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DTMF circuit — Parallax Forums

DTMF circuit

JonathanJonathan Posts: 1,023
edited 2009-01-02 03:19 in General Discussion
Hi All,

I'm messing about seeing if I can come up with the parts to try the DTMF circuit in the BS manual. I have a couple questions about component substitutions.

1) Xfrmr. I have a couple of 800/1800 ohm ones that I could couple back to back. I also have some 50/50 ohm isolation transformers, and a few modem xfrmrs of various values. How close do I need to get?

2) I don't have any SIDACS. I do have some MOVS, part #ZNR10K20IU and ZNR20K20IU, I can't seem to find data sheets on these. How crucial are they? Can I play with it to get going without ad order some, or am I going to blow something?

Thanks!

Jonathan

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www.madlabs.info - Home of the Hydrogen Fuel Cell Robot

Comments

  • MikerocontrollerMikerocontroller Posts: 310
    edited 2008-12-31 23:03
    · I wonder if you can find those parts in a PC modem card or telephone answering machine???
  • LeonLeon Posts: 7,620
    edited 2009-01-01 01:07
    The main reason why those parts are used is to prevent damage to the telephone exchange, and to protect their staff from dangerous voltages. If something went wrong and you damaged something or someone, you'd probably be looking at a lengthy prison sentence.

    Leon

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    Amateur radio callsign: G1HSM
    Suzuki SV1000S motorcycle
  • JonathanJonathan Posts: 1,023
    edited 2009-01-01 16:30
    Leon,

    I find it hard to beleive that a 1:1 xfrmr driven by a Stamp can injure anyone. Mess up phone equipment *mayve*. I think the SIDACS are there to protect the user and stamp from voltages coming down the line. I got it to work with the xfrmrs I have and I have ordered the SIDACS just to be safe.

    Thanks!

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    www.madlabs.info - Home of the Hydrogen Fuel Cell Robot
  • LeonLeon Posts: 7,620
    edited 2009-01-01 17:51
    If it was powered from the mains it could conceivably happen. That's why the phone companies are concerned about people connecting stuff to their lines, and have such strict regulations about it. The FCC enforces them in the USA, it's Ofcom in the UK.

    This covers the sort of things you need to take into account:

    www.cermetek.com/catalog/Telephone-Line-Interface/DataSheet/XE0068DTRevC.pdf

    Leon

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Amateur radio callsign: G1HSM
    Suzuki SV1000S motorcycle
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2009-01-01 18:09
    You could try acousttic coupling. I've been able to dial a phone reliably by holding the receiver's mouthpiece close to the speaker of a Scribbler robot.

    -Phil
  • JonathanJonathan Posts: 1,023
    edited 2009-01-01 18:51
    Phill, I'd rather not have to use an actual phone. All I need to do is dial a number if the alarm goes off. It's working great.

    Leon, thanks for the link, it's a good read. With the SIDACS, I think I'll be ok. The mains power is well isolated from the phone circuit, so I think I'm OK there.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    www.madlabs.info - Home of the Hydrogen Fuel Cell Robot
  • lenswerkslenswerks Posts: 40
    edited 2009-01-02 03:19
    Jonathan,

    Did you consider using a Data Access Arrangement ( DAA ) verses the transformer? A DAA will give you ring detection, hookswitch control and bandwidth for voice grade applications if you ever wanted to include a EMIC - Also provies 1500 volts isolation between tip/ring and the rest of any circuitry.
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