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I can't get why the resistance for a Photoresistor should be increased at night time as said in Photoresistors.
The reason is that I think that to turn on a light it's better not to have a higher resistance so that the electrons can conduct and electron flow takes place which in turn would produce light.Am I wrong in the concept I have stated?Can anyone help me.
Photo-resistors were not intentionally designed to have a high dark resistance. Someone just discovered that some materials had the property that they reduce resistance when exposed to light, so could be used to sense the level of light.
The fact that photo-resistors work they way they do may seem unfortunate, but with very simple electronics, circuits can be made to turn on lights at night. The actual power for the lights does not need to pass through the photo-resistor (and if photo-resistors worked the way you want, they still wouldn't be connected to directly control a streetlight - they wouldn't be able to handle the required current.)
A photoresistor or 'light-dependent resistor' (LDR) is a resistor whose electrical resistance decreases at high light intensity.
In the dark, they have a high electrical resistance of a few mega-ohms, whereas during the day, the resistance drops to a few hundred ohms.
This is a physical fact about LDRs.
Electrical conductivity is the inverse of resistivity.
During the day, the current through the LDR is high, due to its low resistance. Conversely, at night, the current through the LDR is relatively low. This fact can be used to 'drive' a relay coil, for example, so that the relay is energised during the day. By connecting 230VAC to the light circuit via a normally-closed contact from the relay, the street lamp(s) can be made to turn OFF during the day and ON during the night.
Here's a simplified schematic:
In practice, this would be done with solid-state electronics such as transistors, so as to improve efficiency, reliability and cost.
That is how photoreceptors work. If you put in it series with a battery and a led light, the led will turn on when there is light. To make a light that turns on when it is dark you have to make a more complex circuit. See here for the details.
