Digital/Standard Phone Line Tester

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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.

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.

Sources

  • Dan Ts’o

Category:Telephone

 

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