Physics LDR (Light dependant resistors)

in an LDR if in bright light the resistance falls and in darkness the resistance is highest, how come street light light up in the dark (if the resistance is high then)???

Reply 1

Sid2001

You are right in saying that a Light Dependent resistor's resistance increases when it is darker. This is shown in this circuit where a high light intensity is able to power an LED.

Street lights use a component called an NPN Transistor in order for them to turn on in the darkness. Here is a very simple circuit showing that effect. When resistance of the LDR is low (there is a lot of light) the transistor is OFF and therefore the circuit is broken. When the resistance of the LDR is high, the Transistor is ON, allowing current to flow from the battery, to the LED and to ground (negative of the battery).