|
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 |
