Cell phone shielding circuit diagram phone shield

This is a mobile phone shielding circuit that can be used as a mobile phone charger. This simple circuit provides protection against accidental use or theft of your phone. When someone tries to take away the phone, it is capable of producing a loud chirping sound. Added functionality is that the circuit also acts as a mobile charger.

The circuit consists of step-down transformer X1, rectifier diodes D1 and D2 and filter capacitor C1. Voltage stabilizing IC 7812 (IC1), together with noise filter capacitors C2 and C3 provide the regulated power supply.

The mobile phone shielding circuit uses two selected NE555 timer ICs: a very simple multivibrator (IC2), and a second monostable multivibrator (IC3). The multivibrator has timing resistors R1 and R2 but no timing capacitor as it works with stray capacitance. Its pins 6 and 2 are directly added to the protective shield built on a 10cm x 10cm copper clad board.

The stray capacitance inherent in the circuit is sufficient to provide an output frequency of approximately 25 kHz with R1 and R2. This arrangement provides better sensitivity and allows the circuit to be used manually with capacitive effects. The output pulse immediately assigned to the oscillator triggers pin 2 of the monostable multivibrator. The monostable uses low value capacitor C6, resistor R3 and preset VR1 timing.

Using preset/trim VR1, which is slightly higher than the multivibrator, the output frequency of the monostable multivibrator is changed. This puts the circuit into standby mode as long as no hand capacitor is present. Therefore, in standby mode, the astable output will be low. This tends to cause the trigger monostable to go low and the output to go high.

A warning indicator buzzer and LED1 were added, they come on active only when the output of the monostable multivibrator sinks current. In standby mode, LED1 remains "off" and the buzzer is silent. As someone attempts to defend the phone from the shield, his hand approaches the shield and comes into contact with the shield, introducing the hand capacitor in the circuit. Because of this, the frequency of the astable changes, which causes the monoflop pin to go low and its output to oscillate. This will produce a chirping sound from the buzzer and also cause LED1 to flash.

It can even be used as a cell phone charger circuit. It provides 180 mA 6V output through voltage regulator IC 7806 ( IC 4) and resistor R5 to charge the phone. Diode D3 defends output polarity reversal.

Generally, the circuit on the PCB can be wired. Enclosed in an appropriate case, it supplies the charger's output leads. Utilize 10cm × 10cm copper clad foil board or aluminum plate to produce protective umbrella. Hang it up on 15cm of plastic wire to work the circuit. All capacitor information should be short term.

Fine-tune VR1 with a small plastic screwdriver until eventually the buzzer stops sounding by a small amount. Move your hand near the shield and nudge VR1 to the right until the buzzer sounds. Through trial and error, I set it to the highest level of sensitivity, as soon as a hand approached the shield, the buzzer started chirpring and the LED flashed. As an alternative to applying shielding copper cladding, metal phone holders can utilize it as a shield.