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The Rotary Dial Telephone
A simplified schematic diagram of the
traditional rotary dial telephone is shown in
Figure 3-2. The major parts of this telephone
are explained below.
electronic type telephone.
your telephone in Section 6, you will notice
several differences between the traditional and
electronic type telephones.
Hook Switch
When the hook switch is open (on hook) no
current flows in the local loop. The 48VDC
from the battery in the Central Office appears
on the tip and ring input to the telephone set.
When the receiver is lifted, the hook switch
closes and a current of about 20 to 120mA
flows in the local loop. The resistance of the
local loop drops the voltage at the telephone to
about 6 volts. The current picks a relay in the
Central Office which tells other equipment
there that service is being requested. When
the Central Office is ready to accept the
number being called, a dial tone is sent to the
calling telephone. The dial tone stops when
the first digit is dialed.
Dialer
There are two types of dialers, pulse and tone.
Pulse Dialer
Pulse dialing is accomplished by the familiar rotary dial shown in Figure 3-2. The dial is rotated to the stop and
then released. A spring in the dialer returns the dial to its null position. As the dial returns, the dial switch (S2)
opens and closes at a fixed rate. This switch is in series with the hook switch. Opening the switch interrupts the
current in the local loop. A series of current pulses is thus sent out on the local loop as shown in Figure 3-3. The
number of pulses sent corresponds to the digit dialed. Dialing "0" sends ten pulses. The dial pulses are sent at
a rate of 10 pulses per second (100 ms. between pulses). Each pulse consists of a mark interval (loop current)
and a space interval (no loop current). In America, the mark interval is 40 ms. and the space interval is 60 ms.
giving a mark/space ratio of 40/60. In Europe, the mark space ratio is usually 33/67.
Your kit is a newer
When you build
Figure 3-3
-6-
Figure 3-1
Figure 3-2
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