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 Josehf 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.

Step 1: Components:

ICs
-LM741 general purpose opamp x1

Resistors
-680k x2
-1M x1
-27k x1

Transistors
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