DIY Pocket Sized Anti-Theft Alarm !

About: I'm an electronics hobbyist.I have a humongous interest in electronics.I enjoy building circuits,experimenting with ICs and MCus and fiddling and opening up any thing related to electronics or has electronic...
IS somebody  pinching  your stuff and you can't find who it is?don't know who that somebody is?then this instructable is for you to catch them red handed!
in this instructable i will show you how to make a pocket sized  intruder alarm that is lightweight and portable.The project involves 2 parts - the transmitter and the receiver .
the object to be protected is placed between the Transmitter and the receiver .When the object is removed the allows the signal from the transmitter to reach the receiver and sound the alarm.The circuit is light weight and is portable.It cost very less to make as it uses very few components.This project should be easy for you depending on your circuit building and soldering skills .Also i have not uploaded an image of the transmitter. .Thanks to  J
osehf Murchison for helping me troubleshoot my circuit problems.Watch the video below to see the circuit in action.I have entered this for the Pocket Sized Electronics Contest so if you think my instructable is good,Vote for me.

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Step 1: Components:

-LM741 general purpose opamp x1

-680k  x2
-1M     x1
-27k    x1

2N2907 PNP transistor  x1

piezo buzzer  x1(this should be small)

IR photodiode  x1

IR l.e.d   x1

The following is only if you're soldering,
pcb board(small) x2
soldering lead
Dil socket x1

-The image is for representation purposes only.

Step 2: Building the Circuit!(Using the Schematic)

There are mainly 2 parts to the circuit- the receiver  and the transmitter
i'm gonna show you how to make it in a number of steps.
-first insert the dil socket into the pcb board or breadboard(if you want to prototype it)
-then insert the LM741 opamp into the dil socket
-connect pin4 to V-
-connect pin7 to V+
-connect a 680k resistor between V+ and pin 2
-connect a 680k resistor between V+ and pin 3
-Connect a 1M resistor between V- and pin 2
-connect the shorter end(cathode) of the photodiode to V- and longer lead(anode) to pin 3
-connect a 27k resistor between pin6 and base of the transistor.
-connect the emitter of the transistor to to V+
-connect the collector of the transistor to the cathode of the piezo buzzer
-connect the anode of the piezo buzzer to V+
----------------------------- And the reciever is done.

Building the transmitter is easier(i have used a 9Vbattery but you can use a 3v coin cell battery or 2x1.5v aa or aaa cells  to make it smaller and portable)
-if you're going to use a 9v battery then 
-connect a 47 ohm resistor between 
the positive of the battery and the anode of the IR led
and connect the negative of the battery to the cathode of the IR led
if you're using 2x1.5v cells follow the same but if you want you can exclude the resistor

--------------------------------------The transmitter is done

When you turn on both the circuits make sure there is an object in between the transmitter and receiver or else the alarm wil sound off.Set it up the circuits parallel to each other like i have done in the video.

Step 3: Tips ,Tricks Precautions and Sources of Errors!

Tips and tricks
by increasing the range of the IR led(you can find high output IR led's in remotes) you can even guard bigger objects such as laptops and Maybe even your lunch box!!!!!(people steal your lunch, don't they?)
if you want to make it less noticeable you can conceal it in a small paperweight or a small box

Precautions and sources of error:
Make sure you place the transmitter and receiver parallel to each other with the IR led pointing to the photodiode and place an object in between it before you turn it on.
Placing the transmitter and receiver too far apart can cause the alarm to not work at all.So make sure you place it close enough.
Thanks to 

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    19 Discussions


    8 months ago

    Sir, my piezo wont stop buzzing. what's. I dont know what's wrong.

    1 reply

    4 years ago


    I am trying to make this circuit since 4 days, but it just do not work. please help me i have made exactly as what you have shown.


    4 years ago


    I have been trying to make this simple circuit since 4 days, but this is just not working.

    Please help me, I have already replaced all the components of the circuit with new ones, but it didn't work.


    4 years ago on Step 2

    there is a mistake in receiver circuit

    When we connect junction of both 1m ohms and and 680k ohms to 2 terminal and junction of photodiode and 680k to 3 terminal it will work correctly.

    1 reply

    4 years ago on Introduction


    Amazing instructable ! :)

    What can we use instead of 2n2907 ? :)




    5 years ago on Introduction

    nice instructable you did there, can you post how each components in the circuit works like the resistors, transistors and everything. . . thanks in advance !! :)


    6 years ago on Introduction

    Can you please tell me if I want to work it reversely? means when a Object passes or someone passes through it, it beeps.

    2 replies

    Reply 6 years ago on Introduction

    sure azharz !.Here's what you have to do-
    1) instead of the 2n2907(pnp) transistor use a 2n3904(npn) transistor
    2)connect the resistor from pin 6 to the base of the transistor
    3)connect the emitter of the transistor to V+ (positive supply)
    4)connect the collector of the transistor to the positive(longer leg/anode) of the piezo buzzer
    5)connect the negative(shorter leg/cathode) of the piezo buzzer to V- (negative supply)
    and that's all you have to change.Thank you for commenting on my instructable and if you think my instructable is good, please vote for me the pocket sized electronics and the kit contest .cheers


    6 years ago on Introduction


    I have a suggestion for you!

    If you integrate the transmitter and receiver into the same place you can transmit a signal and then measure amount of reflected signal from the object.

    The reflected signal can be 'seen' even with the same LED as transmitted because led's can be used as receivers (instead of photoresistors).

    This way you save one battery that may be low and transmitter would usually not receive but you'll definitely need sophisticated electronics in the receiver to first emit light and then receive the reflection and take action.

    Otherwise with the photoresistor you may need to optically isolate the transmitter not to transmit sideways.

    Anyways, a great instrucatable!

    4 replies

    Reply 6 years ago on Introduction

    well hertzgamma ,that is an excellent suggestion!,in fact i am going to try it today.well i modified what you said a little -instead of the 2n2907 transistor i am going to add a 2n3904 transistor (so that it only buzzes loudly when it does'nt receive any signal) and solder the ir led onto the board and set everything and place an object close to it.When the object is removed the 2n3904 transistor will be activated and it will sound will also save battery space as you said.thanks for the comment and excellent suggestion.


    Reply 6 years ago on Introduction

    @hertzgamma ,i will post a video in a week or so .Thank you for the comment and once again excellent suggestion.Oh and if you could, please vote for me in the pocket sized electronics contest and once again thank you .


    6 years ago on Introduction

    The buzzer will alert you to the fact that your possession has been stolen, but who stole it you cannot tell, unless you're prepared to strip search everybody in the office!

    1 reply

    Reply 6 years ago on Introduction

    well you're right but if you replace my ALarm which works on IR leds and photodiodes instead of a laser securrity system and lead it tot he relay like the link in the video below you can then know who it was without having to strip search everybody!