PDA

View Full Version : dtmf circuit question



den372
01-25-2010, 07:57 AM
I am building a dtmf dialer using the bsp40 and a cm8880. I used the schematic found on the parallax site. My question is the schematic does not include connection to the phone lines. Is it as simple as connecting the audio out of the cm8880 to the black line on the phone or is there a need for a transformer etc? Any help appreciated.

FlyingFishFinger
01-25-2010, 08:11 AM
Yes, you will need a transformer and some capacitors to isolate the DC voltage component on the line. I did something like this about 4 years ago for a science fair project, so forgive me if I don't have the details on me at the moment. There is a Parallax N&V column I referred to back then:
http://www.parallax.com/Portals/0/Downloads/docs/cols/nv/vol1/col/nv19.pdf
This should work for you.

Rafael

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
UC Berkeley '12 EECS
CalSol: Berkeley Solar Car team
www.calsol.berkeley.edu
KJ6AWU

Post Edited (FlyingFishFinger) : 1/25/2010 1:17:04 AM GMT

Franklin
01-25-2010, 08:52 AM
den said...
I am building a dtmf dialer using the bsp40 and a cm8880. I used the schematic found on the parallax site.
Link to the schematic and explain just what you want to do. Use the post reply button and attach your code.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
- Stephen

kf4ixm
01-25-2010, 07:35 PM
This should work for ya!

http://forums.parallax.com/attachment.php?attachmentid=67112



Parts:

2X RJ-11 phone Jacks1X 0.22uF 200V capacitor1X 4.7K 1/2W resistor1X 600 ohm to 600 ohm modem transformer2X 5V 1W zener diodes1X 10K audio taper potentiometer1X RCA audio jack

PJ Allen
01-25-2010, 07:55 PM
"Ma Bell" (or whomever the controlling authority is anymore) gets super irritated if/when you take the Central Office down.

mpark
01-29-2010, 02:46 AM
PJ Allen said...
"Ma Bell" (or whomever the controlling authority is anymore) gets super irritated if/when you take the Central Office down.


Is this a comment about the suitability (or lack thereof) of kf4ixm'c circuit?

allanlane5
01-29-2010, 03:19 AM
Well, IF you built the circuit WITHOUT that 600-ohm to 600-ohm transformer, that would make the Central Office unhappy, yes.

I think as the circuit is shown, it is suitable. Though I might mention, commercial modem makers usually spend a lot of money making sure their phone interface circuit is checked to be suitable for connection to the telephone line.

Oh, one more point -- the Red and Green wires usually are the two 'center' conductors in a 4-wire 'flat' telephone wire, that diagram isn't explicit about that.

PJ Allen
01-29-2010, 06:00 AM
http://www.ce-mag.com/99ARG/Gubish233.html

Do your research on "Part 68".

mpark
01-31-2010, 02:06 PM
PJ, for the benefit of us less hardware-oriented types, could you please summarize what the problem is with kf4ixm'c circuit? Assuming there is a problem, that is—I'm still not sure what your point is.

Leon
01-31-2010, 07:37 PM
I think he means that anyone using it should check that it conforms to Part 68 of the FCC regulations. One thing that comes to mind is that an approved transformer must be used. Telcos are worried about people electrocuting their staff, as well as blowing up their equipment, by being careless with what they connect to their lines.

Leon

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Amateur radio callsign: G1HSM

Post Edited (Leon) : 1/31/2010 12:46:26 PM GMT

kf4ixm
01-31-2010, 09:21 PM
if you use the isolation transformer, i dont think there will be any issues with the fcc. i remember reading somewhere once, that this is what makes telephone interconnections 'ma bell friendly'. as the 600:600 isolation transformer acts like an opto-isolator does in digital electronics. i've used this circuit (from forrest cook)·many times in projects and haven't had anyone knocking on my door! (yet) but just in case, here is my disclaimer...

This posting does not reflect the thoughts or opinions of either myself, my
company, my friends, or my cat; don't quote me on that; don't quote me on
anything; all rights reserved; this document is distribution copyrighted to
the extent that you may not distribute this posting and all its associated parts
freely, make a profit from it or include the article or parts
of it in commercial publications, or as part of any fee-based services or
products; further redistribution’s only allowed unedited and in its entirety
by electronic transfer (anonymous FTP, Gopher, WWW and mail servers), storage
media, and printed copy as long as this notice is included and no monetary fee
is charged; subject to change without notice; text is slightly enlarged to
show detail; resemblance to actual persons, living or dead, is unintentional
and coincidental; all models are over 18 years of age; dry clean only; do not
bend, fold, or mutilate; anchovies or jalapenos added upon request; your
mileage may vary; no substitutions are allowed; for a limited time only while
supplies last; offer void where prohibited; is provided "as is" without any
warranties expressed or implied; user assumes full liabilities; not liable for
damages due to use or misuse; equal opportunity joke employer; no shoes, no
shirt, no jokes; caveat emptor; read at your own risk; may contain material
some readers find objectionable; parental advisory: explicit lyrics; keep away
from pets and small children; limit one-per-family please; no money down; no
purchase necessary; ask us about our trade-in plan; you need not be present to
win; some assembly required; batteries not included; action figures sold
separately; jokes were packed full, contents may have settled during mailing;
sanitized and sealed for your protection; do not use if safety seal is broken;
do not use while operating a motor vehicle or heavy equipment; safety goggles
may be required during use; call before you dig; use only with proper
ventilation; for external use only; if a swelling, redness, rash, or
irritation develops, discontinue use; do not place near a flammable or
magnetic source; keep away from open flames; avoid inhaling fumes or contact
with mucous membranes; contents under pressure, may explode if incinerated;
smoking these may be hazardous to your health; the best safeguard, second only
to abstinence, is the use of a good laugh; text is made from 100% recycled
electrons and magnetic particles; no animals were used for testing, no salt,
MSG, preservatives, artificial color or flavor added; if ingested, do not
induce vomiting, if symptoms persist, consult a someone who cares; are ribbed
for your pleasure; slippery when wet; must be 18 to enter; possible penalties
for early withdrawal; one size fits all; offer is valid only at participating
Internet sites; slightly higher west of the Rockies; allow four to six weeks
for delivery; if defects are found, do not try to fix them yourself, but
return to an authorized service center; please remain seated until the we have
come to a complete stop; objects in the mirror may be closer than they
appear; this disclaimer does not cover hurricanes, floods, earthquakes, and
other Acts of God, sonic boom vibrations, electromagnetic radiation from
nuclear blasts, unauthorized repair, improper installation, misuse, typos,
misspelled words, missing or altered signatures, and incidents owing to
computer or disk failure, accidental file deletions, or milk coming out of
your nose due to laughing while drinking; other restrictions may apply. If
something offends you, lighten up, get a life, and move on.