How do Light dependent resistors work in automatic street lights?
varghese p asked:
I understand that as the light intensity increases the resistance decreases and vice versa in an LDR. But I don’t know why, shouldn’t it be the other way around because in the morning when there is high light intensity shouldn’t it there be high resistance to prevent any current to light up the bulb. And at night when there is low light intensity the resistance decreases to allow more current to light up the bulb. Thanks!
I understand that as the light intensity increases the resistance decreases and vice versa in an LDR. But I don’t know why, shouldn’t it be the other way around because in the morning when there is high light intensity shouldn’t it there be high resistance to prevent any current to light up the bulb. And at night when there is low light intensity the resistance decreases to allow more current to light up the bulb. Thanks!
You are correct that the LDR principals seem to be the opposite of what is acheived in street lights, but that is assuming that they are placed in series witht the bulb. However, in reality the LDR is in a potential divider circuit, such that when there is a low resistace of the LDR (when there is high light intensity), the overall output of the bulb placed in parallel gets a higher resistance than normal, therefore the current does not flow as readily. It is hard to describe but if you google potential divider circuit, it may help. [who] Dr Alexi
By allowing the resistance of the LDR to decrease, this allows the extra current to operate a relay which will cut off the supply to the bulb. Once the resistance increases on the approach of darkness there is insufficient current for the relay to operate and the connection to the bulb will be re-instated.
Nowadays, instead of using mechanical relays a solid state (transistors, etc) circuit is used. [who] A MacLean