Simple Mobile Phone Detector Circuit using Op-Amp

This article shows you how to build a circuit that can sense nearby cell phones.

It picks up on the invisible radio signals that cell phones use when they are turned on and can be used to let you know if someone is using a phone in an area where they should not be like a library or a meeting.

It is important to remember that using this circuit where cell phones are banned might not be allowed.

Always check the rules before using it.

Circuit Working:

Parts List:

The device can discern both incoming and outgoing calls, text messages SMS, internet and video transmissions even if the mobile phone is set to silent mode.

Upon detecting a radio frequency (RF) signal from an activated mobile phone the devices LED starts blinking and continues to do so until the signal ceases.

The diagram illustrates the authors prototype depicting the circuit and its functionality.

The circuit diagram for the mobile phone detector utilizing op amp LM358 It is constructed around the LM358 integrated circuit IC1 and the NPN transistor BC547 T1.

When a mobile phone is active it emits an RF signal that permeates the surrounding space carrying electromagnetic RF radiation from the phone.

Capacitor C1 plays a crucial role in the circuit by capturing the RF signal emitted by the mobile phone.

As the mobile phone emits RF signal energy C1 absorbs it and transmits it to the inputs of IC1, signified by the blinking of LED1.

The range of the circuit can be adjusted using preset VR1 2.2M.

Transistor T1 amplifies the signal obtained at pin 1 of IC1.

This circuit is suitable for 2G networks, GPRS and network search manual and automatic.

However, it may not effectively detect 3G, WCDMA and HSDPA network signals.

Formula:

The typical function of an operational amplifier op amp based mobile phone detector circuit is to detect the electromagnetic signals that mobile phones generate.

The precise formula would rely on the particular design factors, such as the bandpass filters cutoff frequencies, the rectification and filtering components and the comparators threshold.

Generally, conventional formula for RC circuits and filter design may be used to determine the values of the resistors and capacitors in the bandpass filter and low pass filter stages.

The cut off frequency (fc) of an RC low pass filter, for example may be computed using:

fc​ = 1​ / 2πRC

where:

• R is the resistance.
• C is the capacitance.

Additionally, the bandwidth and center frequency of the bandpass filter will dictate which precise frequencies you are picking up.

Notes:

Verify that the antenna can detect the desired frequency range.

The exact requirements (power supply voltage, bandwidth, etc.) should be taken into consideration while selecting the op amp.
By adjusting the op amp circuits gain or reference voltage, the detectors sensitivity may be changed.

This simple synopsis offers a place to start.

You may need to modify component values and the circuit design as a whole based on your unique needs (sensitivity level, detection range, etc.).

How to Build:

Building a mobile phone detector using the op-amp LM358 circuit involves several steps and requires some basic electronic components.

Here’s a simplified guide to help you create the circuit:

• Connect pin 4 of LM358 IC1 to ground.
• Connect pin 8 of LM358 IC1 to the positive supply voltage.
• Connect pin 3 of LM358 IC1 to the junction of R1 and R2.
• Connect the other end of R1 to pin 2 of IC1.
• Connect the other end of R2 to the junction of C1 and VR1.
• Connect one end of VR1 to the junction of R2 and C1.
• Connect the other end of VR1 to the ground.
• Connect one end of C1 to the junction of R2 and VR1.
• Connect the other end of C1 to pin 1 of IC1.
• Connect the cathode of the LED to the ground.
• Connect the collector of the transistor T1 to the positive supply voltage through a resistor R3.
• Connect the emitter of T1 to the ground.
• Connect the base of T1 to pin 1 of IC1.

Power Supply:

• Connect the positive and negative terminals of the power supply to Vcc and GND respectively.

Antenna:

• You may need to attach a small wire as an antenna to improve signal detection.

Testing:

• Power up the circuit.
• Activate a mobile phone within the detection range.
• Observe the LED blinking indicating the detection of the mobile phones RF signal.

Adjustments:

• Use VR1 to vary the circuits range until you achieve the desired sensitivity.

Note:

• Please note that building electronic circuits requires care and attention to detail.
• If you are not familiar with electronics consider seeking assistance or guidance from someone with experience.
• Additionally, be mindful of local regulations regarding the use of such devices.

Conclusion:

It is important to note that the use of such circuits should comply with legal regulations and privacy considerations should be taken into account.

Building and deploying electronic devices should be done responsibly and ethically.

References:

Implementation of Cell Phone Detection Mobile Robot for Restricted Areas Using NodeMCU

CELL PHONE DETECTOR PROJECT