Schematics > Telephone related > Telephone line based audio muting and light on circuit
Telephone line based audio muting and light on circuit
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
Title: Telephone line based audio muting and light on circuit
Source: www.electronic-circuits-diagrams.com
Published on: 2005-02-09
Reads: 163
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