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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
    ask a question

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

    6. 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. 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?

    10. If both LDRs have low resistance(both are under light condition), is there any short circuit?

      • As a matter of fact short-circuit condition is satisfied when both the LDRs are subjected to strong light. In short time they will blow up if nothing(a resistor, for example) limits the current. The designer never thought at this situation.

    11. Question – I have different op-amps, could I substitute one for the 741? Considering using this as part of an alarm system for our motor home while we are living in it

    12. Can I use bc557

      • You can use BC557(PNP) on one condition: emitter should be tied to +V and the buzzer should be between collector and ground(minus). D1 (LED) should be reversed as well. See the picture…

    13. what is the purpose of capacitor in this circuit?

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