Measuring low ac voltage

Baard Toerdal

Hi!
I need to monitor an analogue telephone line and distinguish between two diferent voltage levels; is someone talking or is the line quiet? My initial thought was to use an LM339 comparator as described in Matt Gilliands Microcontroller cookbook 2. but I am unsure if this circuit will work with the signal on the telephone line. The comparator circuit seems to be a DC circuit where the Stamp and the comparator has a common ground. My circuit will need to be connected to an AC signal and give a clear high or low when a certain voltage level is reached, preferably with a way of finetuning the ciruit with a potentiometer.

Baard T.

Chris Savage

Phone lines can run between 5V and 100V depending on whether the phone is off the hook or ringing.· Perhaps a Google search might be in order, I seem to recal some chips that are designed to detect phone line conditions.· Although that was some time ago...



▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Chris Savage

Knight Designs
324 West Main Street
P.O. Box 97
Montour Falls, NY 14865
(607) 535-6777

Business Page:·· http://www.knightdesigns.com
Personal Page:··· http://www.lightlink.com/dream/chris
Designs Page:··· http://www.lightlink.com/dream/designs
·

allanlane5

Ah, I fondly remember the time I was wiring in a phone extension cable. I managed to get across the phone line just as someone called in -- and the phone RANG! My hand felt like it went "BZZZZT!", then it stopped. Then the phone RANG AGAIN! AIEEEE! "BZZZZT!" again.

Be careful with this kind of project, the phone company takes a dim view of you breaking their system, even inadvertently. And the phone line does use 'wierd' voltages. Google for phone-lines or something to find out what you're dealing with.

steve_b

I was visiting a friend that lives out in the boonies and a thunderstorm was passing by.

Well, his telephone would ring every once in a while and I was getting angry at the 1-ringers.· So I asked what the deal was and it was pointed out that thunder happens around the time of the ringing phone.

Turns out lightning was hitting the line somewhere and charging the line.· There wasn't enough load through the cable network to dissipate the strike.

That's certainly a call that'll knock yer socks off! $1.99/minute!

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
·

Steve
http://members.rogers.com/steve.brady
"Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."

SPENCE

PHONE LINES IN USA. NOMINAL ON HOOK OR HANDSET HUNG UP IS -48 DC. THIS HANDS YOU QUITE A ZAP REALLY.

OFF HOOK IS APPROXAMATLY -12 VDC .

RING VOLTAGE IS ABOUT 90 VAC AT 20 HERTZ. (CYCLES PER SECOND).
NOW THAT CAN REALLY SCORTCH YOU.

NOTE THESE NOMINALLY ARE MINUS IN REFERENCE TO EARTH GROUND.

SUGGESTION GOOGLE FOR A PHONE LINE BUSY CIRCUIT WHICH USES OPTICAL COUPLERS. MOST HOBBISTS AND I THINK ALL COMMERCIAL AND PROFERIONALS "GENERALLY USE THEIR STAMPS" ABOVE GROUND SO THE EARTH IS AT 0VDC AND STAMP IS VCC=+5 VDC.

http://www.discovercircuits.com/T/telephone.htm
http://www.ee.washington.edu/circuit_archive/circuits/

HERE IS SOME STARTING LINKS.

73
SPENCE
K4KEP

Baard Toerdal

Well, what if I measure the voltage at the handset speaker. That should be a safe place to check the voltage, nowhere near the 90-5 volt line levels... What are the typical voltage levels needed to drive such a speaker? If the level here is to low, could I amplify the level so that I could get a good reading?

Baard