04-12-2012, 06:11 PM
MOBILE SHIELD
[attachment=42532]
Working
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 multi vibrator (IC2) and the second as a monostable (IC3). The
astable multi vibrator 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.Make the protective shield
using 10cm×10cm copper-clad board or aluminium sheet. Connect it to the circuit
using a 15cm plastic wire. Leads of all capacitors should be short. Adjust VR1 slowly
using a plastic screwdriver until the buzzer stops sounding. Bring the hand close to
the shield and adjust VR1 until the buzzer sounds. With trial-and-error procedure, set
it for the maximum sensitivity such that as soon the hand comes near the shield, the
buzzer starts chirping and the LED blinks. Instead of using the copper cladding for
shield, a metallic mobile phone holder can be used as the shield.
[attachment=42532]
Working
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 multi vibrator (IC2) and the second as a monostable (IC3). The
astable multi vibrator 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.Make the protective shield
using 10cm×10cm copper-clad board or aluminium sheet. Connect it to the circuit
using a 15cm plastic wire. Leads of all capacitors should be short. Adjust VR1 slowly
using a plastic screwdriver until the buzzer stops sounding. Bring the hand close to
the shield and adjust VR1 until the buzzer sounds. With trial-and-error procedure, set
it for the maximum sensitivity such that as soon the hand comes near the shield, the
buzzer starts chirping and the LED blinks. Instead of using the copper cladding for
shield, a metallic mobile phone holder can be used as the shield.