Mobile Detector

Smart phone detector using Schottky diode to detect the cell phone signal. Mobile phone signal is in the frequency range of 0.9 to 3 GHZ. Schottky diodes have a unique property of being able to rectify low frequency dingnal, with low noise rate. When an inductor is placed near the RF signal source, it recieves the signal through mutual induction. the signal is rectified by the Schotttky diode. This low power signal is amplified and used to power any indicator like an LED in this case.

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-

Here a simple BJT BC547 is used in common emitter mode. Since the output signal is of low value, the emitter resistor is not required in this case. The collector resistor value is determined by the value of battery voltage, collector emitter voltage and collector current. Now the battery voltage is chosen to be 12 V (since maximum open source collector emitter voltage for BC 547 is 45V), operating point collector emitter voltage is 5 V and collector current is 2 mA. This gives a collector resistor of approx 3 K. Thus a 3 K resistor is used as Rc. The input resistor is used to provide bias to the transistor and should be of larger value, so as to prevent the flow of maximum current. Here we chose a resistor value of 100 K.

Theory Behind cell phone Tracking System-

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 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. formed by combining N This very low reverse recovery time enables the diode to provide high switching speed and better system efficiency. The low noise is due to the of about 100 per sec.