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Telephone Line
Vigilant |
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Here
is a telephone line vigilant circuit to guard against mis- use of
your telephone lines. It monitors telephone lines round the clock
and provides visual as well as an audio warning (when someone is
using your telephone lines) which can be heard anywhere in the
house. Another advantage of using this circuit is that one comes
to know of the misuse and snapping of the lines (due to any
reason) instantaneously on its occurance. This enables the
subscriber to take necessary remedial measures in proper time.
Various telephone line conditions and audio-visual indications
available are summarised in Table I.
Even when the subscriber himself is using his telephone (handset
off-cradle) while the vigilant circuit is on, the buzzer beeps
once every 5 seconds since the vigilant circuit cannot distinguish
between self-use of the subscriber lines or by any unauthorised
person. Thus to avoid unnecessary disturbance, it is advisable to
install the vigilant unit away from the phone. However, if one
wishes to fit the unit near the telephone then switch S1 may be
flipped to ‘off’ position to switch off the buzzer. But remember
to flip the switch to ‘on’ position while replacing the handset on
cradle.
Irrespective of telephone line polarity at the input to the
circuit, proper DC polarity is maintained across C1 due to bridge
rectifier comprising diodes D1 to D4. The DC voltage developed
across capacitor C1 is used to check telephone line condition as
per Table I. This circuit draws negligible current from telephone
line; thus when it is connected to the telephone line, the normal
telephone operation is not affected. The circuit may be divided
into two parts. The first part comprises zener D9, transistors T1
to T4 and diode D5. It is used to verify whether telephone line
loop is intact or discontinuous. The second part comprising zener
D10 and transistors T5 to T10 is used to check whether telephone
line is in use (or misuse) or not. The zener diode D9 (3.3V)
conducts when phone line loop is intact and not broken. Zener D9
sets control voltage for transistors T1, T2 and T3 to conduct and
for T4 to cut off. As a result, green LED lights but no sound is
heard from the buzzer.
When phone line loop is discontinuous, no voltage is available
across capacitor C1. Thus zener D9 and transistors T1, T2 and T3
do not conduct while T4 conducts. Now green LED extinguishes and a
continuous sound is heard from the buzzer. When telephone line is
alright but is not in use, zener D10 conducts as voltage across
capacitor C1 is quite high. This results in conduction of
transistors T5 and T6 and cutting off of transistor T7 (as
collector of transistor T6 is near ground potential). Thus
positive 9V rail is not extended to the following multivibrator
circuit built around transistors T8 and T9. Consequently, the red
LED is not lit and buzzer does not sound. When phone line is in
use, zener D10 does not conduct. As a result, transistors T5 and
T6 also do not conduct, while transistor T7 conducts. Now +9V is
extended to multivibrator circuit. This multivibrator is designed
such that collector of transistor T9 goes high once every 5
seconds to forward bias transistor T10 and it conducts. Thus at
every 5-second interval a beep sound is heard from buzzer. The
beep sound interval can be increased or decreased by changing the
value of capacitor C3 while the volume can be adjusted with the
help of preset VR3
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Click
Here For Circuit Diagram
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