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Simple Shadow Sensor Alarm Circuit

Last Updated on January 3, 2024 by Swagatam 12 Comments

This shadow detector circuit operates using two LDRs and effectively detects the difference between the light levels and triggers a loud audible warning siren.

Table of Contents
  • Circuit Operation
  • Circuit Diagram

In circuits which uses a single LDR (photoresist), the detection may not be as sharp as with two LDRs discussed here.Operational details of the shadow detector circuit may be studied as follows:

The key elements of this circuit are the two LDRs and the active operational amplifier, which functions as a comparator.

Circuit Operation

As may be seen in the diagram, the inputs of the opamp are carefully balanced using alternately positioned LDRs across the corresponding supply rails in conjunction with the respective resistors.

The two resistors may be possibly replaced with presets for getting a fine adjustment option and for ensuring an optimum balance and a perfect zero logic at the output of the opamp.

In normal light situations that is with no shadow detected (no shade) the two LDRs are able to receive the same amount of light across the sensing input of the opamp which renders a low logic level at the output of the IC.

In an event when one of the LDRs (for instance R1) experiences a shadow or less light than the other (R4), causes the voltage at the inverting input of opamp to go lower than in the non-inverting counterpart, causing the logic at the output of IC to switch to a high logic.

The above action activates transistor Q1, which in turn activates the LED and the relay. The LED allows to acquire a visual warning while the relay activates a siren device.

Preferably you may want to place a semiconductor diode (D1) in parallel with the relay, as shown in the diagram, to protect the transistor Q1 from the relay reverse EMFs.

Circuit Diagram

warning message: electricity is dangerous, proceed with caution
shadowdetectorcircuit 1

Things to be notes:
- The circuit is powered by 9 volt lead acid battery or any similar SMF battery.
- The LDRs should be placed with a separation of about 3cm. for an optimal response and to avoid incorrect triggering.

Parts list for the proposed shadow detector circuit

- 1 Operational amplifier: LM741 (IC1)
- 2 LDRs (photoresistor / LDR) (R1, R2)
- 1 NPN transistor 2N2222 or similar (Q1)
- 1 1N4007 diode (D1)
- 1 red LED diode (D2)
- 9 volt relay (RL1)
- Two 10K resistors (R3 and R4)
- 1 1K resistor (R5)
- 1 resistors 470 (R6)
- 1 100 nF capacitor (C1)

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Filed Under: Sensors and Detectors Tagged With: Alarm, Sensor, Shadow, Simple

About Swagatam

I am an electronics engineer and doing practical hands-on work from more than 15 years now. Building real circuits, testing them and also making PCB layouts by myself. I really love doing all these things like inventing something new, designing electronics and also helping other people like hobby guys who want to make their own cool circuits at home.

And that is the main reason why I started this website homemade-circuits.com, to share different types of circuit ideas..

If you are having any kind of doubt or question related to circuits then just write down your question in the comment box below, I am like always checking, so I guarantee I will reply you for sure!

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Reader Interactions

Comments

  1. li perciro everest says

    June 23, 2022 at 5:08 am

    Please I want practical videos for students.

    thanks

    Reply
  2. Umer says

    April 27, 2021 at 5:03 am

    I want to ask that why the 100nF capacitor is used in this circuit? Is it a necessary component or we can ignore it to make our circuit functional?

    Reply
    • Swagatam says

      April 27, 2021 at 9:48 am

      The C1 must be actually placed directly across the IC supply pins…it is used for cancelling the reverse voltage spikes generated due to the siren operation, and relay clicking…

      Reply
      • Umer says

        April 27, 2021 at 11:03 am

        Thank you very much for the kind help.

        Reply
        • Swagatam says

          April 27, 2021 at 11:10 am

          You are welcome…

          Reply
  3. Adam says

    August 18, 2020 at 8:01 am

    Please I will like a video to show me step by step in constructing Shadow Detector

    Reply
  4. adio kabiru says

    January 29, 2015 at 5:04 pm

    can i use ic 555 timer in place of ic lm741,is it a must i use a relay

    Reply
    • Swagatam says

      January 30, 2015 at 6:42 am

      two LDRs won't be possible with a 555

      Reply
    • Swagatam says

      January 30, 2015 at 6:42 am

      for a big siren, relay will be required

      Reply
  5. yasir ulhaq says

    January 28, 2015 at 7:03 pm

    hello sir,
    your website is awsome. iam working on a project can you please post wifi signal detector circuit

    Reply
    • Swagatam says

      January 29, 2015 at 5:01 am

      thanks yasir, presently I do not have it, i'll post it if I happen to get it….

      Reply
    • yasir ulhaq says

      January 29, 2015 at 4:53 pm

      ok

      Reply

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