same way. To prevent destroying
either or both phones, use an optical
isolator to prevent the current in one
phone circuit from affecting the
other phone circuit (it also prevents
ground loops which can cause
interference and will reduce noise
transferred between the two
systems). Figure 7 shows an optical
isolator circuit for connecting the
VoIP phone line to the four-line
phone's unused line.
The four input diodes insure
correct operation of the circuit
regardless of the phone line polarity (my residential line
always shows as reversed on my store bought phone line
tester). The opto-isolator prevents current from flowing
between the VoIP phone system and the four-line system;
this prevents damage to components in these circuits. If
you do not want the unused four-line indicator to flash,
eliminate the flash oscillator and tie both AND gate inputs
to the opto-isolator output.
If your four-line indicator will operate from the five
volt/11 mA TTL output from the AND gate, you can leave
out the op-amp (you could do this
with your VoIP output but there
would be NO isolation and no
circuit protection). The op-amp
section will require some calculation
based on the output you need from
the isolation circuit to drive the four-line indicator. A value of 1 KΩ for
Rin is a good starting point; calculate
Rf based on the needed output
voltage from Vout/Vin = (1+Rf/Rin).
Build the circuit first on a
breadboard in case you need to
modify (add and/or delete modules)
anything before you are ready to commit to a printed
circuit board (PCB).
While writing this column, I experienced problems
with my residential phone in which after a rain storm, the
phone sounded like something was frying, our Internet
signal was lost, and our phone/answering machine
alternated between "Line in Use" (a.k.a., phone off-hook)
and saying, "At the tone, leave a message." Our telco
charges a large fee if their technician comes out and finds
a problem with the internal residential wiring, so I checked
12 January 2016
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