off? Daylight would trigger it and the bulb is much closer & brighter.
I'm confused.
--
Dave
The Medway Handyman
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I suspect its down to frequencies of light; natural light must produce a certain frequency of light the bulb does not produce, and when the sensor detects it, it turns the light off cus it knows it must be day time.
Steve
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George wrote:

I'm slightly more confused now :-)
Are you/wilkapedia saying it the frequency of the light that triggers the sensor, What would happen with a 'daylight' bulb?
--
Dave
The Medway Handyman
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They turn off for .5 sec every half an hour or so, to see if it's still dark.
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