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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
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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