MUSIC ON HOLD
----- -- ----

Have you ever called a Doctor's office and been told to hang on for a minute?
If so, chances are that while you were waiting you suddenly heard some back-
ground music meant to entertain you and help keep your attention from the
dragging time. Now, with this button, you too can place people calling you on
hold and even let them listen to music until you get back to them. You will
be able to answer the phone in one room of your house, put the caller on hold
and then pick up the reciever in another location. When you pick up the
reciever the second time you will automatically deactivate the music-on-hold
feature and can continue your conversation undisturbed. All these advantages
can be had for less then $10.

ABOUT THE CIRCUIT
----- --- -------

The circuit is a relatively simple one and ideal for the not-so-experienced
hobbyist. It has a total of seven electronic components and can be put to-
gether in less than an hour even by a novice.
  The basic operation of a hold button requires that a high resistance,
about 1200 ohms, be placed across the telephone line while it is in use to
prevent the line from being be disconnected when you hang up the reciever.(in
some parts of the country, lower resistance values may be needed )
  Now, If part of this high resistance is formed by the secondary of an in-
versely connected transistor ouput transformer, then an audio signal can be
coupled into the telephone line for the benefit of the party on hold.
  In operation, this seven-component circuit is connected in parallel with the
telephone line. When the reciever is lifted off the hook, the voltage on the
line is about 5 V. Even if the hold button were activated, this voltage would
be to low to keep it so. If the pushbutton were pressed and the reciever hung
hung up, however, the voltage on the line would rise to about 48 V. R1, T1,
and D1 momentarily form a voltage divider with R2, allowing part of the line
voltage to be applied to the gate of the SCR and triggering it into its con-
ducting mode. The triggered SCR acts as a short circuit that connects the
resistor, LED, transfomer, SCR, and diode series circuit across the phone line,
thus raising the resistance across the line to between 1200 and 1500 ohms and
placing the line on hold. In addition, if an audio signal, such as that
obtained from a radio or tape recorder, is fed into the 8-ohm primary of the
transformer, the signal will be coupled into the telephone line and the person
waiting on hold will hear it.
  When the telephone or any extension is subsequently picked up,the line
voltage drops again to about 5 V and the SCR becomes current-starved, thereby
causing the SCR to stop conducting and effectively opening the circuit and
disconnecting the phone line from the hold mode and the audio source.


DIAGRAM
-------
                      R1  1200 ohm
Green o--------------/\/\/\---------+
                                    !
                                    !
                                 D1 V   LED
                                    -
                                    !
                                    !) !! (------o-------
                             1k ohm !) !! ( 8 ohm       !
                              T1    !) !! (             ----v !  Audio in
                                    !) !! (------o-------------
                                    !
                                    !
                                    o-------!
                                    !       o
                               SCR  V       o/  SW  spst
                                    !\______!
                                    !       !
                                    !       !
                                D2  V       >    R2  820 ohm
                                    !       <
                                    !       >
Red  o------------------------------o-------!

NOTE
----
  The LED must be connected so that the anode goes to the green wire, the
cathode to the red. On D2 its cathode must go to the red wire. Just about any
silicon diode can be used for D2, as well as for the SCR, and LED.

PARTS LIST
----- ----
D1   Light Emitting Diode (LED)
D2   1N914  or any silicon diode
R1   1200 ohm resistor ( may be diff, see text )
R2   820 ohm resistor
T1   1000 to 8 ohm transformer
SCR  RS 1020 ( radio Shack )
SW   SPST momentary-contact pushbutton



Brought to you by:
                   the Toymaker

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