[sci.electronics] Here is a Phone In-Use Light

mark@mips.COM (Mark G. Johnson) (06/28/90)

This device lights an LED when any of the phones on the circuit is in use.
For the application "is one of my *extension* phones being used", a series
connection is not applicable.

The device below is installed in parallel (shunt) with the line, so
precautions have been taken to keep current draw below 500 microamps in the
phone-not-in-use ("on hook") condition.

    1. A low power IC (National LP-339, available mailorder for $0.94) that
       consumes <100 uA is employed.

    2. The indicator LED is _pulsed_.  Otherwise, the following scenario
       might occur: (1) User picks up phone; (2) LED comes on, thereby
       drawing LED current; (3) user hangs up phone; (4) LED current is
       sufficiently large to make central office think phone is still
       off-hook .... phone is never "hung up".


All parts are readily available to the hobbyist (e.g. from Digi Key).

ASCII graphics being what they are, the schematic is presented in 3 parts;
the "rails" part, the "comparator" part, and the "indicator" part.

                          "RAILS" CIRCUIT



  PHONE LINE >>-----+-------+-------------------------------->  VBIG
   WIRE (+)         |       |
                  +---+   +---+
                  |R1 |   | R2|
                  +---+   +---+
                    |       |
                    |       +----------------+--------------->  VPLUS30
                    |      C|                | (+)
                    |     +---+            +---+
                    |     |DZ3|            | C1|
                    |     +---+            +---+
                    |      A|            (-) |
                    +-------+--+----------------------------->  VPLUS1
                               |A            |cross
                             +---+           |over
                             | D2|           |
                             +---+           |
                               |C            |
               Cathode         |             |
  PHONE LINE       +----+      |             |
   WIRE (-)  >>----+ D1 +------+-------------+--------------->  VLOW
                   +----+ Anode



         D1, D2 == Silicon Diode, 100V (such as 1N4002)
         DZ3    == 30 Volt Zener (such as 1N4751)
         C1     == 47 microfarad, 50 Volt electrolytic capacitor
         R1     == 1Meg resistor
         R2     == 200K resistor

                          "COMPARATOR" CIRCUIT


  VBIG >>--------+-------
                C|
               +---+
               |DZ4|
               +---+
                 |A        VPLUS30 >>-------+
                 |                          |
               +---+                        |
               | R3|                  _     |
               +---+                 |  \   |
                 |                   |    \ |
                 +-------------------| -    \
                 |                   |        \
               +---+                 |    IC1   >-----------> IN_USE_BAR
               | R4|                 |        /
               +---+       VPLUS1>>--| +    /
                 |                   |    / |
                 |                   |_ /   |
                 |                          |
  VLOW >>--------+--------------------------+---------



         DZ4    == 10 Volt Zener (such as 1N4740)
         R3,R4  == 500K resistor
         IC1    == National Semi. LP-339 micropower quad comparator   Digi-Key
                       (all pins of other 3 sections connected to VLOW)

                          "INDICATOR" CIRCUIT


                +----+
  VBIG >>-------+ R5 +-----+---------+---------------------+-----
                +----+     |         |                     |
                           |         |                     | Anode
                           |       +---+                +-----+
                           |       | R6|                |     |
                           |       +---+                | LED |
                           |         |                  |     |
                           |         +------+           +-----+
                           |         |      |              | Cathode
                           |     +---|------|--------+     |
                           |     |  (1)    (8)       |     |
                           +------               (6) ------+
                                 |(2)     IC2        |
                                 |                   |
                                 |               (5) ------+
                                 |         (4)       |     |
                                 +------------|------+     | (+)
                                              |          +---+
                                              |          | C2|
                                              |          +---+
                                              |            | (-)
  IN_USE_BAR >>-------------------------------+------------+



         LED    == Light Emitting Diode; plain variety without resistor
         C2     == 400 microFarad, 6V electrolytic capacitor
         R5     == 1.2K resistor
         R6     == 1.5K resistor
         IC2    == National Semi. LM-3909 micropower flasher/oscillator IC


Theory of operation:

    The RAILS circuit creates filtered voltages equal to one diode drop
    (VPLUS1) and also 30 volts (VPLUS30).  Diode D1 in the VLOW leg
    protects against accidentally hooking up the input wires backwards.
    Resistors R1 and R2 are big enough to be sure the DC current is small
    when the phone is on-hook (phone not in use).

    COMPARATOR uses the amazing 94-cent chip from Digi Key.  Powered off
    30 volts, it compares (LineVoltage - 12V) versus 1V.  So, if the
    LineVoltage is above 11 volts, the comparator output "IN_USE_BAR"
    is high.  Conversely, if the Line Voltage is below 11V, IN_USE_BAR
    goes low and activates the LED indicator.

    INDICATOR is just the National LED-flasher chip, with its ground leg
    enabled by the comparator's open-collector output.  When IN_USE_BAR
    is low, current flows and the INDICATOR flashes the LED.

    When the phone is in use ("off hook"), the LED is only lit for brief
    periods [duty cycle is about 1%].  The average current draw of the
    INDICATOR is about 1ma.  So, when the user finally hangs up the phone,
    the indicator's current is small enough for the central office to
    recognize an on-hook condition, the line voltage rises again, the
    comparator senses this, and shuts off the indicator.
-- 
 -- Mark Johnson	
 	MIPS Computer Systems, 930 E. Arques M/S 2-02, Sunnyvale, CA 94086
	(408) 524-8308    mark@mips.com  {or ...!decwrl!mips!mark}