Phone-In-Use indicator

(Circuits -> Telephone)
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From: Richard Steven Walz

    ------+-------------+-------------+----------- pos (tip)
          |             |             |
          |             |            ---
          |             |         R3 | |
          |             |            | |
          |             |            ---
 zener    |             |             |
      /------/         ---          -----
        /  \        R2 | |           \ / LED
       ------          | |          -----
          |            ---            |
          |             |             |
          |     --------+----+     ---+
          |   |/             |   |/
          +---| npn1         +---| npn2
          |   |\                 |\
         ---    -+                 -+
      R1 | |     |                  |
         | |     |                  |
         ---     |                  |
          |      |                  |
    ------+------+------------------+-------------- neg (ring)

Now here's some logic that should work fine with the right zener and the right resistors and a couple of cheap npn's 2n2222A's or 2n3904's (06's?). If you get close to 25 volts with the new smart test boxes, a 20 volt Z may work fine. Choose R1 to limit current through Z and have enough left to turn on npn1 just enough to deprive npn2, choose R2 for that, and you will need to add a resistor R3 to protect the LED from overcurrent as needed, depending on the phone system you have!

You MIGHT need a resistor between the bottom of R2 and the base of npn2 to get it right, but I don't think so because of the B-E 0.7 volt diode junction voltage needed in npn2 to get it out of cutoff.


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