Phone in use indicator 3

It appears that you are using AdBlocking software. The cost of running this website is covered by advertisements. If you like it please feel free to a small amount of money to secure the future of this website.
       ------+-------------+-------------+----------- 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

 

© 1998-2017 – Nicola Asuni - Tecnick.com - All rights reserved.
about - disclaimer - privacy