HOW THE CIRCUIT WORKS
The telephone line analyzer has four identical
indicators of polarity input voltage. Every indicator
includes two resistors and bi-color light emitting
diodes (LED), see Figure 21.
The operation of the bi-color LED is very simple. This
LED has two standard LEDs inside with red and
green colors. When voltage on the anode (A) of the
first single color LED is higher than the voltage on the
cathode (C), current flows through this LED and it will
emit light. At the same time, other single LEDs will
not light. After changing the polarity of the voltage,
the second LED will emit light.
The resistor Rb limits the current so that the LED will
not be damaged. The normal open telephone line
voltage between the tip and ring wires is 48VDC.
The resistors Ra and Rb divide an input voltage to a
lower voltage that the telephone line has when
connecting the telephone to the loop.
-12-
Elenco Electronics has developed a telephone line simulator, Model TT-250, for the TWT-1 Telephone Line
Analyzer. It will simulate the most common field situations. It provides an RJ-11 jack for one or two line testing,
straight and reversed wiring and simulates opens.
It also provides an RJ-45 jack for one, two, three, or four line testing, straight and reversed wiring and simulates opens.
Contact Elenco Electronics (800) 533-2441 for availability and price of the telephone line simulator Model TT-250.
A great teaching tool for the telecommunication field.
If the simulator Model TT-250 is not available for testing and troubleshooting the TWT-1, you can use the method
on page 8, or a wall plate that has a phone connected to it. Remove the telephone line plug from the wall plate.
Establish that it is an RJ-11 or RJ-45 type jack, and insert the correct plug into the wall plate. If you know that it
is a single line phone, number 1 on the line analyzer should be lit green. If it is lit red, the wires are reversed. The
phone could still work because the phone has a circuit built into it to reverse it to the proper polarity.
If you know where there are wall plates with more than one line coming in, then test that wall plate so that you
get familiar with normal readings.
DO NOT LEAVE THE TELEPHONE LINE ANALYZER IN THE JACK FOR ANY LENGTH OF TIME, AS THAT
LINE WILL GIVE A BUSY SIGNAL AND COULD OVERHEAT THE UNIT.
LEARNING TO USE THE TWT-1 TELEPHONE LINE SIMULATOR MODEL TT-250
SCHEMATIC DIAGRAM
Ra
Rb
A
C Green
C
A Red
Figure 21