Automatic Street Light Circuit with TRIAC and LDR

In this circuit you must want to know how street lights circuit automatically on in the evening and off in morning. We want to tell you that nobody manually on/off these light physically. In this article we are describing a circuit for automatic street light or street light controller project by using LDR (Light dependent resistor).
Here is our new simple Electronics project about Automatic Street Light Control System. In this project we did not use any relay for on/off. We have just used a TRIAC as a relay to on and off the Street light Automatically.
1. It is a simple and powerful concept, which uses TRIAC as a switch to switch ON and OFF the street lights automatically.
2. Automatically switches ON lights when the sunlight goes below (e.g. in the evening)
3. It automatically switches OFF lights when Sunlight fall on LDR sensor of this project.

So in this circuit we are going to uses a LDR (Light Dependent Resistor) as a sensor to sense the light. In the day time due to light the resistance of LDR will be very low. So the voltage drop be high. This keeps the TRIAC (BT136) on off-stage. BT136 is a TRIAC that is used to control the main power 110/220V directly. The bulb will remain OFF.
As night falls the resistance of LDR increases and it turns on the TRIAC points M1 and M2. When M1 and M2 of TRIAC joints the current the bulb Turns on. TRIAC is a component that is used to control the light directly 110/220 volts AC main power.
Components Used:-
1. TRIAC: BT134/BT136/BT139
2. Resistor: 220k 1/4 watts
2. LDR: Light dependent resistor
The light Sensor Switch works on 220v AC Directly and if you are not accustomed to working with mains voltage or do not have experience in working with 220v AC Mains Voltage please stay away from this project while it is connected with Main power.
Advantages Street Lights Switch:-
Street lights consume a lot of power in the cities. Many times after sunrise the street lights remain on. Much amount of energy is wasted in this way. The automatic street lights using LDR work according to the light. LDR detects the light. The lights are turned on if the light is Below a specific value.
The system is highly sensitive. When the LDR goes dark the street lights automatically on when the LDR finds light the wall lights off automatically. Once the street lights are placed in this system there is no need to monitor them anymore. They function on their own. They are less costly and reliable. This system can handle heavy loads up to 7A if you use BT139 then it can handle 18A main current. The system can be changed to the manual mode if needed.


Subramanian MK, currently serving as a workshop instructor at Sakthi Polytechnic College, Erode Tamil Nadu. With a career spanning 25 + years, Subramanian MK has dedicated himself to advancing knowledge in Electronics and Communication Engineering (ECE). His passion for exploring new technologies has led to the development of numerous projects, showcasing expertise in IoT and PCB design.

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  1. this circuit will give fluctuation at output? As LDR resistance may vary with small variation in light, the switchind of Triac will also get effecte. How can we remove these fluctuation?

  2. give the operating voltage of Ldr to confirm it will operate at mains level voltage 220v another thing how atriac is turned according to your design ?

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