This is the cell phone shield circuit which can be used as mobile charger. Give protection to your cell phone from unexpected use or theft working with this easy circuit. It is able to produce a loud chirping sound when someone tries to take away the mobile handset. The added function is that the circuit also operates as being a mobile charger.
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The circuit is powered by a step-down transformer X1 with rectifier diodes D1 and D2 and filter capacitor C1. Regulator IC 7812 (IC1) together with noise filter capacitors C2 and C3 gives regulated power source. The cell phone shield circuit uses two NE555 timer ICs: One as being a very simple astable multivibrator (IC2) and then the 2nd as being a monostable multivibrator (IC3). The astable multivibrator has timing resistors R1 and R2 but no timing capacitor since it operates with stray capacitance. Its pins 6 and 2 are directly joined to a safeguarding shield built up of 10cm×10cm copper-clad board.
The inherent stray capacitance of the circuit is enough to supplied an output frequency of about 25 kHz with R1 and R2. This arrangement gives better sensitivity and allows the circuit with hand capacitance effect. Output pulses from the oscillator are immediately assigned to trigger pin 2 of the monostable multivibrator. The monostable utilizes a low-value capacitor C6, resistors R3 and preset VR1 for timing.
The output frequency of the monostable multivibrator is altered utilizing preset/trimmer VR1 such that it is slightly less than that of the astable multivibrator. This makes the circuit standby, as soon as there is no hand capacitance present. So in the standby mode, the astable’s output is going to be low. This tends to
make the trigger input of monostable become low and output become high.
The warning indicator buzzer and LED1 are joined such that they come to be active only when the output of the monostable multivibrator sinks current. During the standby state, the LED1 continues to be “off” and also the buzzer is silent. As someone attempts to take the cell phone from the defending shield, his hand comes close to the shield or makes contact with the shield, which introduces hand capacitance within the circuit. Because of this, the astable’s frequency changes, which makes the trigger pin of the monostable become low and its output oscillates. This generates chirping sound from the buzzer and also makes the LED1 blink.
The circuit can even be utilized as being a mobile charger. It delivers output of 6V at 180 mA through regulator IC 7806 (IC4) and resistor R5 for charging the cell phone. Diode D3 defends the output from polarity reversal.
The circuit could be wired on a general PCB. Enclose it inside a appropriate case with provision for charger output leads. Produce the protective shield making use of 10cm×10cm copper-clad board or aluminium sheet. Hook it up towards the circuit working with a 15cm plastic wire. Leads of all capacitors ought to be short.
Fine-tune VR1 little by little working with a plastic screwdriver until eventually the buzzer stops sounding. Get the hand nearby to the shield and fine-tune VR1 right up until the buzzer sounds. With trial-and-error method, set it up for the highest level of sensitivity such that as shortly the hand comes close to the shield, the buzzer begins chirpring and also the LED blinks. As an alternative to applying the copper cladding for shield, a metallic cell phone holder can be utilized as being the shield.
