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