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    This shadow alarm circuit can sense a moving shadow in a confined area. It can be used to protect things from theft. When somebody approaches the unit, it will give a loud alarm to abort the attempt of theft. The circuit uses the light sensing property of the Photo diode.

    The circuit uses the light sensing property of the PIN Photodiode. The p-n junction of the photo diode gives light current when it is forward biased. IC1 is designed as a voltage comparator. Its non inverting input is connected to a potential divider R1 and VR. By adjusting VR, input current to pin3 can be set to a reference level. The inverting input of IC1 is connected to a photo diode. IC CA3130 is a 15 MHz BiMOS Operational amplifier with MOSFET inputs and bipolar output. The inputs contain MOSFET transistors to provide very high input impedance and very low input current as low as 10pA. It has high speed of performance and suitable for low input current applications.

    CA3130A and CA3130 are op amps that combine the advantage of both CMOS and bipolar transistors. Gate-protected P-Channel MOSFET (PMOS) transistors are used in the input circuit to provide very-high-input impedance, very-low-input current and exceptional speed performance. The use of PMOS transistors in the input stage results in common-mode input-voltage capability down to 0.5V below the negative-supply terminal, an important attribute in single-supply applications.

    A CMOS transistor-pair, capable of swinging the output voltage to within 10mV of either supply-voltage terminal (at very high values of load impedance), is employed as the output circuit.

    The CA3130 Series circuits operate at supply voltages ranging from 5V to 16V, They can be phase compensated with a single external capacitor, and have terminals for adjustment of offset voltage for application requiring offset-null capability. Terminal provisions are also made to permit strobing of the output stage. The CA3130A offers superior input characteristics over those of the CA3130.

    Normally in the light (as set by VR) Photodiode gives voltage to pin2 of IC1.Since this voltage is higher than the voltage set by VR at pin3 ,output of IC1 remains low keeping LED and buzzer off. When a person approaches the photodiode, the shadow causes a reduction in current through the photodiode so that voltage at pin2 decreases below that of pin3. Output of IC1 then goes high and Buzzer sounds.


    The circuit triggers when the light intensity changes without a shadow. It is better to keep the unit in a place where constant light is available

    Shadow Alarm Circuit diagram

    attentionThe author D Mohankumar is not an active member anymore. Please take into consideration that the presented information might not be correct.
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    83 Responses to "Shadow Alarm Circuit"

    1. Sir, can we modify a ckt using LDR…so that it gets activated only at night (if reqd.)… where to place LDR if reqd ?

    2. LDR cannot be used in this circuit. It works based on the photocurrent through the photodiode

      • Raguvaran says: on February 3, 2012 at 12:21 pm

        Hello Sir,
        LDR can be used in this to do the automatic switch on in the nights. If the issue is not with the current leakage then we can connect the LDR parallel to the battery along with the series resistor.Just incase if the current also matters then with a micro relay and a switching transistor we can make this to work with LDR.


      • susanthachandima says: on May 25, 2014 at 6:22 am

        sequrity circuits

    3. S.Haider A. Rizvi says: on April 19, 2010 at 5:44 pm

      Sir I’m a Computer Engineering Student and I have Implemented your Shadow Alarm Design in my Basic Electronics Project. As you know that i have to prove the Alarm by mathematically, I have done the calculations when the photo diode is with out shadow & it’s approximately equal to the measured readings but I ‘m having some difficulties in the calculations when the shadow appears on the photo diode because I can’t figure it out how to calculate please help me on that because I have to submit & give viva on 24th April 2010.

    4. which method you used.Give details to dmohankumarwc@gmail.com

    5. sir,
      I am final year student ece department. i need an circuit of shadow gate system actually i want to do an project based on security system in museum.
      All the gate should access according to the shadow or light in the museum so that when thiefs shadow or light generates then all gates should automatically close and alarm rings.and when the unit or thing is taken from its actual place then light should generate.

      sir please give your opinion about the project and circuit diagram and description about the project


    6. Your Idea is excellent. we can do it using Infrared ray or laser or simply with a photo transistor. My mail is in the comments.Contact

    7. thanks for your reply sir

      we’ll soon approach u

    8. Dear sir,
      we need your guidance in our project. all details are sent to your mail please suggest us. as we to complete our project within 15 days

    9. hello sir,
      we are using lasers for outside security in our project.
      we want to know which type of lasers are better for implementing can we use photo transistor with photodiode or infrared rays.

    10. Laser ray is better since it is pointing and travel in straight line. Use low watt laser pointer which can give a range of 20 meters.Take adequate precautions while handling laser.Direct viewing may harm the eyes. IR rays can also be used with photo transistor or photo diode

    11. sir, is it possible to implement house security system through laser beams.
      we have done shadow alarm successfully can we do laser operation with shadow alarm
      what concept we have to apply for house security system with laser beams.

    12. can this circuit be modified into another application

    13. Sir,is there anyway of modifying the circuit into another application?

    14. pls define all other project related to it and give there component list

      • D.Mohankumar says: on June 12, 2010 at 6:57 am

        Mention your idea/requirements

      • sir,i am looking to alert the people through their cell phone instead of buzzer.would u provide information for it.

      • D.Mohankumar says: on June 15, 2010 at 4:37 pm

        This can be the trigger circuit. But another DTMF circuit is necessary to give telephone alert. look in web for such circuits

      • Najeefa Nikhat Choudhury says: on November 4, 2011 at 4:04 pm

        Hello Sajal,
        I am a CSE student and my project for computer interfacing is similar to what you wanted to do i.e. send a text message after detecting an intruder.Did you complete your project?If so can you help me with some information?

        Thank You.

    15. Sir !! Can U give me the Future Scope and Bibliography for this Shadow Alarm.Please sir it is Urgent….

      • D.Mohankumar says: on June 12, 2010 at 7:00 am

        This circuit can be the front end of optical communication, voice transmission, alarm systems, remote controls etc. Search for bibliography giving key words- photodiode, infrared, optoelectonics, optodevices applications etc

    16. sir,should i look for dtmf recorder/generator?would you prefer a good site providing such circuits

    17. sir,i also have an idea of beeping the buzzer according to the height of the shadow casted.what circuit can i use for this feature.

    18. you can use phodiodes in parallel and placed at different heights. Search in electronicsforu.com – circuit lab for dtmf

      • Sir,
        i am 6th sem ece student. and want to do project on shadow alarm.i want to know a detector will detect a shadow from how many distance.if my hand is 1 feet up from circuit and its shadow is on circuit,then alarm can ring?
        please reply.

      • sir … please help me how does the circuit work… the problem is the buzzer is continues buzzing… what should i do??

      • McCoy McPeter says: on September 4, 2015 at 10:50 am

        Sir, how does Shadow detector work in the night? Must the environment be supplied with constant light?
        What is the usefulness of the led(red) in the circuit?

    19. sir is there any other IC to replace IC CA3130? its very hard to find it…

    20. sir will u please send me about mini ups system. it’s my project and i am not getting the total information.so i need ur help.

    21. sir…… could i use this circuit for shadow detector by giving connection to motors……

    22. can i use photoconductive cell for photo diode.i done this ckt but this is not work.guide me.plz

    23. sir,can i turn on any device using the wave of my hand.Just as the shadow here generates a voltage sufficient enough to trigger the beeper,it may be possible that a shadow of my hand may generate voltage enough to turn a device other that beeper.Probably my cell phone or some other home appliances.PLZ reply urgently


    25. if we are using this shadow alarm to protect valuables at night in jewellery shop or any where….if thief knows about this shadow alarm then he will take care that not to fall shadow on photo diode…. then this becomes disadvantage.

      • Raguvaran says: on February 3, 2012 at 12:24 pm

        This is not for as the primary source of detection. If i need to protect my home from burglars i would rather build a motion detection sensor along with the fiber based pressure sensor and link them with the alarm. This will do the job

    26. i need a breadboard layout and PCB design layout for circuit “shadow activate motion dectector”. please help! thank-you.

    27. Engr, I’m Samuel, a final year student of Abia state university uturu, Nigeria, west Africa. I was assign on a project to design and construct a shadow detector used in Banks. Sir, how do i start? I also need a clearified circuit diagram of this project. Thanks and God bless.

    28. sir we are using a phototransistorl14f1 and a comparator but did not work can u give some suggestion to get the output

    29. Sir am a second year student of a polytechnic,i want to know d basics of electronics design and i have tried d circuit given here,pls is there not any substitute 4 d ic 3130? Pls help me sir

    30. imthe second year student at udsm in Tanzania.i want to design the circuit to control car crossing at the junction instead of using traffic.plz help me sir

    31. sir, i have a question, can we replace the speaker with other devices?


    33. Hi I need to use this circuit interfacing
      so I need an output either 0 or 5 V
      so How I can Reconnect it to acheive this result thank You

    34. sir i am 4th ece student.i want documentation for shadow alarm with ic555 and ldr

    35. nikita singhania says: on January 23, 2012 at 4:16 pm

      can u suggest any other IC other than CA3130…

      • Raguvaran says: on February 3, 2012 at 12:27 pm

        You dont have to stick to CA3130.You can do this with single transistor also. Or try using OPAMP IC 741(which is common and easily available and costs 10 Rs. You can also try with timer IC555.

      • balachandar says: on August 4, 2012 at 10:04 am

        whether anyone got result for this circuit ,using IC741? please frnds i wana implement this project…..

    36. Hello Sir,
      We made all the connections according to the circuit diagram, but our buzzer goes on ringing continuously (even in absence of shadow) when we give power supply to circiuit

      We did try various changes, but to no avail

      What might be the possible problem zones?

      • Raguvaran says: on February 15, 2012 at 5:52 am

        Simple reason could be that your op amp might be saturating.
        did u removed the photo diode(PD) and checked? without PD the input will not get anything and it shouldn’t be producing the output.If this happens then the problem is with ur PD.replace it and most likely it will work. else if it still produces sound without the PD also then vary the 10K pot and set the resistance where it stops beeping.

      • Raguvaran says: on February 15, 2012 at 5:57 am

        February 15, 2012 at 5:52 am

        Simple reason could be that your op amp might be saturating.
        did u removed the photo diode(PD) and checked? without PD the input will not get anything and it should be producing the beeping at output.If this happens then the problem is with ur PD.replace it and most likely it will work.

      • Rishabh Kaushik says: on May 15, 2012 at 5:45 pm

        Same problem please help

    37. Hello i am trying to simulate my circuit on multisim for my college project as the circuit has to be proved that it works before constructing it. However on multisim there is no photodiode component and there isnt the exact moel of the op amp ICI 3130, however im not sure whether this matters because i have tried to replace them with a switch for the pghotodiode and a stamndard 741 op amp. The circuit does not work, have you any ideas to help me?

    38. Hello sir,i want future enhancement for SHADOW ALARAM…………VERY UGRENT

    39. sir i am not able to make a good ppt. Will you please help me over that.

    40. sir,
      i,m a b.tech first year student of eee branch.
      i am doing a project based on the same principle but the circuit which i am doing ,consists of two photodiodes ,and a voltage regulator LM358 and a FET turns on the beeper.
      actualy i found it in the internet
      but i want to modify this circuit can i place a ir led and use it as a transmitter and also make a reciever cicuit please help me with my project

    41. Rishabh Kaushik says: on May 15, 2012 at 8:53 am

      is the connection from terminal 2 of ic to photo diode joint with wire joining r1 and vr

    42. Rishabh Kaushik says: on May 15, 2012 at 5:42 pm

      Same problem. please help. Is the connection from terminal 2 of ic to photo diode joint with wire joining r1 and vr. how to check photodiode. please help urgent

    43. answer ur first question is no. it should not be connected. if u did so then u will hear a continuous buzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz.

      Before trying out a ckt first try to understand the circuit like Wat it does at least block level insight u should be able to get

      • Rishabh Kaushik says: on May 15, 2012 at 7:11 pm

        What about continuos buzzing i am getting. please help

      • Raguvaran says: on May 15, 2012 at 7:56 pm

        adjust the variable resistor and see whether buzzing stops or not.

        if it doesn’t remove the photo diode and see wat happens..

    44. hi sir…can we made a signal instead of alarm sound…..if so..how can make it..????..

    45. Rishabh Kaushik says: on May 17, 2012 at 5:37 pm

      No even if i change vr no effect. even if i remove photodiode still ringing. can i get the pcb layout please

    46. will it work if I replace the alam with a fan and how can change DC to AC?
      plz send me the circut diagram.

    47. Sir thanks for this one, I would like to know what type or name of the photodiode to be used? thank you

    48. sir

      i want the full details of shadow alarm project i.e components,process,cost and features.plz suggest me.its very urgent.

      thanking u sir ,waiting for ur reply.

    49. how to control the alarm according to the distance?

    50. good day sir…

      if a photodiode have a value include…??

      and may have a copy of information about the circuit or details about the circuit …

      pls its ASAP..

      thank you…

    51. sir im ece student i need a components list for shadow alarm..
      waitng for ur reply

    52. Sir,

      Plz clear me the range of this circuit..

      Irfan Ahmad

    53. i am appalled at how many ask questions then do not follow the thread. You are brilliant!!!! thank you for the tip on the 741 or 388 in the shadow idea.i am leaving the medical field (not quite) to enter cyberorganics. we need to chat sometime,would a hysterisis ckt allow me to dtermine proximity of the different types ofshadows. build me a cicuit that measures poximity ans (speed) frequency and i will walk next door to my father’s 35 lawyer firm and then retire…serious

    54. sir,what is the photodiode tht we have to use ?

    55. Vr means voltage regulator I think so
      What is the range of it?

    56. we are working on a mini project(Shadow sensing alarm circuit).we are using LDR &IC 741 .we want to add some additional features to it like measuring the length of the shadow etc,.we have 5 days left to complete help us sir.

    57. can i use ic 741

    58. Sir, I m a class 12 student and I have to submit a working model in physics so can u suggest me a simple and cheap model.

    59. This circuit is not work properly , buzzer always produce sound ….and there is no recover.

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