My computer is in the basement and this device tells me if the phone line is in use. I have inserted a N/O switch in the battery connection so that the batteries will last longer as sometime my sons spend a lot of time on the phone. Prior to using my modem I press the switch to find out if the line is busy.
2N3906 33K 2N3904
2.2 meg /----------/\/\/\---+ /------+
|/ | |/ | ***
Tip o--/\/\/-----+-------| PNP +----| NPN \ 220
\ |\ | \ /
/ 330K | | \
\ | +-------------+ |
Ring | | | | ---
o--/\/\/-----+ | O - | / \ Led
2.2 meg | | 3V | |
| | O + | |
| | | | |
+-------------------------------------+ |
| | |
+-------------+-------------------+
Sources
- Aurel Boisvert