LDR (Photoresistor) is a type of variable resistor which Increase its Resistance when the Light is Decreases and its Resistance Decreases When Light is Increases . It means in dark place or when no light falls on LDR, the resistance of its is very HIGH and in Sufficient light the resistance of LDR is very LOW .
This resistor works on the principle of photo conductivity. These devices depend on the light. The most obvious application for an LDR is to automatically turn on a light at a certain light level.
Construction And Working Of Circuit
Construct the circuit as shown in fig . Two NPN transistor are used to switching operatiobn. The LDR is connected between supply voltage and Base terminal of transistor Q2 . The Emitter terminal of this transistor is connected with ground through a variable resistor or potentiometer (POT) . This variable resistor is used to adjust the sensitivity of circuit.
At Day When Light falls on LDR then conductivity increased and current flows between supply to base of transistor and then transistor Q1 will on . Transistor Q1 And Transistor Q2 are connected as NOT Gate. When transistor Q1 is on by conducting even small current on base, The output of transistor Q2 is LOW (OFF).
At night when there is no light falls on LDR, then high resistance will create and there is no current flows on base of transistor Q2. As there is no supply comes on base of transistor Q1 , this is in OFF state, When this transistor has LOW input then the output of Transistor Q2 will HIGH When transistor Q2 is in HIGH mode then is energised (turned on) the relay connected with its collector and Electric Lamp will Switched ON.
Assemble the circuit at any general pcb board and set the adjustment of pot according to required sensitivity . Fix the LDR out of cabinet for detecting the light.
LDR (medium size)-1
POT or PESET-
Relay 6v -1