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}