Cell Phone Signal Detector

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Op Amp Cell Phone RF Signal Detector

 Revisions 
10-01-2012: Dr Henry Louie had the idea to create a simple cell phone signal detector for Laboratory experiment. 
10-16-2012: Circuit and materials created by Gary Fernandes.
06-12-2013: Added  Dr. Henry Louie's "Intro to Engineering" that includes assembly instructions that start on page 12

See Assembly Instructions Here; start on page 12:  http://www.seattleu.edu/scieng/ece/Cell_Phone_Signal_Detector_Assembly_Instructions.html

Online CircuitLab Circuit Schematic: https://www.circuitlab.com/circuit/uq4jyh/my-lm358-cell-phone-rf-detector/

I don't recommend this circuit for new LTE-4G phones. Should work fine for 3G phones when this circuit was originally created.

 The circuit seems to work fine for 3G phones on any wireless carrier or newer 4G phones on the USA Verizon, AT&T, and Sprint carriers. Perhaps it works well on T-Mobile in some cities. The older phone shown in the video only supported 3G GSM mode and that seems to detect well. With a newer Samsung Note 2 phone in CDMA LTE mode and on wireless carrier T-Mobile this circuit did not detect well but other have made it work well on Verizon and other wireless carrier or in other cities. When I placed the phone in GSM mode the circuit worked on fine on T-Mobile just like the video shows. When time allows I'll create a better circuit and post it or please post your circuit here (if it works better). 

This beginner "hobby" electronics circuit is a quick  fun breadboard circuit that uses an operational amplifier (Op-Amp) to sense the presence of an activated cell phone from a distance of several meters.  The simple circuit can detect any activity of a mobile phone such as incoming or outgoing voice, voicemail, texting, and data. 

If a cell phone signal is detected the circuit will blink a LED and or a sound buzzer.  Keep in mind that the phone must be transmitting and the only time a phone is transmitting is when it is receiving or sending voice, text, internet or data.

Sensitivity can be adjusted on the circuit by adjusting the potentiometer. This circuit is made very simple using the least amount of components. The components used are very inexpensive and readily available.

Schematic

Online Schematic

 

R5 Potentiometer (POT):

R5 must be very carefully set by the user to a threshold above the background noise present in the environment and yet below that of the radiated noise from a nearby cell phone. Adjust the POT until the LED is off with only an occasional flicker. Again adjust the potentiometer very very very delicately until the LED or buzzer is just barely off and only randomly blinking or buzzing from only the light background radiated noise of your environment. Then your circuit should detect 2G-3G cell phone activity easily. 

Antenna: Experiment with the antenna length.  You can search the internet on antenna design, tutorials, and calculators to find the best length or shapes taking into account the frequency involved and chip-breadboard layout. 

When I get a chance I'll build one that is more receptive to higher frequencies of 4G carriers. The circuit above was tested on a slower 3G T-Mobile service.

Basics of How It Works

Come to our offices or if you don't like traveling long distances........   R1 and R2 form a voltage reference connected to U1-A Pin3.   R3 and C2 form a RC circuit (You must understand basic RC circuits). Any sudden energy picked up by the antenna connected to Pin2 causes U1-A output Pin1 to change (you must read up on the basics of Operational Amplifiers and Comparators). Pin1 output remains changed during the period of RC or the antenna energy (cell phone radio waves) is removed. R5 creates a reference voltage on Pin6 and R5 must be very carefully set by the user to an appropriate threshold above the background noise present in the environment and below that of the radiated noise from a nearby cell phone. The setting of R5 is critical and must be adjusted with the greatest of small adjustment tweaks to find this zone above the background noise yet not above the noise of a nearby cell phone. Many students don't get this adjustment correctly because R5 is adjust with such small tweaks. Since Pin1 is connected to Pin5, U1-B output Pin7 turns on the LED and/or Buzzer.  Try to understand Voltage Dividers circuits (example R1 & R2, R5) and Resistor Capacitor (RC) circuits and Op Amp Comparator circuits.  

Assembly Instructions start on page 12: 

http://www.seattleu.edu/scieng/ece/Cell_Phone_Signal_Detector_Assembly_Instructions.html

Use instructions above with photo below for best practice.

Example Circuit 1
 
Example Circuit 2
This circuit works as good as circuit 1.

Enjoy!

Where to buy parts

You may be able to purchase the whole kit from Electronix Express http://www.elexp.com/ . Try emailing their sales department and include the parts list below. The parts list below uses all Electronix Express part numbers for easier purchases.

Other great distributors 

Distributor Electronix Express - Parts List

Electronix Express - LM358 Cell Phone Detector Custom Kit

ItemReference DesignatorPart DescriptionQuantity UsedElectronix Express 
Part Number

1.

U1

LM358AN, Dual OpAmp Single Supply 8-Pin Dip, Thru-Hole,

Qty 1

10358

2.

R1, R2, R3

6.8M Resistor, Carbon, 6.8M Ohm, 1/4 Watt, 5%,  Thru-Hole, Bulk,

Qty 3

130056.8M

3.

R4

1K Resistor, Carbon, 1K Ohm, 1/4 Watt, 5%, Thru-Hole, Bulk,

Qty 1

130051K

4.

C1, C2, C3

0.1uF Capacitor, Ceramic,  0.1uF 50 Volt, Z5U, 20% Radial, Bulk,

Qty 3

14MN050.1U

5.

R5

500K Ohm Resistor Potentiometer Trimmer, 0.5Watt, 10%, 1 single turn, 2.5mm pin spacing, Thru-Hole, Bulk,

Qty 1

18STC500K

6.

D1

LED Green Color T-1 3/4, Thru-Hole, Bulk,

Qty 1

08L53GD     

7.

9V

9 V Snap Connector Clip, 4 Inch Length 26 AWG Lead Wire

Qty 1

2801BSNAP

8.

9V

9 Volt Battery,

Qty 1

2800RG9

9.

 

Breadboard 170 Tie Point

Qty 1

03MB101

10.

 

22 AGW Hook Up Jumper Wire Kit

Qty 1

2700MJW70

11.

 

Wire Cutter Stripper, 26-16AWG

Qty 1

060220

12.

 

Small DC Buzzer, Built in Driver Circuit

Qt1

26PZ06 

My Generic Parts List

Use this parts list for purchasing from any distributor. It is the same parts but uses generic part numbers.   

Op Amp Cell Phone RF Signal Detector  Revision 10-16-2012
My cost

Schematic Reference Designator

Part

Description

Quantity Used in Circuit

Cost Each

Cost Total

Note

U1

LM358AN

Dual OpAmp Single Supply 8-Pin Dip, Thru-Hole

1

$0.19

$0.19

 

R1, R2, R3

6.8M Resistor

Resistor, Carbon, 6.8M Ohm, 1/4 Watt, 5%, Thru-Hole, Bulk.

3

$0.03

$0.09

This value may be substituted with what you have available, such as 1M – 10M. 

R4

1K Resistor

Resistor, Carbon, 1K Ohm, 1/4 Watt, 5%, Thru-Hole, Bulk.

1

$0.03

$0.03

This value may be substituted with what you have available, such as 330 – 5K.

 

C1, C2, C3

0.1uF Capacitor

Capacitor, Monolithic Ceramic, 0.1uF 50 Volt, Z5U, 20% Radial, Bulk

3

$0.06

$0.18

This value may be substituted with what you have available.

R5

500K Resistor Potentiometer Trimmer - Thumb and screwdriver adjustment

3352 - 3/8 ” Trimming Potentiometer 500K Ohm, 0.5Watt, 10%,  Thru-Hole, Bulk, Single-Turn, Cermet, Thumb and screwdriver adjustment

1

$0.90

$0.90

This value may be substituted with what you have available, such as 10K – 1M. Or inexpensive resistor could replace this.

 

D1

LED Any Color

LED T-1 3/4, Thru-Hole, Bulk

1

$0.05

$0.05

Any color or size.

--

9 V, Snap Connector

9 V Battery Snap Clip, 4 Inch Length 26 AWG Lead Wire

1

$0.30

$0.30

 

--

9 Volt Battery

9 Volt Battery

1

$1.00

$1.00

 

--

Breadboard

170 Tie Point Breadboard

1

$0.95

$0.95

 

--

22 AGW Wire

Hook Up Jumper Wire, 22AWG, Solid, Red

Hook Up Jumper Wire, 22AWG, Solid, Black

Hook Up Jumper Wire, 22AWG, Solid, Yellow

Pre-stocked

 

0

You can purchase 25’ spools of different colored wire for a few dollars each or purchase “jumper wire kits”

--

Wire Cutter Stripper

Wire Cutter Stripper, 26-16AWG

Pre-stocked

 

0

 

--

Miniature Potentiometer Trimmer Tool

Miniature Potentiometer Trimmer Tool - Screwdriver

 

 

0

$0.40 Used to adjust trimmer if using a non-thumb adjustable trimmer.

--

Optional Buzzer

 

 

 

0

I used a 14 year old buzzer I found lying around. I’ll purchase a buzzer online and provide a part number in the future.

Total Cost

$3.69