Automatic Street Light Controller


It is very basic concept of controlling the light source based on the intensity of light. The main objective of the project is to turn on the ac bulb automatically when the light intensity decreases and turn off the bulb when the light intensity increases. We may have numerous advantages of this project not only in the field of automation but in energy/power conservation.

Components Required

LDR1Photo sensor
Relay112 volt relay
Screw connector12 pin
Male header12 pin (for ldr)
Ribbon female connector12 pin (for ldr)
Female Power Jack1(like Arduino)
Adapter112v ,1amp

Circuit Diagram

Working Principle

For understanding the working of this project, we must first understand the working of light dependent resistor (LDR).

LDR is special type of Resistor whose value changes as per the intensity of light.

From this curve between resistance and lux we can figure out that as the intensity of light increases the resistance decrease.

CASE (1)

When the light falls in LDR the resistance of the LDR decreases and the voltage across BE junction of 1st transistor will be greater than 0.7v thus BE is forward biased.Once the BE junction is forward bias the transistor may operate either in active region or in saturation region. According to the circuit diagram we have calculated that (Ic>Isat) which is the condition for transistor to be in saturation region. In the saturation region the voltage across the collector emitter junction will be almost equal to 0 and there will no be enough voltage across the base of second transistor Q2 to drive the second transistor ON.

Transistor Characteristic Curves | Electrical Academia

CASE (2)

When there is no any light falling in the LDR (i.e dark) then the resistance of the LDR increases to mega ohm (high value). So, all the voltage will be dropped across the resistor and there will not be enough voltage at the base of the 1st transistor to drive the transistor thus, Q1 remain OFF. But the transistor Q2 will be ON because the base of the second transistor Q2 get a certain voltage (>0.7v) from the upper loop. Thus relay on the collector junction of the second transistor will also be on where the load is connected.

The use of potentiometer is to adjust the sensitivity that mean in how much darkness you want to on the relay.

PCB Design

I have designed the PCB of my above circuit in Easy Eda software you can use any which you feel comfortable.



Thus, we have achieved our objectives that is our bulb is automatically ON during night time and automatically OFF during day time using LDR.

Youtube Link

You can watch the full video about this project in this link below:

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