Tutorial 11: Light Activated Alarm

A light activated alarm is built in this tutorial. When enough light falls on the LDR in this circuit, an alarm tone is played on the loudspeaker.

Light Activated Alarm Circuit Diagram

Light Activated Alarm Circuit Diagram


13 thoughts on “Tutorial 11: Light Activated Alarm

    • I did not simulate it in software, I built the circuit on breadboard and it works. Simulation software has its limits — build the real circuit to see if it actually works.

    • Hi Aracely
      Sure, any of the breadboard circuits can be built on a PCB or strip-board. You will just need to do the layout yourself.

      • I also had anothe question… I can’t seem to find the 100n capacitor, can I use a .1uf instead? And by any chance do you know where I can get a ldr. Cause all I have found are cds.

        • Hi Aracely
          0.1uF is the same as 100n.
          I got my LDRs from RS Components, but where to get them depends on where you live.

  1. why is that it is not working on my circuit ive constructed it but doesnt work…can u please show some other videos..tia

  2. Sir, i was making light activated alarm from article post on your website .
    I made circuit with your articles guidance but its speaker sounding but it does not stops in absence of light . I have checked it many times but iam unsuccessful .
    Can you please tell about me probable mistakes that i have done , as result its LDR is not working on the circuit board.
    Please send me your answer as soon as possible. Thankyou

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