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Simple Automatic Street Light System

A simple automatic 220V automatic street light circuit is explained in the following article which is built around a light activated circuit concept. The circuit can be used for automatically switching ON and switching OFF a roadway lamp or group of lamps in response to the varying ambient light levels.

The electrical unit once built can be used for switching OFF a lamp when dawn breaks and switching it ON when dusk sets in.

How it Works

The circuit can be used as an automatic day night operated light controller system or a simple light activated switch. Let’s try to understand the functioning of this useful circuit and how it is so simple to construct:

Referring to the circuit diagram we can see a very simple configuration consisting of just a couple transistors and a relay, which forms the basic control part of the circuit.

Of course we cannot forget about the LDR which is the prime sensing component of the circuit. The transistors are basically arranged such that they both complement each other oppositely, meaning when the left hand side transistor conducts, the right hand side transistor switches OFF and vice versa.

The left hand side transistor T1 is rigged as a voltage comparator using a resistive network. The resistor at the upper arm is the LDR and the lower arm resistor is the preset which is used to set the threshold values or levels. T2 is arranged as an inverter, and inverts the response received from T1.

How the LDR Works

Initially, assuming the light level is less, the LDR sustains a high resistance level across it, which does not allow enough current to reach the base of the transistor T1.

This allows the potential level at the collector to saturate T2 and consequently the relay remains activated in this condition.

When the light level increases and becomes sufficiently large on the LDR, its resistance level falls, this allows more current to pass through it which eventually reaches the base of T1.

How the Transistor Responds to LDR

The transistor T1 conducts, pulling its collector potential to ground. This inhibits the conduction of the transistor T2, switching OFF its collector load relay and the connected lamp.

Power Supply Details

The power supply is a standard transformer, bridge, capacitor network, which supplies a clean DC to the circuit for executing the proposed actions.

The whole circuit can be built over a small piece of vero board and the entire assembly along with the power supply may be housed inside a sturdy little plastic box.

How the LDR is Positioned

The LDR must be placed outside the box, meaning its sensing surface should be exposed toward the ambient area from where the light level is required to be sensed.

Care should be taken that the light from the lamps does not in any way reach the LDR, which may result in false switching and oscillations.

simple day night automatic street light circuit using transistors and relay

Parts List

R1, R2, R3 = 2K2,

VR1 = 10K preset,

C1 = 100uF/25V,

C2 = 10uF/25V,

D1 ---- D6 = 1N4007

T1, T2 = BC547,

Relay = 12 volt, 400 Ohm, SPDT,

LDR = any type with 10K to 47K resistance at ambient light.

Transformer = 0-12V, 200mA

PCB Design

PCB design for simple street light circuit

Using opamp IC 741

The above explained automatic darkness activated street lamp circuit can be also made using an opamp, as shown below:

automatic day night street lamp circuit using opamp and relay

Working Description

Here the IC 741 is designed as a comparator, wherein its non-inverting pin#3 is connected to a 10k preset or pot for creating a triggering reference at this pinout.

Pin#2 which is the inverting input of the IC is configured with a potential divider network made by a light dependent resistor or LDR and a 100K resistor.

The 10K preset is initially adjusted such that when the ambient light on the LDR reaches to the desired darkness threshold, the pin#6 goes high. This is done with some skill and patience by moving the preset slowly until pin#6 just goes high, which is identified by the switching ON of the connected relay and the illumination of the red LED.

This must be done by creating an artificial darkness threshold light level on the LDR inside a closed room and by using dim light for the purpose.

Once the preset is set, it may be sealed with some epoxy glue so that the adjustment remains fixed and unchanged.

After this the circuit may be enclosed inside a suitable box with a 12V adapter for powering the circuit, and the relay contacts wired with the desired road lamp.

Care must be taken to ensure that the lamp illumination never reaches the LDR, otherwise it may lead a continuous oscillations or flickering of the lamp as soon as it is triggered at twilight.

Need Help? Please leave a comment, I'll get back soon with a reply!


  1. Thanks sir for your explanation i appreciate your kindness. What value of cap do i add across relay coil.

  2. Hi.
    Actually i need your Contact number. Can you give your number? Or message me to this num. i'll call you. 9600582084

  3. Hi,

    Please feel free to contact me through my email ID

  4. Sir i want to make smart street light this with the help of sensors the intensity of light inc. And decreases...plzz help

  5. Harsimran you can try the following design:

  6. Sir,
    what do you mean of (across relay coil)? does it mean (in parallel with the relay coil)?

  7. yes I meant parallel to the coil

  8. Which pin of relay connect to supply and transitor?

  9. Whow to connect realys pin to suppy and other components. Please explain in detail i have dast demo day after tomorrow.

  10. The relay coil terminals will connect with those points

  11. the center of the preset will go to T1 base, the other pins across R1 and ground.

  12. I am trying to use BT136 and Transformerless-power supply like

    But confused here " connect center lead to load and load to mains one point and mains other point to circuit ground."

    What do you mean by "circuit ground" negative side of C2 ( if yes then how a 40 watt filament AC B22 base Indian Bulb could run) or Phase or Neutral of Mains of Transformer less power supply.

  13. circuit ground is the line which is common to the triac right lead and the circuit negative. This line also gets connected with one of the mains inputs (preferably to neutral).

    The other mains input (preferably phase) goes to the load wire as explained in your above comment.

  14. 220 ohm caught fire!!!

    I have tried the following method to avoid Chattering of Relay at afternoon using BT136

    ""replace the relay with a BT136 or similar triac.

    do the following mods:

    remove relay.
    connect collector of T3 to positive directly.
    connect emitter of T3 to ground via a 1K resistor.
    connect triac extreme right lead to emitter via a 220 ohms resistor
    connect extreme left lead of triac to ground ( I have tired but not work then Connected to Phase)
    connect center lead to load and load to mains one point and mains other point to circuit ground.( as well as Neutral)
    circuit ground is the line which is common to the triac right lead and the circuit negative and the circuit negative (Left is already connected to negative as you told earlier) . This line also gets connected with one of the mains inputs (preferably to neutral).

    Since Center pin of Triac is already connected to neural as well as negative, Left Pin of triac again if connect to negative or ground or neural circuit not completed so I connected Left pin of Triac to Phase and middle pin to neural or negative via load of a 40 watt bulb, I used a transformer less power-supply using 474k capacitor. But lamp lighten but within a second 220 ohm resistor caught fire I immediately turned off switch.

    It will be great help if you post a diagram using BT136 and transformer less power-supply.

    Thanks in advance

  15. do not use bridge rectifier for the transformerless power supply, you can take the help of this article diagram:

  16. Can the circuit be built without a control switch?

  17. please elaborate your requirement?

  18. Beni
    Sir! thankyou very much for your webesite.The AC is changed to which type of lamp may feet to it now to get enough light?

  19. thanks Beni, the lamp can be seen connected with the AC mains via the relay contacts, so the lamp will be an AC lamp

  20. sorry,i asked you before watching it good.thanks again

  21. HI SWAGATAM how can i add like more 5 LAMPS 20W each to the circuit

  22. Hi Davis, yes you can accommodate more number of lamps as long as it's below the relay's maximum current handling capacity.

  23. HI SWAGATAM the circuit works fine when i used transformer power supply and used 1 watt for resistors but when i replaced the transformer with the cheap transformerless supply as you said then the relay could activate but bulb could not light yet am planning to use transformeless supply to be able to connect 10 bulbs 15w each rated 0.09A since my relay can support up to 7A with 240V . help me what is the problem with this supply to be able light the bulbs, thanks

  24. Hi Davis, 15 watt LED cannot be operated with a transformerless power supply, so I would suggest you to continue with your transformer based power supply.

  25. Hi Swag,
    I am using a triac with diac to control 4w lamp. 120vac feeds lamp. Lamp to 100k variable resistor to diac as well as LDR to common. The circuit works fine. When I add a step down transformer 120vac to 12vdc in parallel with lamp circuit still works a so does transformer.
    If I remove lamp however and only have transformer, circuit does not operate properly no matter value of VR. Let me know how to send circuit diagram if it helps. THX

  26. Hi Pete, it could be because the triac /diac design which you have incorporated is very basic and is not optimized for handling inductive loads, therefor it might not be able to switch a transformer, but is working well with a resistive load such as a bulb...

    or may be its because the transformer being an inductor load offers very high resistance initially, which stalls the triac operation, and connecting a bulb in parallel solves this issue...

  27. you cn send it to

    homemadecircuits @

  28. Hello sir your work is very impressive .i am from pakistan and i want to know the components working function in on and off condition

  29. Hello Mudasir, the circuit explanation is already given in the article, please read the article to learn the details.

  30. Sir I have this circuit and my daughter presented it to their school as project, it make me laugh when she said all of her classmate is astonish with it, I think they don't know about ldr, tnx u very much for this kind of tut.

  31. sir do u have this kind of circuit that is operated only with 12v battery, and use led strip, tnx sir

  32. That's great Jindro, I am glad your daughter could successfully use this circuit as a school project!

  33. Hi Jindro, I'll try to update the required design soon in the above article

  34. hello sir good day
    I want some help regarding automatic street light control circuit but wen it uses it 741 and ldr as the main control board and compartible with up to atleast 100watt bulb

  35. Hi Olupot, I'll rty to include the design soon....

  36. ok sir thanks you let me know wen it's done

  37. I have updated the design, you can check it out...

  38. In Using opamp IC 741 circuit diagram, can I use 12v transformer less power supply which is made by 684J capacitor? Also this circuit can run 12 to 36w AC LED bulb?

  39. If you have confirmed with a 12V adapter then you can try with a transformerless power supply

  40. 36 watt led bulb should work.


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