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    This musical light alarm circuit is very simple, uses only 7 components, a LDR and a 3.6 V battery or 3 x 1.2 volts rechargeable batteries. The well-known UM66 is used as the sound generator and will give a pleasent wake up alarm.

    As you probably know the LDR is a light dependent resistor. Normally the resistance of an LDR is very high, sometimes as high as 1MΩ, but when they are illuminated with light resistance drops dramatically. In the circuit adjust the 220KΩ preset to the desired sensitivity, meaning adjusting the threshold point where the alarm start singing.

    When there is light on the light dependent resistor the T1 transistor will start conducting and powers the UM66 musical integrated circuit. The produced musical note will be amplified by transistor T2 and fed into the 8Ω speaker.

    On the UM66 IC are different numbers, each number giving a different musical note (in this example we use UM66T). You may use 2 x 1.5V batteries but 3 x 1.2V NiCad or NiMH are better because you can recharge them.

    Light alarm electronic circuit schematic

    musical light alarm circuit schematic

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    39 Responses to "Light Alarm Circuit with LDR"

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    1. MOHIBULLAH KHAN says: on August 24, 2011 at 11:52 am

      what are the values of T1 and T2?

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    2. MOHIBULLAH KHAN says: on August 24, 2011 at 12:12 pm

      please tell me the values of transistors? as soon as possible PLEASE

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    3. aLOksuPari says: on October 21, 2011 at 10:23 pm

      T1=BC107 or BC147
      T2=SL100
      Have fun frnds. . .
      :)

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    4. sir which transister can use insted of bc107,sl100

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    5. what should be the value of LDR?
      PLZ REPLY

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    6. How to long life the battery for standby ?
      thanks.

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      • aLOk suPari says: on March 3, 2012 at 6:13 am

        If u use nokia BL-5C battery (3.7 volts), then the life of battery is 25-30 days,

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    7. sorry for late reply.
      Thanks for information, Alok.

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    8. aLOk suPari says: on April 4, 2012 at 7:12 pm

      sorry frnds . . .
      the replacement of BC107 is BC147
      nd no replacement for SL100

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    9. flyingbird says: on May 1, 2012 at 8:27 pm

      Tried the same circuit, but didn’t work. There is no current to speaker. Anyone has tried this circuit, does it really work?

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    10. sweety says: on May 17, 2012 at 7:44 am

      but it rings continuosly again manual work is required to switch off .is there is any possible way for switching off automatically

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    11. Values of T1 and T2

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    12. will it work???can v use t1 as bc548

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    13. i cannot understand this circuit because this alarm buzzer at day & also night reply fast

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    14. aLOksuPari says: on October 6, 2012 at 12:34 pm

      @shubham : this circuit is just for fun bro. . . . :-)

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    15. aLOksuPari says: on October 6, 2012 at 12:37 pm

      @sunny : no!!!
      cz bc548 is a npn tr and bc547 is pnp tr. . . .

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    16. is there a UM66 component or replacement that creates a siren sound? pls reply ASAP

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      • and how to make a circuit that LED and speaker is parallel? like they both activate when it is dark? pls pls pls reply! thank you so much!

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      • Jim Keith Jim Keith says: on November 1, 2012 at 4:11 pm
          member

        DigiKey stocks (11) siren devices:
        digikey.com/product-search/en/audio-products/sirens/721819?k=siren&stock=1

        But the UM66 is not listed anywhere–probably obsolete

        To add the LED, eliminate R1 (tie collector of T1 to Vcc), and connect LED with series 1K resistor between emitter of T1 and gnd. Good luck. No other info is available.

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      • Jim Keith Jim Keith says: on November 1, 2012 at 4:13 pm
          member

        Increase voltage to 6V or 9V.

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      • can i directly change UM66 with XL-5530-LW300-S-R?? no need to change anything else in the circuit? if you can show a diagram like this one with output speaker and LED and using XL-5530-LW300-S-R instead of UMM66, PLEASE PLEASE PLEASE DO. our project is due two days from now. :(

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      • Jim Keith Jim Keith says: on November 10, 2012 at 12:00 am
          member

        I have a good idea for a good, simple circuit using your siren module –will post in a few hours –watch for new post titled:

        LIGHT ACTIVATED SIREN CIRCUIT

        Hopefully webmaster will be present to list it promptly –if not, please provide email address so I can send directly.

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      • please i am interested to show your simple circuit

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    17. bunny bunny says: on June 16, 2013 at 4:22 pm
        member

      as t2= SL100 is an Indian transistor can we use C139 for it! and also please tell that which will be the emitter,collector and base of the transistors?
      Please reply ASAP. i am making this circuit and made it as same as shown but it is not but it is not running!
      also if you have any diagram of placing components on the project board or the other PRC board so please upload or mail at this ID sdabdullah16@gmail.com!!
      I will be very thankful to you in ur replying this. As i am making it for my physics project. so your anticipation is needed!! regards

      Bunny

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    18. jasmine says: on July 16, 2013 at 8:47 am

      Simple and awesome work…i had a doubt while checking the ckt…can we replace sl100 wit bc107..n why?plz do reply

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    19. Why u use collector as common…instead of emitter as common one??

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    20. can u please send me the procedure to make much more simpler circuits? or if u know any other type of circuits on topics related to 12 class tht can help me in making my 12 project?

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