Detecting a telephone RING 2 Electronic Circuit

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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
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