Phone in use indicator 3

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——+————-+————-+———– 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.

Sources

  • Richard Steven Walz mwandel[at]bnr.ca

Category:Telephone

 

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