View Full Version : dtmf circuit question

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.

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:
This should work for you.


UC Berkeley '12 EECS
CalSol: Berkeley Solar Car team

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

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

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



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.

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?

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

Do your research on "Part 68".

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.

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.


Amateur radio callsign: G1HSM

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

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...

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