Protect your mobile phone from unauthorised use

MOBILE SHIELD
Protect your mobile phone from unauthorised use or theft using this simple circuit. It can generate a loud chirping sound when somebody attempts to take away the mobile handset. The added feature is that
the circuit also works as a mobile charger. The circuit is powered by a step-down transformer X1 with rectifier diodes D1 and D2 and filter capacitor C1. Regulator IC 7812 (IC1) along with noise filter capacitors C2 and C3 provides regulated power supply. The circuit utilises two NE555 timer ICs: One as a simple astable multivibrator (IC2) and the second as a monostable (IC3). The astable multivibrator has timing resistors R1 and R2 but no timing capacitor as it works with stray capacitance. Its pins 6 and 2 are directly connected to a protecting shield made up of 10cm×10cm copper-clad board. The inherent stray capacitance of the circuit is sufficient to given an output frequency of about 25 kHz with R1 and R2. This arrangement provides greater sensitivity and enables the circuit with hand capacitance effect. Output pulses from the oscillator are directly given to trigger pin 2 of the monostable. The monostable uses a low-value capacitor C6, resistors R3 and preset VR1 for timing. The output frequency of the monostable is adjusted using preset
VR1 such that it is slightly less than that of the astable. This makes the circuit standby, when there is no
hand capacitance present. So in the standby mode, the astable’s output will be low. This makes the trigger
input of monostable low and output high. The warning LED1 and buzzer are connected such that they become active only when the output of the monostable sinks current. In the standby state, the LED1 remains ‘off’
and the buzzer is silent. As somebody tries to take the mobile phone from the protecting shield, his hand comes near the shield or makes contact with the shield, which introduces hand capacitance in the circuit. As a result, the astable’s frequency changes, which makes the trigger pin of the monostable low and its output oscillates. This produces chirping sound from the buzzer and also makes the  LED1 blink. The circuit can also be used as a mobile charger. It provides output of 6V at 180 mA through regulator IC 7806 (IC4) and resistor R5 for charging the mobile phone. Diode D3 protects the output from polarity reversal.
The circuit can be wired on a common PCB. Enclose it in a suitable case with provision for charger output

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