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Telephone
Line Based Audio Muting and Light-On Circuit
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Very
often when enjoying music or
watching TV at high audio level,
we may not be able to hear a telephone ring and
thus miss an important incoming phone call. To overcome
this situation, the circuit presented here can be used.
The circuit would automatically light a bulb on arrival
of a telephone ring and simultaneously mute the music
system/TV audio for the duration the telephone handset
is off-hook. Lighting of the bulb would not only indicate
an incoming call but also help in locating the telephone
during darkness.
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On
arrival of a ring, or when the handset is off-hook,
the inbuilt transistor of IC1 (opto-coupler) conducts
and capacitor C1 gets charged and, in turn, transistor
T1 gets forward biased. As a result, transistor T1 conducts,
causing energisation of relays RL1, RL2, and RL3. Diode
D1 connected in anti-parallel to inbuilt diode of IC1,
in shunt with resistor R1, provides an easy path for
AC current and helps in limiting the voltage across
inbuilt diode to a safe value during the ringing. (The
RMS value of ring voltage lies between 70 and 90 volts
RMS.) Capacitor C1 maintains necessary voltage for continuously
forward biasing
transistor T1 so that the relays are not energised
during the negative half cycles and off-period of ring
signal. Once the handset is picked up, the relays will
still remain energised because of low-impedance DC path
available (via cradle switch and handset) for the in-built
diode of IC1. After completion of call when handset
is placed back on its cradle, the low-impedance path
through handset is no more available and thus relays
RL1 through RL3 are deactivated.
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shown in the figure, the energised relay RL1 switches
on the light, while energisation of relay RL2 causes the
path of TV speaker lead to be opened. (For dual-speaker
TV, replace relay RL2 with a DPDT relay of 6V, 200 ohm.)
Similarly, energisation of DPDT relay RL3 opens the leads
going to the speakers and thus mutes both audio speakers.
Use ‘NC’ contacts of relay RL3 in series with speakers
of music system and ‘NC’ contacts of RL2 in series with
TV speaker. Use ‘NO’ contact of relay RL1 in series with
a bulb to get the visual indication. |
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