musical light alarm circuit schematic

Light Alarm Circuit with LDR

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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  • anam

    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?

  • Arun

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

  • jasmine

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

    • Alok supari

      cz bc107 is not a power transistor.
      the base volt will burn normal tr.

  • bunny

    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!!
    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


  • 104

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

    • Jim Keith

      DigiKey stocks (11) siren devices:

      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.

    • ritesh

      sir please send me some idea how to make LDR circuit please as soon as possible…

    • 104

      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. 🙁

    • sayeed

      please i am interested to show your simple circuit

    • Jim Keith

      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:


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

    • Jim Keith

      Increase voltage to 6V or 9V.

    • 104

      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!

  • aLOksuPari

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

  • aLOksuPari

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

  • shubham

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

  • sunny

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

  • KK

    Values of T1 and T2