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Digital/Standard Phone Line Tester

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  • Nicolae Sfetcu
    Digital/Standard Phone Line Tester (From Dan Ts o) Radio Shack sells a similar device without the high current function. It detects one or two lines on an
    Message 1 of 1 , May 30, 2002
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      Digital/Standard Phone Line Tester

      (From Dan Ts'o)

      Radio Shack sells a similar device without the high current function. It detects one or two lines on an RJ-11 and tells you its polarity. It costs $6. The schematic is:

          o-------+----------+
                  |          |
                  \          |
          Line 1  /680      \ / red/green LED
                  \ .5W     ---
                  |          |
          o-------+----------+
      

      The circuit for Line 2 is identical. Note that each red/green LED comes standard as reverse wired (red LED "forward", green LED "reversed).

      Based on the above, I think an appropriate modification to include a high current indicator would be: (I've tested it)

                20
         o--+-\/\/\/--+-----------+----------+
            |         |           |          |
            |         |           \         ---
            +--->|----+           /         / \  red/green LED
             red/green            \ 680      |
               LED                | .5W      |
         o------------------------+----------+
      

      You can adjust the 20ohm resistor value to set what is considered "high current". 20ohms lights the LED at around 90ma. Total parts costs under $4, or if you use Radio Shack's $6 line tester and add the above resistor and LED, then under $7. (I'm not faulting IBM for charging $30).

      This whole issue really bothers me because it means that I can't bring a PCMCIA modem with me on trips and count on it working at any given hotel. That means I should carry around my pocket modem just in case. So then what's the point of having the PCMCIA modem!

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