Cell Phone Detector
Cell Phone Detectors, commonly referred to as mobile phones, have become the ubiquitous electronic gadgets of our time. With the advancement of communication technology, the demand for cell phones has surged dramatically. These devices transmit and receive signals within the frequency range of 0.9 to 3 GHz. In this article, we introduce a straightforward circuit that can detect the presence of an active cell phone by monitoring these signals.
I have developed two cell phone detector circuits—one using a Schottky Diode and a Voltage Comparator, and the other employing 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 functioning principle of the Cell Phone Detector circuits revolves around the identification of RF signals. In the Schottky Diode circuit, the Schottky Diode plays a crucial role in detecting the mobile phone signal due to its distinctive ability to rectify low-frequency signals with minimal noise interference. When an inductor is placed close to the RF signal source, it induces a signal, which is then rectified by the Schottky diode. This resulting low-power signal can be amplified and utilized to activate various indicators, 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
In this circuit, the Op-amp component serves as the RF Signal Detector, whereas the Transistor section functions as the indicator. The capacitors, in conjunction with the antenna, are employed to detect RF Signals when a cell phone initiates or receives a call or sends or receives a text message.
The Op-Amp interprets these signals by transforming the increase in current at its input into voltage at its output, resulting in the activation of the LED.
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 detection circuit comprises an inductor, a diode, a capacitor, and a resistor. For the inductor, a value of 10uH has been selected. The chosen detector diode is the Schottky diode BAT54, known for its ability to rectify low-frequency AC signals. To eliminate AC ripples, a 100nF ceramic capacitor has been chosen as the filter capacitor. Finally, a load resistor of 100 Ohms is implemented in the circuit.
Amplifier Circuit Design
In the common emitter mode, we employ a basic BJT BC547 transistor. In this configuration, there is no need for an emitter resistor since the output signal has a low value. The value of the collector resistor is determined by factors such as the battery voltage, the collector-emitter voltage, and the collector current.
To elaborate further, the battery voltage has been set to 12 V, which is within the safe operating range of the BC547 (given its maximum collector-emitter voltage of 45V). The collector-emitter voltage at the operating point is established at 5 V, with a collector current of 2 mA. This configuration leads to the selection of a 3 K ohm collector resistor, resulting in Rc being a 3 K ohm resistor.
The input resistor serves to provide bias to the transistor and should have a higher value to restrict the maximum current flow. This design decision ensures proper transistor operation.
Comparator circuit Design
In contrast, we opt for the LM339 in this scenario. To establish the reference voltage at the inverting terminal, a voltage divider is put into play. Given that the amplifier’s output voltage is modest, we set the reference voltage at a lower level, approximately around 4V. This adjustment is achieved by combining a 200 Ohm resistor with a 330 Ohm potentiometer. Additionally, a 10 Ohm resistor is employed as a current-limiting component in the output.
Mobile Phone Tracking Circuit Operation
Under normal circumstances, when there is no RF signal present, the voltage across the diode is negligible. Although the transistor amplifier increases this voltage, the resulting output voltage remains lower than the reference voltage provided to the comparator’s inverting terminal. Consequently, the output from the OPAMP registers as a low logic signal due to the smaller voltage at the non-inverting terminal compared to the inverting terminal.
However, when a mobile phone approaches the signal, the choke induces a voltage, and the diode demodulates the signal. The common emitter transistor then amplifies the input voltage. This amplification leads to a disparity between the output voltage and the reference output voltage. Consequently, the OPAMP produces a logic high signal, causing the LED to illuminate, thus indicating the presence of a mobile phone. To ensure accurate detection, it is advisable to position the circuit a few millimeters away from the object to be detected.
Theory Behind Cell Phone Tracking System
Mobile Phone Signal Detection using Schottky Diode
The signal originating from a mobile phone is classified as an RF (Radio Frequency) signal. When a mobile phone is in proximity to the circuit, its RF signal induces a voltage in the inductor through mutual induction. This results in the generation of an AC signal with a high frequency, typically in the GHz range, which is subsequently rectified by the Schottky diode. The output signal is then smoothed or filtered by the capacitor.
Schottky diodes, which are specialized diodes created by combining N-type semiconductor material with a metal component, are particularly well-suited for this application. They are known for their low noise characteristics and high-frequency capabilities. These diodes possess a unique attribute of being able to conduct at exceptionally low forward voltages, typically falling within the range of 0.15 to 0.45V. This feature allows the diode to switch or transition more rapidly, leading to improved system efficiency. Additionally, their relatively short reverse recovery time, which is on the order of approximately 100 seconds, contributes to their low-noise performance.
Signal Amplifier using BJT
The most frequently utilized amplifier is the BJT (Bipolar Junction Transistor) when configured in its common emitter configuration. A transistor amplifier operates on the principle that the input base current is magnified to produce the output collector current, typically by a factor denoted as β. In this setup, the emitter serves as the common terminal.
Biasing for the circuit is achieved by employing a voltage divider circuit created through the combination of two resistors. When the transistor is properly biased in the active region, which means the emitter-base junction is forward-biased while the collector-base junction is reverse-biased, even a modest base current yields a significantly larger collector current.
LM339 as Comparator
The LM339 is an integrated circuit featuring four individual comparators, but in this application, we are utilizing only a single comparator. When the voltage present at the non-inverting (+) terminal exceeds that at the inverting terminal, the output voltage becomes high. Conversely, when the voltage at the inverting terminal is greater, the output voltage drops to a low state.
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.