Question

if a LDR is a resistor whose resistance decreases with increasing light intensity, then,
when there's light, resistance will decrease then current will flow, then it will operate the street lamp, then wouldn't it light up the lamp in the daytime?
so how will a street lamp be switched on when the day light is low? since resistance will be high....

Answer 1

In street lighting systems, the LDR is connected to a controller circuit, which measures the resistance and controls lights according to the resistance.

So, the LDR is not connected in series with the street light bulbs, as you presumed, if I understood your question correctly. An LDR wouldn't even be capable to handle the large currents of street lamps.

To get the idea, there are some simple circuits, which turn on and off a LED when put in dark (or in bright light):
http://www.reuk.co.uk/Light-Dependent-Resistor.htm

In street light controller circuits, there is usually a microcontroller (= small computer) reading the LDR resistance. The controller will, for example, integrate the resistance over a certain time period, in order to prevent that the street lights go off if car lights accidentally hit the sensor :-).

Answer 2

http://www.howstuffworks.com/environmental/energy/question363.htm

Answer 3

The schematic diagram you linked has two mistakes:
* Firstly, the transistor circuit driving the relay needs a energy source. You shound put a battery between the emitter and the rightmost wire (which is connected to relay coil and the LDR).
* Seconly, the places of the resistor and the LDR must be swapped. Now the circuit will light the bulb when it is bright outside.

Anyone majoring in electronics can confirm I'm right :).

Answer 4

hmmmm those links look very complicated to me.
anyway, can i conclude that,
a LDR depends on the intensity of light ie high light intensity, low resistance, -> current flows. circuit is on. but, it doesn't really mean the light bulb will work it might just mean that it's controlling some other parts of the circuit that makes the bulb turn off instead? it's actually connected to some other complicated device?

Answer 5

Yes, the LDR has to be connected in a complicated device, in order to drive a bulb or lighting system.

Answer 6

Ok, thx! :)