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    The light activated switch circuit can be used for switching OFF a particular lamp or group of lamps in response to the varying ambient light levels.
    The unit once built can be used for switching OFF a lamp when dawn breaks and switching it ON when dusk sets in.

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

    Schematic of automatic light activated switch circuit

    light activated switch circuit schematic

    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. The circuit can be used as an automatic street light controller system or a simple light activated switch.

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    41 Responses to "Light Activated Switch"

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    1. Jim Keith Jim Keith says: on July 31, 2012 at 3:27 pm

      Not Recommended–there are a number of problems–power source not indicated, relay contact misconnection, resistor omitted…

      • Kuberkoos Kuberkoos says: on August 1, 2012 at 1:07 pm

        The mains power is left out on purpose. It is for the constructor to determine, as mains voltages vary from country to country.

      • Surprised at your response , Build one the circuits we provide are all working models and not concepts for protypes. These are tested circuits which are doing well commercially.

        voltages ARE ALWAYS SUBJECT TO COUNTRY AND INPUT VOLTAGES ARE LEFT TO VIEWERS DISCRETION

        COMPONENTS ARE ALWAYS THE ONES THAT ARE MOST USED AND COMMONLY AVAILABLE

        We share some designs to help the beginner make simple circuits and provide PCB designs which are not displayed by the site for reasons best known to them.

        ALL DESIGNS PROVIDED BY US ARE WITH PCB’S ABSOLUTELY SIMPLE WITH THE BEGINNER IN MIND AND SAFETY PARAMOUNT

      • Jim Keith Jim Keith says: on August 1, 2012 at 4:46 pm

        You should have 220Ω resistor from base of 2nd transistor to common to reduce base voltage–1st transistor may not be able to saturate sufficiently to turn off 2nd transistor–it may work, but unlikely that 2nd transistor is fully turned off.

        Also look at relay contact details–any good experimenter can see typo, but should be corrected.

      • Jim Keith Jim Keith says: on August 1, 2012 at 4:47 pm

        woops 2.2K resistor, not 220Ω

    2. Very nice and elegantly simple, this will serve as a very nice porch light controller.

      Thnak you very much.

    3. Good day Mr. Sebastian Kushero. I was wondering if you could help me make a small device for a key chain. I once had a key chain that when you clap it emits a sound and has an LED making it easy for you to find out where you loss your key in the room. It is the best key chain I had. It can be used on any important item. Hope to hear from you soon

    4. Dear Onitz

      We are into Renewable energy and within that we produce energy efficient devices that are simple, effective and economical. ]

      You would find a lot of clap switch devices its basically a transmitor/receiver circuit . We would send a link when we see and build something that is within our philosphical thinking.(Simple ,smart, effective and economical

      Thanks for the post

      Onitz you can send us your email to care@care-india.com

    5. Kuberkoos Kuberkoos says: on August 2, 2012 at 6:12 pm

      Hi Sebastian,
      I Think what Onitz really requires is a sound activated switch that drives a LED and/or a buzzer. I cannot find anything in my database, but I am sure one of our readers will have something that will work
      Regards
      Kuberkoos

      • I agree. He would have the receiver on the key chain and the transmitter to beep the key s would be the clap .

      • I used to have one of those wistle key minder,it works in a quiet environment but when there are so many sound sources like TV and stereo from the living room, it just keeps beeping, and turn out to be annoying, so I just stop using it.
        I have encountered some high end key minder, there is a transmitter that is kept in one place and when you need to find a key, you have to activate the transmitter and the misplaced key would beep and flash an LED, this is more practical, except that you have to keep the transmitter in one place.

      • With sound activated circuits this is a major hitch.

    6. There are some designing errors in this diagram which makes it non-functional
      or barely functional.

      Firstly, the contacts of the relay is wrongly connected. The COMMON is left open so the circuit is not completed. Tie COMMON to NC.

      Secondly, you didn’t named the components, like R1, C1, T1, D1…etc, so we do
      not know what we are talking about.

      Thirdly…
      The resistor(2.2K) in the base of final transistor is useless. The diode in
      emitor of the first transistor is useless too. Moreover, this diode won’t let the final transistor to block.

      In order to activate the relay firmly, some amount of hysteresis should be employed in order to prevent chattering.

      For better results, tie together the two emitors and complete the circuit to ground(V–)using a 10Ω(4.7Ω…68Ω) resistor. This is the the simplest way to implement a Schmitt Trigger, this approach being chatter-free. And get rid of that capacitor(10µF). It won’t do any good.

      • Hi,

        You are right with the transistor diode, it could be eliminated and it will not affect the function of the circuit, however the capacitor, provides the delay between switching on and switching off, the hysteresis, that you require, between two levels of lighting condition.

        well the relay is probably a typo error, any amateur builder, can easily understand, that the relay is there as a switch, and wire it appropriately.

        regards

      • Mihai

        You seem more of a theorist than a builder Awake from your books and start building before shooting from the hip.

        Let us confirm that you are not even an amateucircuit builder as any amateur who has actually built circuits would know how to read ciruits. I am suprised you have not shot the idea of the AC 110v or 240 V yet maybe you can do it in another mail.

        Why must one name them as R1 and D1 and c1 Is there some sort of a LAW. pal keep things simple and stiop picking holes where none exist. and try to show your theory , these circuits by us are for people who enjoy building circuits and not for someone who wants to exhibit his knowledge and shoot idea and confuse people..

        It would be better if you can contribute and display your skills in circuits by showing us what you have built.

      • And again who says it a Schmitt Trigger, and why do you presume it to be so.

        Build the circuit and then come back

      • It MUST BE Schmitt Trigger. In case you don’t know, this kind of circuit is used whenever a slow varying signal(or analog–LDR, in your case) need to be converted to a step signal(to activate the relay properly, in this instance).

        Got it?!

        By the way, I built my first circuit of this kind in 1979. It wasn’t Schmitt trigger, and it didn’t work properly. Shortly afterwards, I got the idea and I made it with hysteresis. It worked like a charm.

        cheers!

        Mihai.

      • MAYBE YOU FAILS TO OBSERVE THE 2K3 IN SERIES FROM THE POSITIVE RAIL AND OH YES YOU ARE A DINOSAUR STILL LIVING IN THE 80′S REFUSING TO CHANGE. AND STANDING GIVING ADVISE WHERE NONE IS ASKED.

        THIS IS THE INTERNET AND WHAT YOU GET FREE YOU TAKE OR IGNORE LIKE AIR AND SUNLIGHT OR BE READY FOR PAYING

      • the capacitor is there to address that, simple problem of hysteresis, why should we make a simple circuit so complicated. Mihai, we are here for fun of the hobby, the elegance of this, is its simplicity….we are hobbist and we enjoy the kind of circuits that challenges us to think and decipher the nicks and ticks of a circuit…..

        enjoy..

      • For arguments sake, I built the circuit, it works, dim the light and it switches on and bright the lights then it switches off, or you can do it in reverse, just wire the relay appropriately…a Schmitt trigger is when you want a precise knee to switch a circuit… in this case, who cares if it switches on a little bit before sunset , or it switches off when it is still a little bit dark outside….it is not a critical knee…the thing is it worked… and I’m happy about it..

        Thanks
        sebastian

    7. Some of us hobbyist enjoy the free gifts of life and not harp on less air or sunlight wish people like Mihai grow up and appreciate simplicity works and there is no need for theory maybe they should write a book rather than pick holes

      • We have to understand that he want to talk about areas that he feels he is strong, most of us fall in that trap. experience taught us to appreciate and understand the working elements, until it comes to a point detrimental, to intended function, some simple circuits are appreciatively functional and I see the elegance in them.

    8. i am a beginner so can you please explain me how the circuit works after the input current is given…..my first project and at any cost i want to do it.
      kindly help me.

    9. HOw can I build in a 15 second delay at ON and OFF? I want to build something like this in my car so that the light come on when it gets dark, but need some delay time, in case you go into a tunnel for a few seconds. It has to be dark for at least 15 seconds continuously and then turn the light off and once it is on, it has to remain on untill there is enough light for at least 15 seconds…

      Thanks,
      W

    10. care at care-india.com

    11. gud day sir,
      i made the circuit. but it is always on. what do u think the problem with it?

    12. its working now.. my problem now is its low sensitivity of light.. help me please..

      thanks for any response.

    13. I want a working principle of this project light activated by switch… means how it works..??.. so pls rply..

    14. @Sebastian Kushero
      Can you please give the working of this circuit?

    15. Small Question – will it flicker during the transistion of Light.??

      • YES, it will flicker(relay chatter) for sure during the light to dark or dark to light transition. This circuit doesn’t have hysteresis implemented, so don’t waste your time with it. Try something else. If you don’t find something viable, I might help you with this.

        Cheers,
        Mihai.

    16. Hi Mihai,
      Please do let me know if you have any other option for the same.
      It would be a very good help to me.
      My Email id is : ajaykumargaur@gmail.com

    17. hi sir..i would like to ask fr ur opinion regarding the ldr curcuit..i build a curcuit consist of ldr,dry cell,relay n etc for my project where we need to cut the curcuit once the ldr no detect any light source..however,the curcuit is not happen as v expected n i dunno why because the motor keep moving although i placed the ldr in a dark place..fyi,im preparing this as a stopping mechanism fr the chemecar competition(moving car using chemical reaction)..so v got prob in stopping the curcuit..any idea??

    18. use a relay to operate the motor.

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