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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.

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  • \$\begingroup\$ If you know basics of op-amp, then I can give an answer using op-amp. \$\endgroup\$
    – user22180
    Nov 26, 2014 at 6:34
  • \$\begingroup\$ @user22180:Sorry I'm just going to study about it. \$\endgroup\$
    – justin
    Nov 26, 2014 at 6:35
  • \$\begingroup\$ Would Electrical Engineering be a better home for this question? \$\endgroup\$
    – Qmechanic
    Nov 26, 2014 at 6:59
  • \$\begingroup\$ @Qmechanic:How to move it to EE.Should I delete the post here and post it in EE.Is there any other way? \$\endgroup\$
    – justin
    Nov 26, 2014 at 7:01

3 Answers 3

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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.)

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  • \$\begingroup\$ that's an answer I was looking for. \$\endgroup\$
    – justin
    Nov 27, 2014 at 11:06
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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:

enter image description here

In practice, this would be done with solid-state electronics such as transistors, so as to improve efficiency, reliability and cost.

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  • \$\begingroup\$ :Is Electrical conductivity proportional to light? \$\endgroup\$
    – justin
    Nov 26, 2014 at 6:20
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    \$\begingroup\$ @justin Electrical conductivity is the 'inverse' of resistivity. Since the photoresistor has 'low resistance' under light, its conductivity is high during the day. \$\endgroup\$
    – theo
    Nov 26, 2014 at 6:30
  • \$\begingroup\$ :Okay.If it's conductivity is high during the day wouldn't street lamp shine during daytime too?Because I think light of an lamp is brought by the electron flow or we can say conduction isn't it? \$\endgroup\$
    – justin
    Nov 26, 2014 at 6:34
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    \$\begingroup\$ @justin The current through the photoresistor is not made to go through the lamp, but through a 'relay' which 'drives' the lamp, since the lamp requires a different voltage and current to operate... See the diagram I've just added for clarification. \$\endgroup\$
    – theo
    Nov 26, 2014 at 6:49
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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.

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  • \$\begingroup\$ I'm just asking why couldn't a photoresistor with lower resistance used for street lamps during night time? \$\endgroup\$
    – justin
    Nov 26, 2014 at 6:27
  • \$\begingroup\$ I am not aware of any material that can do that. But you can invert the effect with the circuit I linked. \$\endgroup\$
    – Wolphram jonny
    Nov 26, 2014 at 6:31

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