Cell Phone Detector often known as mobile phones are the most commonly used electronic devices nowadays. The demand for cell phones has risen tremendously as communication technology has advanced. A cell phone broadcasts and receives signals in the 0.9 to 3GHz frequency band. By monitoring these signals, this article presents a simple circuit to identify the presence of an activated cell phone.
I created two cell phone detector circuits, one with a Schottky Diode and a Voltage Comparator and the other with a BiCMOS Op-Amp.
- Basic Principle of Mobile Phone Detector Circuit
- Circuit 1: Simple Cell Phone Detector Circuit
- Components Required
- Circuit 2: Cell Phone Detector using Schottky Diode
- Cell Phone Detector Circuit Design
- Detector Circuit Design
- Amplifier Circuit Design
- Comparator circuit Design
- Mobile Phone Tracking Circuit Operation
- Theory Behind Cell Phone Tracking System
- Cell Phone Detector Circuit Design
- Cell Phone Detector Circuit Applications
- Limitations of Mobile Phone Detector Circuit
Basic Principle of Mobile Phone Detector Circuit
The Cell Phone Detector circuits work on the premise of detecting RF signals. The Schottky Diode is used to detect the mobile phone signal in the Schottky diode circuit because it has the unique attribute of being able to correct low frequency signals with low noise rate. Mutual induction receives the signal when an inductor is positioned near the RF signal source. The Schottky diode rectifies this signal. This low-power signal can be amplified and used to power any indicator, such as an LED.
Circuit 1: Simple Cell Phone Detector Circuit
The first circuit of the cell phone detector is a simple implementation using an Op-amp and a few other passive components.
- CA3130 Op-Amp
- Resistors – 2.2MΩ x 2, 100KΩ, 1KΩ
- Capacitors – 22pF x 2, 0.22nF, 47pF, 100µF
- BC548 NPN Transistor
- Connecting Wires
- 9V Battery
The Op-amp part of the circuit acts as the RF Signal Detector while Transistor part of the circuit act as the indicator. The capacitors collection along with the antenna are used to detect RF Signals when a cell phone makes (or receives) a phone call or sends (or receives) a text message.
Op-Amp reads the signals by converting the rise in current at input to voltage at output and the LED will be activated.
Circuit 2: Cell Phone Detector using Schottky Diode
- V1 = 12V
- L1 = 10uH
- R1 = 100Ohms
- C1 = 100nF
- R2 = 100K
- R3 = 3K
- Q1 = BC547
- R4 = 200 Ohms
- R5 = 100 Ohms
- IC1= LM339
- R6 = 10 Ohms
- LED = Blue LED
Cell Phone Detector Circuit Design
Detector Circuit Design
The detector circuit consists of an inductor, diode, a capacitor and a resistor. Here an inductor value of 10uH is chosen. A Schottky diode BAT54 is chosen as the detector diode, which can rectify low frequency AC signal. The filter capacitor chosen in a 100nF ceramic capacitor, used to filter out AC ripples. A load resistor of 100 Ohms is used.
Amplifier Circuit Design
In common emitter mode, a basic BJT BC547 is utilised. The emitter resistor is not required in this scenario because the output signal is of low value. The collector resistor value is determined by the voltage of the battery, the voltage of the collector emitter, and the current of the collector.
The battery voltage is now set to 12 V (because the BC 547’s maximum open source collector emitter voltage is 45V), the operating point collector emitter voltage is set to 5 V, and the collector current is set to 2 mA. This results in a 3 K collector resistor. As a result, Rc is a 3 K resistor. The input resistor provides bias to the transistor and should be of a bigger value to prevent the maximum current from flowing. Here we are.
Comparator circuit Design
As a comparison, the LM339 is employed. A potential divider is used to set the reference voltage at the inverting terminal. Because the amplifier’s output voltage is low, the reference voltage is set low, on the order of 4V. This is accomplished by using a 200 Ohm resistor and a 330 Ohm potentiometer. As a current limiting resistor, a 10 Ohm output resistor is employed.
Mobile Phone Tracking Circuit Operation
When there is no RF signal, the voltage across the diode is insignificant in normal conditions. Despite the fact that this voltage is increased by the transistor amplifier, the output voltage is less than the reference voltage provided to the comparator’s inverting terminal. The output of the OPAMP is a low logic signal since the voltage at the non inverting terminal is smaller than the voltage at the inverting terminal.
When a mobile phone comes close to the signal, the choke induces a voltage, and the signal is demodulated by the diode. The common emitter transistor amplifies the input voltage. There is a difference between the output voltage and the reference output voltage. As a result, the OPAMP’s output is a logic high signal, and the LED begins to illuminate, indicating the presence of a mobile phone. The circuit must be set back a few millimetres from the object to be detected.
Theory Behind Cell Phone Tracking System
Mobile Phone Signal Detection using Schottky Diode
The signal from mobile phone is a RF signal. When a mobile phone is present near the circuit, the RF signal from the mobile induces a voltage in the inductor via mutual induction. This AC signal of high frequency of the order of GHz is rectified by the Schottky diode. The output signal is filtered by the capacitor.
Schottky diodes are special diodes made by mixing N type semiconductor material with a metal, and they are often low noise, high frequency diodes. These diodes have the rare ability to conduct at very low forward voltages, ranging from 0.15 to 0.45V. This allows the diode to transition at a faster rate and improve system efficiency. The relatively short reverse recovery time of roughly 100 seconds accounts for the low noise.
Signal Amplifier using BJT
BJT or bipolar junction transistor in its common emitter form is the most common amplifier used. A transistor amplifier works on the fact that the input base current is amplified to the output collector current by a factor of β. Here the emitter is the common terminal.
The circuit is biased using a voltage divider circuit formed by combination of two resistors. When a transistor is biased in active region, i.e. the emitter base junction is forward biased and the collector base junction is reverse biased, a small base current results in a larger collector current.
LM339 as Comparator
LM339 is a comparator IC containing 4 comparators. Here we are using only one comparator. When the voltage at non inverting (+) terminal is higher than the voltage at inverting terminal, the output voltage goes high. When the voltage at inverting terminal is higher, the output voltage goes low.
Cell Phone Detector Circuit Applications
- This circuit can be used at examination halls, meetings to detect presence of mobile phones and prevent the use of cell phones.
- It can be used for detecting mobile phones used for spying and unauthorized transmission of audio and video.
- It can be used to detect stolen mobile phones.
Limitations of Mobile Phone Detector Circuit
- It is a low range detector, of the order of centimetres.
- The Schottky diode with higher barrier height is less sensitive to small signals.