Home Security System Using Cell Phone




The project is about a home security system in which a cell phone is used as a device that will alert the owner of the house when an intruder enters the house.

Step 1: About the Instructable

The owner just have to turn on the system before leaving his house.When an intruder enters the house in the absence of the owner the owner receives a call from a registered number which is the phone number of the hacked mobile phone at his/her house.

Step 2: How Does the Circuit Work?

Cell phones have switches which when pressed short the terminals (gold plated) hence a number is seen on the screen

so what i have done is put my cell phone number on speed dial on the phone i hacked and when this number is pressed for a while i get a call.

so basically what we r doing is a circuit that will short the phone terminals. when a intruder enters the house.

IC LM358 is a dual op amp.

The output of an op amp is difference of the input times the gain of the op amp. ie. it compares the 2 inputs.

NOTE: refer datasheet for more details about the IC.

pin 2 of IC 358 is threshold a potentiometer is connected to it so it forms a voltage divider and a particular voltage is set up at pin 2 which the op amp compares with voltage at pin 3.

to pin 3 an LDR is connected it behaves in following way.

1) when light shines on LDR its resistance decreases hence a more positive voltage is set up at pin 3 which forces the output to be high.hence there is a potential difference at the terminals of the number on the cell phone.

2)when the light is cut off by the door which is opened by the intruder the LDR resistance increases hence a more negative voltage is set up at pin 3 which forces output to be low (0v).

in the equivalent op amp circuit shown Avid is approximately 0v when output is low and resistance Ro is very small hence the output is shorted to the ground thus shorting the 2 terminals of the cell phone.

Step 3: Materials (electronic Components) Required

1) A Cellphone having press buttons (i used an old one which i had).

2) A 10K ohm resistor.

3) A 10K ohm potentiometer.

4) An LDR (Light Dependent Resistor) 10K ohm or A 2 legged IR sensor (IR sensors are better bcoz they are not affected by surrounding light).

5) An LM358 (Dual op-Amp) IC.

6) An Optical LASER (I used a red LASER). In case you are using IR sensors use an IR LASER.

7) wires.

Step 4: Step 1:Hacking the Cell Phone.

The cell phone i used is an old nokia phone.

Remove the battery then remove the front cover then the keypad an then the transparent cover over the keys.

You will see a thin white film on the key terminals dont worry just remove this film using forceps but remember only cut the film on a single key from 0 to 9. I just pulled it out in excitement and now i have problems starting the device i have to manually start it by shorting the terminals using any metallic object.

Step 5: Step 2: Checking the Polarities

Insert the battery back into the cell phone and turn it on.

I wanted to use key 3 for the speed dial so i was checking the terminals on key 3.

Put a multimeter on dc voltage measurement then touch the probes to the 2 terminals on the key then reverse the probes on the key terminals either way the multimeter will show a reading but it will show +ve DC voltage when the probes are on correct -ve ,+ve terminals.

In my case the inner one was positive and the outer one negative.

Solder a red wire to the +ve terminal and a black wire to the -ve terminal.

Step 6: Step 3: Soldering the Circuit.

According to the circuit diagram solder the circuit.a small dotted board is enough as there are hardly any components.

NOTE: the circuit remains same if a 2 legged IR sensor is used.

Solder the red wire from the cell phone key to the output of the op-amp IC ie. at pin 1 and the black wire to the ground.

Solder the LDR to a small piece of board and solder wires to it (LDR does not have polarity so it can be connected any way to the main board).

Step 7: Step 4:The Power Supply.

Use a cell phone charger just cut the jack and solder 2 wires to it which will be connected to the main circuit board.

The red one is +ve (Vcc on circuit diagram).

and black one is -ve (Ground on circuit diagram).

Step 8: Step 5: Placing the Circuit.

Place the sensor and LASER on opposite sides of a door.

focus the LASER on the Sensor such that LASER falls on the sensor when door is closed and it is cut off from the sensor when door is open.

Insert any active sim card into the cell phone and register its number on your cell phone as 'security' or anything you like.insert the battery and turn the cell phone on.

turn on the power supply to the circuit.

Step 9: Step 6: Calibrating the Circuit

Check video

We had kept a 10K ohm potentiometer for the circuit which will set the threshold for the op-amp.

Turn the potentiometer in either direction and check for a point at which

1)LED lights when door is closed and

2) it turns off when door is closed.

if you have reached the threshold point now don't change it.

set your phone number as speed dial on the key you have selected on the hacked phone.

press the home (call end) button to clear the screen.

The project is done.

now you just have to turn it on when you r going out.

Step 10: Step 7:Extending the Circuit

Any intruder will never come through a given door so every door in the house needs this circuit.

the main boards will increase the times you have doors and the output of each and every op amp should be AND'ed (connected to inputs of AND gate (Digital IC 7408) and the final output should be connected to the mobile +ve terminal so only 1 cell phone is required.

Step 11: Step 8: the Final Product

There you have it.

Do vote if u've liked my instructable and happy hacking.



  • 1 Hour Challenge

    1 Hour Challenge
  • Sensors Contest

    Sensors Contest
  • Fandom Contest

    Fandom Contest

75 Discussions


1 year ago

Hello Brother, great project. Appreciate for sharing it

Does the laser need to be interrupted for a specific amount of time so that speed dial can work? What if the interruption was for a short time say less than one second? How do we modify to incorporate this? Please advise

1 reply

Reply 1 year ago

Sorry for the late reply

Thank you

the current system has problems if the interrupt is brief.

You can add a 555 timer IC in astable mode and set the timing for around 5 seconds, its guaranteed to trigger everytime


1 year ago


I want to buy your project within 2 days please send your contact info @ chiragjoshicj16@outlook.com

MRo47md salman ahmed

Reply 1 year ago

I don't have the thing now. but I can definitely make one for you. Where are you from ?

md salman ahmedMRo47

Reply 1 year ago

I am from Hyderabad...
I want to buy this device ...
Please sent your contact no. & Email I'd..


2 years ago

Hi there,

This is an interesting project. May I suggest something simple?

If you solder a reed switch (as suggested by rfitzgerald) directly to the speed dial button, will it work? You will avoid building all the circuit you have shown here.


1 reply

3 years ago

does it need a charger supply as input..? can we use a 9v battery?


3 years ago

Please give your whatsapp no i want to ask something

1 reply

3 years ago

Thank u...it really working . I am a beginner in electronic... and i made it for my room


3 years ago on Introduction

good job ,is there other other projects for old mobiles .


3 years ago on Introduction

I need to make something like this but i lack the materials....would i use my radio?


3 years ago on Introduction

How long does it require to make the call?

Because if an intruder gets into our home he'll not stay in one place and I think that it will keep on pressing the key every time the circuit is broken.(If we use mirrors to extend the laser ray) Plz tell the sollution!

2 replies

Reply 3 years ago

you can use mirrors to extend the ray. Make it like a network as seen in movies for top class security systems. it takes approx 5 to 10 secs to place a call. And intruder might not have knowledge of what hides behind the door so might not check.


3 years ago on Introduction

Is it really working . I am a beginner and i am doing it for a school project...