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    There have been lot of problems in street lights. Major problem in some places is every evening a person has to come and switch ON the street light and it should be again switched off in morning. Yes, this may not be the situation in everywhere but exists in many places.

    So this problem can be overcome by using a simple circuit. Below shown circuit will be automatically switched ON and OFF during night and morning times respectively.

    Automatic light schematic

    Automatic street light

    This circuit will probably not function so please do not build it! The author is not a member of the website therefore his projects are not trustworthy!

    In above circuit R1 can be used to adjust the sensitivity. And the working of the circuit is very simple. The LDR will have very low resistance during day time so the transistor Q1 will be in OFF condition. And during night time the resistance will be very high so automatically the transistor Q1 will be ON.

    The Q1 is PNP transistor and the emitter of Q1 is given to base of Q2. So the Q2 transistor will be ON only if the transistor Q1 is ON. The TRIAC is used in the circuit to make is circuit complete. As the TRIAC will allow voltage to pass from either directions only when there is a certain threshold voltage in gate terminal. And the gate of TRIAC is controlled by transistor Q2.

    So totally the lamp will be ON during night time and will be again switched off during day light. To change the sensitivity of the circuit to light adjust R2.

    If you have any doubts, do not hesitate to comment below. We will come to you with an appropriate solution. :)

    ask a question

    42 Responses to "Automatic Street Light Circuit"

    1. Wouldn’t it be better to use something that doesn’t require external power, and that can be used for compact fluorescent lamps too? Maybe with a relay or something..

    2. but once a triac fires, even if we remove the gate voltage it will still conduct till the supply is turned off. so how will it be outomatic

      • Srihari Rao says: on July 14, 2012 at 4:23 pm

        @jojopidosa once the TRIAC is triggered it continues to conduct until the gate supply is above certain threshold. If the gate current drops below threshold then the device will not conduct, thus making it automatic.

      • jojopidosa says: on July 16, 2012 at 7:08 pm


    3. Hi, sorry but how can the circuit work? i mean, Q2 is a NPN junction type, so base current will flow into the base, and Q1 will try to drain it from the Q2 base, Q2 will never be activated right? sorry if i’m wrong, but please explain that part…and LDR can not reach too low resistance value because Q1 base will drain an high current from the 9V

      (I’m portuguese, so, sorry for my english!)

      • Rasim Mahmood says: on October 9, 2014 at 5:19 pm

        Yes I totally agree with you the transistor Q1 is PNP type and can never be turned on when LDR resistance is high. If any how transistor Q1 is turned on then Ground is provided to the base of transistor Q2 which is NPN. Impractical circuit….

    4. This circuit is wrong and looks like a project without checking.

      First, the 9 V battery is shorted by the 5 volts zener diode D2, you need a limiting resistor in series with D1 (which absorbs only 0.6 volts) to avoid this problem, but there would be a constant drain current the battery because the voltage difference battery-zener. Better yet, spare the zener diode.

      Second, for the triac operation, Q2 has to be ON, and Q1 OFF. The Q2 transistor (NPN type) will be ON only if the transistor Q1 (PNP type) is OFF: With Q1 ON, shorts the base of Q2 (NPN type transistor) to ground and Q2 is OFF. Furthermore, with Q1 OFF, requires a bias current from the base of Q2 so that Q2 is ON, it is necessary a polarization resistance of the base of Q2 to positive supply.

      Please check the projects before publishing.

    5. hi there.. im so gLad 2 see this site… but will it be better if u use IDEC SmartRelay to make sure there’s no problem about the scheme and the design… 2nd to avoid short circuit, and effective too co’z it’s a reLay w/ timer and just to program…

      that’s what im working on…

    6. There seems something very much amiss.
      The Ground of the AC line and the DC line seem to merge and there is not isolation seems to be hazardous. Has this been built

    7. Plz sir can hi replace R2 with vr100 and D1 to IN4148 and Q1 to BD140 plz help…help

    8. Where is the limiting resistor for zener diode (D2)? You can’t apply 9 Volts across a 5V zener diode without limiting the current!

      And where is the load resistor for Q1? It should be tied from emitor to positive supply(+5V).

      I wouldn’t believe that you can turn ON this type of triac with only 5mA in the gate. Would you?

      • Jim Keith says: on August 12, 2012 at 11:39 pm

        I think that D1 should be a resistor–reverse polarity protection is provided via the zener forward bias characteristics. A better solution would be to keep D1 and eliminate D2–this will reduce battery current and provide more gate current at the same time.

        I agree with you that there should be an emitter resistor.

        On the triac sensitivity, some devices may may work, but not all–there is definitely a sensitivity problem and perhaps a more sensitive triac should be selected. Check out the Igt sensitivity specification:

      • I already did(Igt for BT136). This type of triac can’t be triggered with this circuit. No way! Gate of the triac is connected via R2–1kΩ to Q2, whose colector is tied at +5V. The maximum current provided by Q2 is less than 4mA!!! But only if the base of Q2 is pulled up to +5V by means of a resistor–say 4.7kΩ. Anyway, this circuit sucks. Don’t bother testing it. It’s waste of time.

      • Mihai it is very possible that this circuit doesn’t work. The author is not publishing anymore on electroschematics.com because of the repetitive problems that other users have reported about his circuits. That is why we have the disclaimer http://electroschematics.com/disclaimer/


    9. What is R1 and LDR…….

      • Jim Keith says: on August 19, 2012 at 6:06 pm

        R1 is a potentiometer (wired as an adjustable resistor) and LDR is a light dependent resistor (cadmium-sulfide photocell).

        Together they form a resistance voltage divider whose output voltage is a function of light intensity.

      • want to build this cct fo r my project work. somebody help me with the details and the documentation.. tns.

      • Don’t bother to built it. Some mistakes in the diagram makes it nonfunctional. Find something else.

    10. Sir plz give me detail working of circuit before trnsister as early as possible.thanks

    11. I made this circuit but it is acting as a light detector instead of automatic street light circuit..what can be the possible problem in my circuit..plz help.

    12. without external supply to the lamp how we can give 230v a.c to the lamp

    13. sir i made the automatic street light ckt but it cant give output. i think their are problems in triac triggering ,please help me sir

    14. Is it necessary the use of two resistors(r1 & r2) to change the snsitivity

    15. Hi All!

      On the drawing

      Q1 is a PNP BJT and Q2 is an NPN…
      They are completely wrong..

      As a start to the right direction they should be swapped so

      Q1 is NPN and Q2 is PNP…

      Also as others have already mentioned having D2 is pointless in the circuit.. I believe C1 isn’t needed either…

      Next thing could be to drop R2 resistor to about 350 – 400 ohms

      Than maybe…

    16. getachew anteneh says: on April 10, 2014 at 10:19 pm

      what do we use instead of LDR

    17. I m studying in 6th sem BE Electrical Engg. I want to work on this project. Is this is sufficient as a big project for last 2 sem? can we use directly solar panel to run thia circuit? plz give me whole detail about this project. and also give me some new latest project ideas …….which is batter? IDP or UDP?

    18. Plz Explain me bout how to work this circuit

    19. how does the transistor work in this circuit.

    20. sandeep chowdary says: on January 18, 2015 at 11:07 am

      Iam a Btech 1st year Electronics student.I had intrest on this project.in how many days we can construct the circuit and we can make trial!!!?? please give me a suggestion to start my first engineering project.

    21. am the first year physics student, how can the cut off of the circuit be overcomes in this circuit

    22. Sir I want to full explanation regarding Automatic intensity controlled steet looking ght with cost.

    23. how it is working .. what r d advantages

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