In normal condition, when there is no RF signal, the voltage across the diode will be negligible. Even though this voltage is amplified by the transistor amplifier, yet the output voltage is less than the reference voltage, which is applied to the inverting terminal of the comparator. Since the voltage at non inverting terminal of the OPAMP is less than the voltage at the inverting terminal, the output of the OPAMP is low logic signal.
Now when a mobile phone is present near the signal, a voltage is induced in the choke and the signal is demodulated by the diode. This input voltage is amplified by the common emitter transistor. The output voltage is such that it is more than the reference output voltage. The output of the OPAMP is thus a logic high signal and the LED starts glowing, to indicate the presence of a mobile phone. The circuit has to be placed centimeters away from the object to be detected.
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 formed by combining N type semiconductor material with a metal and are typically low noise diodes, operating at a high frequency. These diodes have a unique property of conducting at a very low forward voltage between 0.15 to 0.45V. This enables the diode to provide high switching speed and better system efficiency. The low noise is due to the very low reverse recovery time of about 100 per sec.
Q1. Why we need this mobile phone detector?
Ans1. The mobile phone detector circuit is used to detect the presence and use of mobile phone to communicate in restricted areas and zones. During exams, board meetings and while travelling in airplane we can detect mobile phones.
Q2. What major component is used in this project?
Ans2. The major component used in this project is IC CA3130 (op-amp) and 555 timer IC.
Q3. What is the role of CA3130 IC?
Ans3. CA3130 IC is an op amp that combines the advantage of both CMOS and bipolar transistor used in the input circuit to provide very-high-input impedance, very-low-input current, and exceptional speed performance.
Q4. What is the range of this mobile phone detector system?
Ans4.The range of this system is 3.3 to 10 cm.