Phone in use indicator 4

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The circuit I built gives a visual indication at each extension when any extension is off-hook. It is line-powered, and the maximum number that can be used on our system is three. Since they all draw power at the same time to light the LEDs, any more indicators would cause an off-hook condition. Some changes could be made to reduce the current draw, to allow using more indicators, but the brightness of each led would suffer. The LEDs I used are tiny, but amazingly bright on just a couple milliamps. I picked them up from a surplus catalog, I can’t remember which one. If you were to use battery power for the circuit, you could use almost any number of indicators. I had use only for three, and I did not want to worry about replacing batteries. If I remember correctly, our pbx required a load of about 20 milliamps before the line failed to hang up. This circuit draws about 5 milliamps when off- hook, much less when on-hook. It senses the drop in line voltage from about 46 volts to 6 volts when an extension is picked up. The zener voltage should be well above the off-hook voltage of your system, and well below the on-hook voltage. The transistors are small high-voltage npn types I had on hand. The LED also flashes with the ring voltage. Putting a suitable MOV across the line is a good precaution to prevent lightning damage.

     (+)------------+---------------+--------------------------+ 
      green         |               |                          | 
                    /               /                          / 
                    \  2200         \ 100K              100K   \ 
                    /               /                          / 
     1N4148         \               \                          \ 
          +---------+               |                          | 
       ___|___   ___|___            |                       ___|___/ 
         / \     \     /   LED      |              10V     /  / \ 
       /_____\   __\ /__            |             ZENER     /_____\ 
          |         |               |                          | 
          +---------+               |                          | 
                    | c             |                          | 
                     \  |           |                          | 
              MPSA42   \|-----------+------------- c           | 
                       /|        ___|___           \  |        | 
                     /  |          / \     MPSA42    \|--------+ 
                    | e          /_____\             /|     ___|___ 
                    |      1N4148   |              /  |       / \ 
                    |               |             | e       /_____\ 
                    |               |             |            | 
     (-)------------+---------------+-------------+------------+ 
     red                                                       1N4148 

Sources

  • Kenneth Carver kcarver[at]nmsu.edu

Category:Telephone

 

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