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Watch-Dog
for Telephones
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of the telephone security devices available in market
are simple but quite expensive. These devices provide
blinking or beeping type line-tap/misuse indications.
Quite often they do not offer guaranteed protection against
unauthor-ised operation. A very simple and uni- que circuit
of a telephone watch-dog to safeguard subscriber telephone
lines against any fraud is described here. This little
circuit keeps continuous watch over the telephone lines
and sounds an alarm in case of any misuse. In addition
it transmits a loud tone through the telephone lines to
prevent further misuse. When switch S1 is turned on, the
normal (on-hook) telephone line voltage at the output
of bridge-rectifier diodes D1 to D4 is approximately 48
volts, which being well above the break-down voltage of
zener diode D5, the diode conducts. As a result transistor
T2 gets forward biased. This effectively grounds the base
of transistor T1 which is thus cut off and the remaining
circuit does not get any power supply. In this state,
only a small (negligible) current is taken by the circuit,
which will not affect the telephone line condition. However,
when handset of any telephone connected to the telephone
lines is lifted (off-hook), line voltage suddenly drops
to about 10 volts. As a result, transistor T2 is switched
off and transistor T1 gets forward biased via resistor
R1. Now, the astablemultivibrator built around timer IC1
starts oscillating and the speaker starts sounding. Output
of the astable multivibrator is also connected to the
base of transistor T1 through capacitor C5. As a result,
only a loud (and irritating) tone is heard in the ear-piece
of the unauthorised telephone instrument. This circuit
can be constructed on a veroboard using easily available
low-cost components and it can be connected to any telephone
line without the fear of malfunctioning. No extra power
supply is required as it draws power from the telephone
line for operation. Note: Please disconnect the gadget
when you are yourself using the telephone as it cannot
distinguish between authorised and unautho- rised operation
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