Light Depending Resistor or Photo Resistor
LDR or Light Depending Resistor or Photo Register is an important device in electronics today. It is used to sense the intensity of light as the resistance of the LDR is changed in terms of change in light.
When the light fall on a LDR it’s resistance goes low. When the light fall on the LDR reduced, i.e. in dark situation resistance of the LDR goes high.
As the resistance of this device depends on the intensity of the light so it is called Light Depending Register (LDR) or Photo Register. Typical value of resistance of a LDR in dark situation is about several megaohms (MΩ ), and resistance in Normal Light is about few ohms.
LDR is use to control a circuit depending on the presence of light or of darkness. or we may say that it can be use to sense presence of light of absence of light. so it can be used as light sensor circuit.
To make a Street Light auto control, i.e. light starts to glow in evening and turnoff in the morning automatically, we have to use LDR sensor switch. Where a transistor can be make on or off depending on its base voltage.
A street light circuit basically consists of a switching section and a sensor section. Here resistance R1 and LDR use to form a voltage divider network which is act as a sensor. It provides base voltage to the switching transistor.
Transistor is act as a switch in between street light power source and ground.
We know that for a transistor if the base voltage increase above the Bese Emitted junction voltage of transistor then the transistor goes to active mode and collector current starts to conduct from collector to base, and we say that the Transistor is ON. Street Light ON
If the base voltage Vb decrease then base emitter junction voltage VBE then we called the transistor is cut off or transistor is in OFF and no current will flow from collector to emitter. Street Light OFF
In the day time when light fall on the LDR, it’s resistance decrease to 100 ohms (approximately). So the voltage at base Vb comes to 0.00059 volt. Which is far bellow the VBE.
So the transistor remain in cut off state. As no current will pass through the transistor street lights remains OFF.
We can calculate the value of Vb from the resistance of the voltage divider network formed by the resistance 100Kelo ohms and the LDR resistance…
Vb=R2/(R2+R1)*Vcc
In the night when there is no light, only dark exists on the LDR, it’s resistance Increase to 1000 Kilo ohms (approximately). So the voltage at base Vb comes to 5.45 volt. Which is far above the VBE
Then transistor starts to conduct. As the current will pass through the transistor street lights starts to glow.
To construct a practical Street Light circuit we have to use high power transistor or MOSFET or electro magnetic relay considering the maximum current ratings of the street light circuit.