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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.
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.
As 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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Click
Here For Circuit Diagram
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