Digital/Standard Phone Line Tester

(Circuits -> Telephone)
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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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