[sci.electronics] power from phone line

murray@jumbo.dec.com (Hal Murray) (12/05/89)

Suppose you have some remote instrumentation, like a seismometer, 
or weather station, and you are transmitting that data back to a 
central collection point via a phone line.

Can you scrounge any power from the phone line? I'm not thinking of 
many watts, just enough to run a few slow CMOS chips.

Is there an obvious book I should know about that answers things like 
this?

psfales@cbnewsc.ATT.COM (Peter Fales) (12/06/89)

In article <14230@jumbo.dec.com>, murray@jumbo.dec.com (Hal Murray) writes:
> Can you scrounge any power from the phone line? I'm not thinking of 
> many watts, just enough to run a few slow CMOS chips.
> 
> Is there an obvious book I should know about that answers things like 
> this?

Check "Understanding Telephone Electronics" from your nearest Radio
Shack.  It is an interesting book, very high level in some areas, very
detailed in others, but as I recall it has some good discussions of
your question.

-- 
Peter Fales			AT&T, Room 5B-420
N9IYJ            		2000 N. Naperville Rd.
UUCP:	...att!ihlpb!psfales	Naperville, IL 60566
Domain: psfales@ihlpb.att.com	work:	(708) 979-8031

larry@kitty.UUCP (Larry Lippman) (12/07/89)

In article <14230@jumbo.dec.com>, murray@jumbo.dec.com (Hal Murray) writes:
> Suppose you have some remote instrumentation, like a seismometer, 
> or weather station, and you are transmitting that data back to a 
> central collection point via a phone line.
> 
> Can you scrounge any power from the phone line? I'm not thinking of 
> many watts, just enough to run a few slow CMOS chips.

	The answer is yes.  Since you did not specifify where you are
referring to a dial subscriber line or to a leased line, I will briefly
cover both issues.

	If you are referring to a conventional subscriber telephone line
at the remote site which you dial using the public switched telephone
network, then you can obtain between 1 and 2 mA from the line when it
is on-hook.  Some simple Ohm's law says this is about 100 mW.  Using a
carefully designed DC-DC converter, one can convert this voltage to say,
7.0 volts to trickle-charge a battery.  The battery can then store this
energy to provide a much higher current to power a modem, microprocessor
and related circuitry during the intermittent intervals when the telephone
line is automatically answered and data is exchanged.

	The precedent for the above circuit design using a DC-DC converter
is in telephone subscriber line carrier circuits, such as the Continental
AML.

	If you are referring to a leased line, then you can superimpose
a certain amount of DC power provided that you use dual-winding transformers 
or inductors to provide DC/signal separation.  In this day and age,
telephone companies will generally not provide leased line circuits having
DC continuity beyond the serving central office.  If you are able to
obtain a leased line circuit, you are generally limited to 100 mA of
current and no more than +/- 135 volts from either conductor to ground
(i.e., 270 volts conductor-to-conductor).  This type of circuit will give
you LOTS of power - even with a long loop resistance of say, 3,000 ohms.

> Is there an obvious book I should know about that answers things like 
> this?

	Not that I am aware of.  I'm afraid that experience is the only
teacher here.

<> Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp.
<> UUCP  {allegra|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry
<> TEL 716/688-1231 | 716/773-1700  {hplabs|utzoo|uunet}!/      \uniquex!larry
<> FAX 716/741-9635 | 716/773-2488      "Have you hugged your cat today?" 

sorka@ucscb.UCSC.EDU (Alan Waterman) (12/07/89)

One thing you forgot to mention is that you get a lot more power out
of the line when it is ringing. I got the worst shock of my life when
I held the red and green wires(one in each hand) while the line sent
a ring signal. It almost knocked me out. It mush have been high voltage
and low amps because it went through me real easy.

ems@Apple.COM (Mike Smith) (12/07/89)

In article <6300@lindy.Stanford.EDU> sorka@ucscb.UCSC.EDU (Alan Waterman) writes:
>
>One thing you forgot to mention is that you get a lot more power out
>of the line when it is ringing. I got the worst shock of my life when
>I held the red and green wires(one in each hand) while the line sent
>a ring signal. It almost knocked me out. It mush have been high voltage
>and low amps because it went through me real easy.

Vague memory from too many decades ago... I think the ring signal is 
pulsed DC at about 50 V and the current is 'all the inductor can carry'...

Most lethal is a LOW current modest voltage signal that ocilates at
a particularly sensitive frequency to the heart muscle.  I've forgotten
the frequency and if it was AC or pulsed DC. 
-- 

E. Michael Smith  ems@apple.COM

'Whatever you can do, or dream you can, begin it.  Boldness has
 genius, power and magic in it.'  -  Goethe

I am not responsible nor is anyone else.  Everything is disclaimed.

brian@ucsd.Edu (Brian Kantor) (12/07/89)

Typical phone lines in the USA have around 48 to 52 volts DC on them
when the phone is on-hook (hung up, eh?).  That drops to "talking
battery" of about 4 to 10 volts when the phone is off-hook.  During
ringing, about 90 to 120 volts of 20 Hz alternating current is applied
to the line.

To power something off the phone line, you have to determine whether
the device needs constant power or only needs power when the phone is
off-hook.  You see, if you draw any significant amount of current (a few
milliamps or so), the central office equipment senses that as a phone
being off-hook.

Devices that require constant power generally derive it from the phone
line by using extremely low-power circuitry (almost always CMOS stuff),
and feeding the incoming line through some high-value resistors,
rectifiers, regulators, and filtering components to keep the ringing
voltage and lightning surges from frying the device.

Devices that only need power when the phone is to be off-hook can grab
more power, but they need to do it through a coupling circuit to allow
the audio interface to the phone line to survive.  The same sort of
protective devices and isolators are needed.

You can buy complete modules to interface to the phone line, but I've
not seen any marketed which also provide device power.  Probably you
could twist one into doing that, though.

A few minutes spent with a voltmeter and a telephone line is a rewarding
experience, and educational too.  A scope will let you see some of the
timing and frequencies used as well.  Try not to zap your ass on the
ringing voltage.
	- Brian

richm@amc-gw.amc.com (Rich Moran) (12/08/89)

}
} 	If you are referring to a conventional subscriber telephone line
} at the remote site which you dial using the public switched telephone
} network, then you can obtain between 1 and 2 mA from the line when it
} is on-hook.  Some simple Ohm's law says this is about 100 mW.  Using a
} carefully designed DC-DC converter, one can convert this voltage to say,
} 7.0 volts to trickle-charge a battery.  The battery can then store this
} energy to provide a much higher current to power a modem, microprocessor
} and related circuitry during the intermittent intervals when the telephone
} line is automatically answered and data is exchanged.
} 

This is fine if you aren't worried about FCC regs.  Part 68 specifies a minimum
5M-ohm input impedance for DC on tip-ring at voltages below 100V.  Even with
a 56V battery at the CO, that's less than a milliwatt.

rich
-- 
=============================================
Rich Moran
richm@amc.com
=============================================

johnd@sco.COM (John Dubois) (12/08/89)

In article <3542@kitty.UUCP> larry@kitty.UUCP (Larry Lippman) writes:
>	If you are referring to a conventional subscriber telephone line
>at the remote site which you dial using the public switched telephone
>network, then you can obtain between 1 and 2 mA from the line when it
>is on-hook.
...
><> Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp.
><> UUCP  {allegra|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry
><> TEL 716/688-1231 | 716/773-1700  {hplabs|utzoo|uunet}!/      \uniquex!larry
><> FAX 716/741-9635 | 716/773-2488      "Have you hugged your cat today?" 

     How much current can you be sure you can draw before the phone
company decides the line is bad?  For example, if I was going to design a
commercial product, how much could I use?  Is there a specific minimum for
on-hook line impedance?

	John DuBois
	johnd@sco.com
	spcecdt@ucscb.ucsc.edu

larry@kitty.UUCP (Larry Lippman) (12/08/89)

In article <5651@internal.Apple.COM>, ems@Apple.COM (Mike Smith) writes:
> Vague memory from too many decades ago... I think the ring signal is 
> pulsed DC at about 50 V and the current is 'all the inductor can carry'...

	Ringing voltage is typically 20 Hz AC @ 105 volt RMS upon which is
superimposed -50 volts DC (with respect to ground).  The DC component is
present to facilitate operation og a "ring-trip" relay or solid-state
circuit.

	The telephone ringing supply is traditionally current limited to
about 100 mA by a resistance lamp (i.e., ballast lamp).  Resistance lamps
are still used in much ESS apparatus due to their inherent simplicity and
reliability.  Newer ESS apparatus uses solid-state circuits for ringing
supply current limiting.

	Ringing voltage is not applied to a telephone line through an
inductor, so there is no current limiting by that means.

<> Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp.
<> UUCP  {allegra|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry
<> TEL 716/688-1231 | 716/773-1700  {hplabs|utzoo|uunet}!/      \uniquex!larry
<> FAX 716/741-9635 | 716/773-2488      "Have you hugged your cat today?" 

reynhout@wpi.wpi.edu (D Andrew Reynhout) (12/08/89)

In article <5651@internal.Apple.COM> ems@Apple.COM (Mike Smith) writes:
>In article <6300@lindy.Stanford.EDU> sorka@ucscb.UCSC.EDU (Alan Waterman) writes:
>>
>>One thing you forgot to mention is that you get a lot more power out
>>of the line when it is ringing. I got the worst shock of my life when
>>I held the red and green wires(one in each hand) while the line sent
>>a ring signal. It almost knocked me out. It mush have been high voltage
>>and low amps because it went through me real easy.

>Vague memory from too many decades ago... I think the ring signal is
>pulsed DC at about 50 V and the current is 'all the inductor can carry'...

   Correct me if I'm wrong, but my memory from altogether too recently is that
the ring signal is a 90+VAC signal, at 17hz.  Current, I have no information
on.  The shock was never particularly bothersome to me, and I've been hit more
times than I can recall.  Of course, electric shock affects different people
differently.

   As an aside, a local paper once printed an article originally appearing in
the NYTimes, explaining how to wire your own extension telephones.  They didn't
bother to mention the ringing signal.  I wrote in, noting that the application
of such a jolt to a person wearing a pacemaker, for example, could be more than
irritating...possibly deadly.  I suggested taking one extension off-hook
(48VDC) before attempting anything.  There were a number of other glaring
errors in the article, like the completely incorrect explanation of the 
two-four wire possibilities.  They printed it, and forwarded it to the NYT, 
who is rumoured to have also printed it... my journalistic debut.. :-)

   Andrew
-- 
  Andrew Reynhout   (Internet: reynhout@wpi.wpi.edu)

  "Maybe if we pretend this never happened, they'll all just...go away."
                                           - Laurie Anderson

jeffp@phred.UUCP (Jeff Parke) (12/09/89)

In article <5651@internal.Apple.COM> ems@Apple.COM (Mike Smith) writes:
>...
>Most lethal is a LOW current modest voltage signal that ocilates at
>a particularly sensitive frequency to the heart muscle.  I've forgotten
>the frequency and if it was AC or pulsed DC. 

It turns out that the most lethal frequencies for putting out your heart are
in the 40-100 Hz range.  60 Hz isn't a good one.  Most deadly would be a
small (threshold) current, since larger currents can actually get the heart
going again - all depending on duration of contact and factors about the
subject heart's own susceptibility.

Early defibrillators (devices that deliver large current/voltage to start up a
fibrillating heart) actually used 60 Hz, but were not especially effective.

Modern, effective defibrillators are all DC; they deliver a 5 to 15 ms long
single monophasic or biphasic waveform.
-- 
...dataio!pilchuck!phred!jeffp {Jeff Parke}
Genie  : JEFFP  | DELPHI : JEFFPARKE |  CIS : 71511,1512

larry@kitty.UUCP (Larry Lippman) (12/09/89)

In article <548@scorn.sco.COM>, johnd@sco.COM (John Dubois) writes:
> >	If you are referring to a conventional subscriber telephone line
> >at the remote site which you dial using the public switched telephone
> >network, then you can obtain between 1 and 2 mA from the line when it
> >is on-hook.
> 
>      How much current can you be sure you can draw before the phone
> company decides the line is bad?  For example, if I was going to design a
> commercial product, how much could I use?  Is there a specific minimum for
> on-hook line impedance?

	The maximum on-hook current draw depends upon the type of central
office apparatus and the set point parameters for the Automatic Line
Insulation Test (ALIT) apparatus.  Most ALIT apparatus, however, checks
for leakage from tip & ring to ground, and NOT leakage from tip to ring.
Under most circumstances, 1 mA is a "reasonably safe" amount of leakage
(corresponding to a 50,000 ohm leak) which should not interfere with
dialing, ringing or supervisory signaling.  ALIT apparatus is generally
programmed for a leak threshold ranging between 50,000 and 150,000 ohms,
depending upon the telephone company practice and condition of the cable
plant.  The primary purpose of ALIT is to give early warning of cable
failures, and not subscriber line apparatus troubles.

	If you are going to design a commercial product, then you are
in trouble because Part 68 will not allow ANY usable amount of leakage
from tip to ring.

In article <1052@amc-gw.amc.com> richm@amc-gw.amc.com (Rich Moran) writes:
> This is fine if you aren't worried about FCC regs.  Part 68 specifies a min.
> 5M-ohm input impedance for DC on tip-ring at voltages below 100V.  Even with
> a 56V battery at the CO, that's less than a milliwatt.

	If the remote monitoring line is an off-premises extension (OPX)
of a customer-owned PABX, then it is exempt from any FCC regulations.  We
do not know exactly what the original poster had in mind, so we'll give
him the benefit of the doubt. :-)

<> Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp.
<> UUCP  {allegra|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry
<> TEL 716/688-1231 | 716/773-1700  {hplabs|utzoo|uunet}!/      \uniquex!larry
<> FAX 716/741-9635 | 716/773-2488      "Have you hugged your cat today?" 

roy@phri.nyu.edu (Roy Smith) (12/10/89)

In article <3547@kitty.UUCP> larry@kitty.UUCP (Larry Lippman) writes:
> If you are going to design a commercial product, then you are in trouble
> because Part 68 will not allow ANY usable amount of leakage from tip to ring.

	Is this a new rule?  Universal Data Systems (now part of Motorola,
I think) used to make a nice little 1200 bps modem called the 212LP, where
the LP meant Line Powered.  We had a couple of them.  The power budget must
have been really tight because this was a real bare-bones unit.  No dialer
or auto-answer.  To call out, you had to dial the number yourself on a
phone then flip the talk/data switch on the front of the modem.  But it
definately didn't have any power supply.  I suppose it's possible that it
drew power from the RS-232 connection, but my impression was that it was
driven from the phone line.
--
Roy Smith, Public Health Research Institute
455 First Avenue, New York, NY 10016
roy@alanine.phri.nyu.edu -OR- {att,philabs,cmcl2,rutgers,hombre}!phri!roy
"My karma ran over my dogma"

wtm@neoucom.UUCP (Bill Mayhew) (12/10/89)

The on-hook allowable leakage for a telephone instument is in the
low microamp region for obvious reasons.  I know that in the past,
and maybe the present, that line sharing in rural areas that gives
private service to two parties on the same line used leakage
current.  One party got regular baseband service.  The second party
operated on a carrier system AM modulated somewhere around 50KHz -
100KHz.  The carrier system interface at the subscriber's drop
contained a nicad battery to supply ringing power, as baseband
ringing is impossible (without a gas tube frequency-selective
ringer in this case).  The nicad also supplied talk power for the
local loop.  When the set was on-hook a small leakage current was
used to recharge the nicad pack.

A couple years ago, I read about a neat system developed in Italy.
They used a fiberoptic connection from the CO to the subscriber
residence.  The subscriber's set was recharged from the laser power
received from the CO.  Obviously, this system wasn't real effective
for subscribers more than a few Km distant from the CO.

One method of getting free power from the phone that I've yet to
see exploited is to save the power from the ring signal.  I've seen
a number of instruments that draw 0.25B or more power when ringing.
It would be a farily simple matter to attach an isolation
transformer to the line for squirreling the ring power into a
nicad.  As long as the remote seismograph were called on a regular
basis, the ringing could keep the battery fresh, if the device
permitted the line to ring a number of times before going off hook.

I have an old UDS manually operated modem that entirely phone line
powered.  It contains 60 some 4000 series CMOS ICs, and even
generates RS-232C compliant output voltage for the computer
interface by using it's off-hook loading to power itself.  Pretty
neat piece of work.  Too bad that it couldn't go off hook or dial
automatically.  All of the modem's voltages were derived from
clever resistor divider networks; only the -6 volts for the RS232
output was derived from a switched capacitor.  At least, that's my
recollection.


Bill

larry@kitty.UUCP (Larry Lippman) (12/11/89)

In article <1989Dec9.220311.15928@phri.nyu.edu>, roy@phri.nyu.edu (Roy Smith) writes:
>> If you are going to design a commercial product, then you are in trouble
>> because Part 68 will not allow ANY usable amount of leakage from tip to ring
> 
> 	Is this a new rule?  Universal Data Systems (now part of Motorola,
> I think) used to make a nice little 1200 bps modem called the 212LP, where

	Perhaps I did not make myself clear.  I was referring to "leakage",
which means the amount of current flow in an ON-hook (i.e., telephone line
idle) state.  In an OFF-hook state, one can draw as much current as the
subscriber loop will allow, typically between 20 and 100 mA.

	It appears that the orignal poster was referring to designing a
telemetering device for data acquisition which would require power on a
*continuous* basis, and therefore would draw power from the telephone line
while it was ON-hook.

<> Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp.
<> UUCP  {allegra|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry
<> TEL 716/688-1231 | 716/773-1700  {hplabs|utzoo|uunet}!/      \uniquex!larry
<> FAX 716/741-9635 | 716/773-2488      "Have you hugged your cat today?" 

sheriffp@spock (Peter Sheriff) (12/11/89)

     Ringing in North America is a 90 Vac 20 Hz voltage. This is enough
to make your eyes water (I speak from experience).

     Line voltage (when not ringing) is a nominal 48 Vdc. Be careful
if you do use this as a source for any sort of equipment other than
a 'phone. If you pull more than a few milliamps, the equipment in the
phone office will respond as though the phone is off-hook and after
a short time (if no dialing takes place) it will bring up an alarm.
This is a low priority alarm so it may take them some time to get round
to testing it but when they do, they will detect the current drain and
could do something like a) cut off your service (actually, when the
alarm activates in the central office, the voltage is cut off at that
time).
b)Send a repairman around to your home (unlikely)
c)Just leave it as it is (you have no service anyway until you remove
the cause of the problem).

                                 Pete

john@zygot.ati.com (John Higdon) (12/12/89)

In article <1989Dec9.220311.15928@phri.nyu.edu> roy@alanine.UUCP (Roy Smith) writes:
>Universal Data Systems (now part of Motorola,
>I think) used to make a nice little 1200 bps modem called the 212LP, where
>the LP meant Line Powered.  We had a couple of them.  The power budget must
>have been really tight because this was a real bare-bones unit.  No dialer
>or auto-answer.

We used to use these on phone systems for remote programming and
diagnostics. The 300 bps version (at least) would do auto-answer just
fine. If the 1200 bps won't then one would have to assume that it takes
more power to run the 212 protocol, no?


-- 
        John Higdon         |   P. O. Box 7648   |   +1 408 723 1395
    john@zygot.ati.com      | San Jose, CA 95150 |       M o o !

jones@hood (Clark Jones) (12/15/89)

In article <3550@kitty.UUCP> larry@kitty.UUCP (Larry Lippman) writes:
>	Perhaps I did not make myself clear.  I was referring to "leakage",
>which means the amount of current flow in an ON-hook (i.e., telephone line
>idle) state.  In an OFF-hook state, one can draw as much current as the
>subscriber loop will allow, typically between 20 and 100 mA.
>
>	It appears that the orignal poster was referring to designing a
>telemetering device for data acquisition which would require power on a
>*continuous* basis, and therefore would draw power from the telephone line
>while it was ON-hook.

Hmm... maybe he could have a capacitor array that could support his system
for a couple of minutes, and then have his box dial its own number
every minute or so, and then "listen" to the busy signal for a while,
recharging the caps... or maybe if the "time" number in his area is free,
it could ring that up once in a while.  ;-) ;-) ;-) ;-)

Disclaimer:  The opinions expressed above are mine and not those of Schlumbrger
because they are NOT covered by the patent agreement!

wb8foz@mthvax.cs.miami.edu (David Lesher) (12/15/89)

Discussion on difference in power available 
{on hook, offhook, ringing}:

The box does not need to call anybody. Just have it go off hook
regularly (say every 30 seconds) for only as long as it take the
CO to complain, then go back on hook for a while, etc.
I suspect the CO would start griping to the CO-humans soon, but
maybe not. It WOULD improve your power budget!
--
A host is a host & from coast to coast...wb8foz@mthvax.cs.miami.edu 
no one will talk to a host that's close..............(305) 255-RTFM
Unless the host (that isn't close)......................pob 570-335
is busy, hung or dead....................................33257-0335

emmo@moncam.co.uk (Dave Emmerson) (12/20/89)

In article <14230@jumbo.dec.com>, murray@jumbo.dec.com (Hal Murray) writes:
>[] 
> Can you scrounge any power from the phone line? I'm not thinking of 
> many watts, just enough to run a few slow CMOS chips.
> 
Theoretically, yes, a little. Legally NO, apart from anything else, it
IS theft!

Dave E.

Nagle@cup.portal.com (John - Nagle) (12/22/89)

     You can indeed get a little power from the phone line, and there
are standards on how much you can draw.  When off-hook, you can 
draw over 100 MA, in fact.  All sorts of telephone-related devices
run off phone line power nowadays.  

     Bear in mind that all sorts of interesting surges and spikes
appear on phone lines.  Ring is nominally 88VAC, 20Hz.  Much larger
spikes can occur during electrical storms, and are considered part
of the normal operating environment.

     On the other hand, nothing particularly bad will happen if
you short the phone line for brief periods.

     Equipment that attaches to the phone line is supposed to be
FCC registered.  Automatic testing from the central office will
detect as faults lines which present unreasonable loads.  The
telco will probably think that they have a cable fault, and if
they spend time and effort chasing one down and discover that
your equipment is causing a false fault indication, you may
be billed for the repair costs.  This is especially true for
something that presents a DC load during on-hook periods insufficient
to take the circuit off-hook.  That, to the central office, looks
like water in a cable, and tends to be investigated.

				        John Nagle

irwin@m.cs.uiuc.edu (12/28/89)

/* Written 10:21 pm  Dec  4, 1989 by murray@jumbo.dec.com in m.cs.uiuc.edu:sci.electronics */
/* ---------- "power from phone line" ---------- */
>Suppose you have some remote instrumentation, like a seismometer, 
>or weather station, and you are transmitting that data back to a 
>central collection point via a phone line.

>Can you scrounge any power from the phone line? I'm not thinking of 
>many watts, just enough to run a few slow CMOS chips.

>Is there an obvious book I should know about that answers things like 
>this?
/* End of text from m.cs.uiuc.edu:sci.electronics */

A few years a go, the University of Illinois was doing some research
on tracking wild animals, in Northern U.S. and in Canada. They would
locate them by aircraft, fly low over them, shoot them with a stun
gun dart, then land in a clearing, hike back and tab the animal with
an ear tag. They also mounted a collar with a radio transmitter on
it.

They had phone pairs ran up to the top of high ground, spaced some
50 miles apart. Connected to the phone pair was an RCA 1802 CMOS
microprocessor, with ROM, RAM and a few CMOS radio receivers. They
put the transmitters on various frequencies, based on the type of
animal. A Bear was on one frequency, Elk on another, Moose on another,
Deer on another, etc.

During idle time, the RAM and radio receivers were powered down, just
a minimum amount of circutry was active. This would allow the phone
pair to recharge the nicads. When they called the phone pair, it would
activate the 1802, with the small amount of ROM and stack memory, they
would tell the 1802 to output to an I/O port, which would enable the
main RAM and they would then download software to it. They could then
with the software in RAM, activate various I/O port configurations,
which would enable a given receiver. If the receiver was receiving
a signal, they would know that Mr. Bear was within radio distance,
if he wasn't, they would disable that receiver and go on to the next
one.

With a grid of phone/microprocessor stations, they could track the
animals. By finding which stations were receiving a given frequency,
they could plot a vector to figure quite closely where they were.

Though I was not involved with this, I know the person who was, that
built the hardware and wrote the software for the project. He even
went into the "wilds" on a few occasions, to help with the stations
being set up and bagging the animals to tag/collar them. He still
lives locally and I could put anyone in contact with him that might
desire to talk with him via phone.

Me, I thought it was a very neat project.

Al Irwin
Univ of Illinois
Dept of Comp Sci
irwin@m.cs.uiuc.edu