TRAFFIC CONTROL TELEPHONE SYSTEMS Circuit - Sam Hallas

Page 8
Common Parts
One 4304-A Selector.
(For the 4001-A or the 4002-A Selector Sets.)
Three Dry Cells.
One No. 4110-A Protector.
Alternative patterns of telephone sets and the method of ordering any piece of
apparatus will be found in our Catalogue "Railway Systems (Apparatus)" D/TD17DI.
NOTE.-Way stations equipped with the 4305-B instead of the 4304-A Selector (see
under Common Parts above) will require from one to three extension bells and
additional telephone sets (with dry cells and battery boxes) the quantity depending
upon the number of extension sets connected to the Selector. Additional
No. 4110-A Protectors may also be required.
Circuit Description
Figure 1, facing page 8, is a schematic diagram showing the Controller's circuit
together with the two way station circuits. References mentioned in the following
description apply to this figure.
The controller operates the selector key having the same code, which we will
assume is 3-11-3, as that of the selector at the way station he wishes to call.
On the release of the key, contacts K1 and K3 are closed, and remain closed
during the revolution of the code wheel. The closing of these contacts operates the
BR relay via 10 volts positive, B4 terminal, call key contacts (Time Sending Set), K1
to K3 of Selector Key, winding of BR relay to B3 terminal and 10 volts negative. The
operation of BR connects the line battery to line, thus:-line battery positive, B1
terminal, circuit breaker, line battery switch, SR contacts, retard coils, BR contacts,
line switch, line protectors thence to L1 or L3 or both. The circuit to the other line is
made via line battery negative, B2 terminal, line battery switch, SR contacts, retard
coils. BR contacts, line switch, line protectors and L2 or L4 or both.
All the selectors (each with its tuning capacitors in series) at the way stations
are in parallel across the line so that when the line battery is first applied they receive
an impulse which clears them from any code pins on which they may be held, so
preparing them for the reception of the code. When the selector key spring K1 rides
over the first tooth of the code wheel, it makes contact with spring K2 (at this stage
K1, 2, and 3 are all in contact) and operates the SR relay via 10 volts negative, B3
terminal, winding of SR, call key (Time Sending Set), K2 to K1 (Selector Key), Call
Key (Time Sending Set), B4 terminal and 10 volts positive. The operation of SR
reverses the line battery connections to the line, thus reversing the polarity of the line