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 schema presented here can be used. The schema 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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Circuit diagram :

Telephone line Based Audio Muting and Light-On Circuit Diagram 
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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 antiparallel 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’ con-tact of relay RL1 in series with a bulb to get the visual indication. 

Author : Dhurjati Sinha - Copyright : EFYmag

Audio Controlled Mains Switch

Audio Controlled Mains Switch Circuit Diagram. It is often useful for audio or video equipment to be switched off automatically after there has been no input signal for a while. The function of the on-off switch in such equipment is then taken over by switch S2 in the accompanying diagram. It remains, however, possible to  switch off manually by means of Si. Automatic  switch-off occurs after there has been no input  signal for about 2 minutes: this delay makes it possible for a new record or cassette to be placed in the  relevant machine.

 

The audio input to the proposed schema may be  taken from the output of the relevant TV set, amplifier, or whatever. The input earth is held at + 6 V  with respect to the schema earth by potential divider  Ri-R2-R3-R4. The two 741s function as comparators: the output of ICi goes high when the in- put signal is greater than + 50 mV, whereas the out- put of IC2 goes high when the input signal  becomes more negative than -50 mV. Resistors  R6, R7, and R8 form an OR gate that drives transistor Ti. If the output of either ICi or IC2 is logic  1, Ti conducts.

 

Circuit diagram :

 

Audio Controlled Mains Switch Circuit Diagram

The 555  operates as a retrigger able monostable,  whose period is determined by Rio and Ci. The  device is triggered when its pin 2 is earthed by the  closing of S2. Its output, pin 3, then remains high  for 1 to 2 minutes, depending on the leakage cur- rent of the 555. The monostable resets itself as soon  as the potential across Ci exceeds a certain value.  As long as there is an input signal to the schema, Ti conducts and Ci remains uncharged. As soon as  the audio signal ceases, Ti switches off, and Ci  charges until the potential across it is sufficient to  reset the 555. The monostable may also be reset by  closing Si, which connects pin 6 of the 555 to + 12 V.

 

When IC3 is reset, Ci is discharged via its pin 7. Resistor Rrn serves as protection, because without it Ti could short-schema the supply lines. When the output of IC3 goes high, T2 conducts,  the relay is energized, and the relay contacts switch on the mains voltage as appropriate. To counter the induced potential when the relay contacts close, which could damage T2, diode Di has been connected in parallel with the relay coil.