TT1
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5
That’s why even when your local power goes out, your telephone still
works. Surprising the things you can learn from these kit manuals.
Well, back to the circuit. To preface the next section it should be understood
that a PNP transistor has an arrow on the emitter whch is pointing in, where
as a NPN transistor has an arrow on it’s emitter which is pointing out. Just as
the arrows are reversed, the operation of these two different types of
transistors is reversed. An NPN transistor is considered turned “on“ when the
voltage at it’s base is more than .7 volts greater than the emitter voltage. PNP
transistors are turned on when the emitter voltage is more than .7 volts greater
than the base voltage.
The voltage at J1 is applied, through a 10:1 voltage divider (R3 and R4 or R5
and R4 depending on how the jack is hooked up) to the base of Q4 turning Q4
on. The diodes (D2,3,4 and 5) are arranged so that even if the jack is hooked
up backward the unit will still work properly. When Q4 is turned on, the
collector (leg without the arrow) is pulled down. This voltage is also at the base
of Q2. This voltage is too low to turn Q2 on. So with Q2 turned off, it’s collector
is allowed to rise to the voltage coming in from the tape recorder remote jack at
P1 and through power switch S1. Typically this voltage is in the range of 3 to
18 volts. This voltage is also applied to the base of Q1 turning it off. With Q1
turned off, no voltage passes through it to the base of Q3 leaving Q3 turned off
(NPN remember?). To activate the tape recorder the remote line must be
pulled low, this is the purpose of Q3. With Q3 turned off the line to the tape
recorder remote jack is not affected and the tape recorder stays in stand-by
mode. Therefore the second part of the circuit (the audio section) to P2 does
nothing at this time. D1 is used to protect the TT1 circuitry from reverse voltage
or possible voltage spikes from a tape recorder motor. We will go through the
audio section when we discuss the next mode of operation (the “off-hook” or
“active” mode).
ACTIVE MODE
When the phone receiver is “off-hook” (picked-up), either by an answering
machine or by someone answering or making a call, this completes the
telephone circuit. When the circuit is completed the voltage now at J1 is in the
range of 3 to 8 volts. This reduction in line voltage is due to something called
“line loss”. This is when a circuit is completed and electrical current flows
through the wires in the circuit, the wire tends to act like a resistor. The longer
or smaller diameter the wire, the more the resistance. So now, with this smaller
voltage at J1, let’s run through the TT1 circuit. With 3 to 8 volts at J1,
then going through the 10:1 voltage divider (R3+R4 or R5+R4) by the time this
voltage gets to the base of Q4 the voltage is not enough to turn Q4 on. With
Q4 turned off the collector of Q4 and also the base of Q2 rise to the voltage
(through R1) to the voltage supplied by the tape recorder remote jack. This
Summary of Contents for TATTLE-TALE TT1
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