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  • Shadow sensors are widely used to detect the movement of a person in a confined area. Many circuits published earlier have a serious drawback that only one light dependent resistor (LDR) is used for shadow detection.
    Needless to say, this is not a perfect solution!

    Shadow Sensor Circuit Schematic

    smart shadow sensor alarm schematic

    Described here is a simple but improved circuit of a smart shadow sensor alarm, which can register a shadow when there is a light difference. Here two 5mm LDRs are used with the popular Op-amp LM741CN to drive an active piezo-sounder when a “valid” shadow is detected. The whole shadow sensor circuit can be powered from four 1.5V AA cells (6VDC), or similar dc supply sources.


    • Place LDR1 & LDR2 1 inch apart (LDR1 —— 1 inch —— LDR2)
    • Diode D2 is not a crucial component. It is for input supply polarity protection only
    • Current consumption of the lab prototype is 2 to 3 mA (standby) & 20 to 30mA (Active). Major “current consuming” component is the Piezo-Buzzer!
    • The Piezo-Buzzer can be replaced with a 6V low-current electro-magnetic relay to drive external loads like an electric hooter. Remember to add a freewheeling diode (in anti-parallel configuration) across the relay coil
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    11 Responses to "Shadow Sensor Alarm"

    1. Thank you for this circuit. It works very well. But I want to ask you what is the usage of the switch? Nothing has changed Whether it is on or off.
      Thank you again.

    2. gopi sainadh says: on February 9, 2014 at 5:58 am

      it’s not working,how can i make it to work???

    3. Pls i need help on this circuit. The alerm refuse to go off until shadow fall on it… But i want it to turn on wen shadow fall on it. Solution please

      • Ganesh K says: on August 6, 2015 at 2:11 pm

        Please use a pnp transistor like bc557 instead of npn transistor.

        Use the appropriate biasing circuit of pnp.

    4. the circuit did not work, the alarm remain ON

    5. rohini sagare says: on August 9, 2014 at 10:45 am

      it seems a very good mini project for opamp based hope it will work successfully

    6. rohini sagare says: on August 9, 2014 at 11:06 am

      what is the application of it??????

    7. the project works very well!!
      but i would like to ask what the capacitor is for.
      and can i change the value of the 2 resistors (10kohm) in the inverting input? and what are they for?

    8. why you are using two LDR in circuit..it can also work on single LDR..??

    9. Abdul Khaled says: on August 22, 2015 at 9:28 am

      Please ensure me.

      In-front of my home main door, I want to use it as security alarm, when a mysterious/Stranger come to my door lock, that time the mysterious/Stranger need to detect physically or by his/her shadow that time alarm will be active. then when alarm will active we can caught the Stranger. Let me know, can I use it, as I want?

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