🚀 go-pugleaf

Here are some of my design notes on a super-cheap circuit that I built
about 7 years ago.

The circuit lights an LED when the telephone is off-hook.  It runs off a
9V battery, to avoid having to figure out telephone line potentials.

It shouldn't be hard to improve on this circuit!

The notes at the bottom explain some of the Bad Things (tm) about the
design, such as overstress of the CMOS IC.  In spite of the poor design,
the circuit worked for several years of daily use, and still works today.

I'd be curious to see some improved (yet simple) designs.

Roger Petersen
roger@sr.hp.com


################################################################################
#
# File:         phone_off_hook
# RCS:          $Header: $
# Description:  Design of Telephone Off-Hook Indicator
# Author:       Roger Petersen
# Created:      June 1985 or so
#
################################################################################



			  Phone Off-Hook Indicator
			  ------------------------

Overview - What is it?
----------------------
Runs off 9V battery.
Plugs into phone jack.
Lights an LED when any phone on the line is off-hook.


Phone Information
-----------------
Measuring the voltage across the telephone line shows (typical numbers):

	On Hook:	40 to 50 VDC
	Off Hook:	4 to 6 VDC
	Ringing:	100 VAC

The "standard" impedence of a telephone, when off-hook, is 680 ohms.
Hanging a 680 ohm resistor across the telephone line will drop the
voltage from 48V to about 5V, causing the line to go "active".
This is how HOLD switches work.
This probably means that it is bad to load down the phone line when
the phone is off hook.  I wouldn't want to hang less than a 100Kohm
load across it.  Should probably measure this, and see how it affects
the on-hook voltage.

I haven't seen any official documentation on these numbers.
They're empirically derived.

The next question is:  What are these voltages referenced to?  If anything?
It's possible that the most positive phone wire is tied to the
GND in your house, or else maybe the neutral wire in your 120VAC outlet.
So measuring the phone line voltages with respect to your household GND
should show 0V and -48V when the phone is on-hook.
But I don't know.  It's probably best to not rely on this behavior.



Circuit Design - Off-hook Indicator
-----------------------------------
Could probably use some sort of transistor design,
but I'm a digital weenie.

I used a CMOS 4049 Hex Interter.  This part (supposedly) has
high drive output.  And since it's CMOS, it can operate with
Vcc from +3 to +15V.  And it has a high input impedence.


                                                            +9V
                                                             |
Phone+   -----+                                              |+
              |                                             LED
              R 2.7Mohm                                      |-
              |                                              |
              |                                              R 680 ohms
              |         5 |\ 4          3 |\ 2               |
              +-----------| >o------+-----| >o---------------+
              |           |/ 4049   |     |/ 4049            |
              |                     |                        |
              |                     |                        |
              R 0.56Mohm            |  14 |\ 15              |
              |                     +-----| >o---------------+
Phone-  ------+                           |/ 4049
	      |
	      |
	     GND


R = resistor.  Those other things are inverters.
Connect 9V battery across +9V and GND, above.
Tie all unused inputs (pins 7,9,11) of the 4049 to GND!  Don't let 'em float.
Tie Vcc (Pin 1) of 4049 to +9V.
Tie GND (Pin 8) of 4049 to GND.

Voltage going into pin 4 of 4049 is:
	Phone voltage	Voltage at pin 4
	6V		1V
	48V		8V
	100V		16V


Fancy Features
--------------
Not all phone jacks are wired the same way.  Some have the two wires reversed.
In the old days, before touch-tone, it didn't matter.
In the early days of touch-tone, some phones didn't dial when the polarity
was backwards.  Now days, most phones don't care any more.

But the circuit above does.  It requires the phone wires to be connected
as shown.  If you connect them backwards, it won't work.  The light will
just stay lit.  And the 4049 may eventually be damaged.  (4049's seem
pretty resilliant).  So it would be nice to have an easy way to switch
the phone wires


Design Analysis
---------------
The 4049 probably takes a lot of abuse in this design.

When the phone rings, the 4049 probably sees bursts of 16V.

When the battery goes low, the voltage on pin 5 of the 4049 may
exceed Vcc on the 4049, which is probably bad.

It shouldn't be hard to improve on this circuit.