Digital/Standard Phone Line Tester Electronic Circuit

From CircuitsArchive

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