Here is how to construct a circuit that can detect mobile phones in the area.
It can detect the invisible radio signals that cell phones produce when they are turned ON and alert them if someone is using a phone in private places like a conference room or a library where they should not be.
One should be aware they might not be able to use this circuit where mobile phones are prohibited.
Before using it always remember to go through the rules and regulations.
Circuit Working:
Parts List:
| Category | Item | Quantity |
|---|---|---|
| Resistors ( All resistors are 1/4 watt unless specified) | ||
| 100k | 1 | |
| 220k | 1 | |
| 1k | 1 | |
| Preset 2.2M | 1 | |
| Capacitors | ||
| Electrolytic 1μF 16V | 2 | |
| Electrolytic 100μF 16V | 1 | |
| Semiconductors | ||
| Op amp IC LM358 | 1 | |
| Transistor BC547 | 1 | |
| LED 20mA 5mm | 1 | |
| ON/OFF Switch | 1 | |
| Antenna | 1 | |
| Battery 1.5V | 3 |
Even when the mobile phone is in quiet mode the device can detect incoming and outgoing calls, text messages, SMS, internet and video transmissions.
The LED in the devices begins to flash when it detects a radio frequency (RF) signal from an active mobile phone and keeps blinking until the signal stops.
The circuit and its operation are shown in the circuit diagram
The mobile phone detector circuit design is based on the op-amp LM358
The LM358 integrated circuit IC1 and the NPN transistor BC547 T1 act as its foundation.
When a mobile phone is turned ON it sends out an electromagnetic radio frequency (RF) signal that spreads across the surrounding area.
By collecting the RF signal that the mobile phone generates capacitor C1 plays an essential role in this circuit.
LED1 blinks to indicate that C1 receives and sent the RF signal energy from the mobile phone to the inputs of IC1.
The preset VR1 2.2M is used to change the circuits range.
The signal received at pin 1 of IC1 is enhanced by transistor T1.
GPRS, 2G networks and both manual and automatic network types are used with this circuit.
But it might not be able to pick up 3G, WCDMA and HSDPA network signals.
Formula:
Detecting the electromagnetic signals produced by mobile phones is the common purpose of a circuit based on an operational amplifier
The exact formula would depend on specific design elements including the threshold of the comparator, the rectification and filtering components and the cutoff frequencies of the bandpass filters.
The values of the resistors and capacitors in the bandpass filter and low pass filter stages can often be found using a standard formula for RC circuits and filter design.
For example the cut off frequency (fc) of an RC low pass filter can be calculated using the following formula:
fc = 1 / 2πRC
where:
- R is the resistance.
- C is the capacitance.
Additionally the bandwidth and center frequency of the bandpass filter will decide which precise frequencies one is picking up.
Notes:
It will also depend on the bandpass filters center frequency and bandwidth to decide which specific frequencies one chooses.
Check to see if the antenna can pick up the specified frequency range.
When choosing the op-amp the specific needs like power supply voltage, bandwidth etc should be kept in mind
The sensitivity of the detector gets modified by adjusting the reference voltage or gain of the op-amp circuit.
This simple summary provides a starting point.
Depending on specific needs one might need to change the values of the components and the circuit design overall sensitivity level, detection range etc.
How to Build:
To build a Simple Mobile Phone Detector Circuit using Op-Amp following are the connection steps to keep in mind:
- Connect pin 4 to ground of the IC1 LM358.
- Wire pin 8 of LM358 IC1 to the supply voltage that is positive.
- Solder the third pin of the LM358 IC1 to the junction of R1 and R2.
- Connect R1s opposite end to IC1s pin 2.
- The opposite end of R2 connect to the point where C1 and VR1 connect.
- Connect one end of VR1 to the junction of R2 and C1.
- Insert VR1s opposite end into the ground.
- Connect one end of C1 to the R2 and VR1 junction.
- Connect the other end of C1 to IC1s pin 1.
- Make sure the LEDs cathode is grounded.
- The collector of the transistor T1 connect to the positive supply voltage through a resistor R3.
- The emitter of T1 should be connected to the ground.
- Connect the base of T1 to pin 1 of IC1.
Source of Power:
- Connect the power supplies positive terminal to Vcc and its negative terminal to GND
The antenna:
- It can be useful to use a little wire as an antenna in order to enhance signal detection.
- Turn ON the circuit.
- Turn ON a cell phone that is within detection range.
- Look for the flashing LED that indicates the mobile phones RF signal has been detected.
Modifications:
- To get the correct sensitivity adjust the circuits frequency using VR1 preset.
Take note:
- Please be aware that designing an electrical circuit involves careful consideration of every detail.
- If anyone is unfamiliar with electronics think about asking an experienced person for help or advice.
- Keep in mind local laws governing the usage of these devices as well.
Conclusion:
One should remember that it is crucial to keep in mind that using such circuits requires extra attention respect to legal requirements and the use of safety protections.
There are legal and safety requirements for this type of design and its operation which one should follow
References:
Implementation of Cell Phone Detection Mobile Robot for Restricted Areas Using NodeMCU