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- From: roger@hpnmdla.sr.hp.com (Roger Petersen)
- Newsgroups: sci.electronics
- Subject: Re: Phone line monitor
- Message-ID: <7380090@hpnmdla.sr.hp.com>
- Date: 5 Nov 92 04:38:33 GMT
- Article-I.D.: hpnmdla.7380090
- References: <16894F0EE.X1FT@VM1.CC.UAKRON.EDU>
- Organization: MID, Home of the Happy Dinosaurs
- Lines: 137
-
-
- 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.
-