Circuit That Dials A Telephone?

Discussion in 'General Electronics Chat' started by ajm113, Nov 10, 2011.

  1. ajm113

    Thread Starter Member

    Feb 19, 2011
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    I want to know how alarm systems call for a monitoring station through the phone line and I was thinking what better possible way then to build a circuit that generates tones so that I can modify it to call my cell phone for example and mores code me a message.

    Is there any circuits out there that show how to do this?

    Thanks, Andrew.
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    Google modem.
     
  3. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    You are looking for a DTMF Encoder, and the isolation and switching from a modem if using a standard phone line, and a bit more complicated setup if using a cellular link unit made for alarm interfacing.

    What you are looking for exists in many forms. The one I've played with uses pre-recorded messages, so it calls you and plays back message #x, x depending on what set the alarm off, or just to call and give a timestamp of when the alarm was armed and disarmed. Higher end units, essentially voice modems will also answer the incoming call, and after you enter your PIN# on the keypad, it will give the status of the system "Status OK" or "Sensor X active", etc.

    One place a friend owns has the one that calls him, but he can't call it. The inbound option was something like a $300 upgrade a few years ago. It also uses a cellular link anyway, as burglars typically cut the phone lines assuming that will stop the alarm from calling the cops (That hasn't worked in over a decade).
     
  4. ajm113

    Thread Starter Member

    Feb 19, 2011
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    Sorry for the long reply, but working two jobs has been keeping me at bay 24/7 almost. >_>

    And I know a schematic of a modem would work, but it's kinda a more of a leap then a walk in the park for me.

    @thatoneguy

    The hole setup where it can interface with the phone is very coo! :D But let me ask this question, how can I generate "tones" so that I can make a call with my circuit then? I'm sure there must be a schematic somewhere that works like a phone and all it does is send "pings" through the line. If this request appears to be a little more advanced for a beginner playing with telephone lines, can someone recommend me a good link about how telephone lines work and perhaps shows example shamatics?
     
  5. MrChips

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    Oct 2, 2009
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    You can look into the history of modems by starting with the Pennywhistle.
     
  6. mjiggidy

    New Member

    Nov 13, 2011
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    The tones are the result of the DTMF signalling (Dual-tone multi-frequency signalling) the telephone system uses. Each tone is a combination of two sine waves, which creates an interference pattern the DTMF decoder on the other side is able to decode.

    This page has some information on how that works, including a useful chart that shows which two sine waves are combined for each number on the keypad.
     
  7. ajm113

    Thread Starter Member

    Feb 19, 2011
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    Oh thank you for the information! The page explains everything very well. I think I may put this project to the side and work on something else, not that I don't get it or understand it, its just one of those things that become less appealing then you thought it was.

    Thank you again guys! :)
     
  8. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    For a bit of extra information, cell phones do not "dial" using DTMF to access the desired number. The tones are generated for user feedback at the handset and to work through voice menus once connected.

    When entering a number and hitting "Send" to call, the call information is all digitized with modern cell phones. This means you can't hold a "Dialer" next to your phone and make calls, though that was possible in the infancy of portable phones 25 years ago.
     
  9. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    In what areas of the USA?

    DTMF dialling is standard in many countries, and was standard in the USA pretty much everywhere. If a specific area has changed to digital dialling and does not support DTMF dialling anymore then ALL the phones in that area must be replaced with new digital dial phones, which is an enormous hassle and expense to that phone company.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    He's talking about cellular communication. Land lines still work using DTMF.
     
  11. thatoneguy

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    As Wookie stated, I was referring to cellular phones, not POTS (Plain Old Telephone System or "land line") telephones, which will work if you hold a dialer near the microphone when you hear a dialtone, with the exception of new payphones. There was a time in the late 80's when you could make free calls from a pay phone if you had a DTMF generator.
     
  12. THE_RB

    AAC Fanatic!

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    Thank you Sgtwookie and apologies Thatoneguy, you know I read your post about 3 times when I was replying to it, and still completely missed the word "cell" there, I just read it as "phones".

    Sorry everyone! My mistake.
     
  13. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Technologically brainwashed like the rest of us, eh?

    I honestly do not understand why they don't add a "TXTing" interface to home phone lines. At least receive only for short messages. It wouldn't be too much overhead on an otherwise idle line and switchboard.

    The downside of this is we'd have a new generation of kids posting new threads with titles like"Nd N E 1 2 HLP W/CKT". I mean, more threads like that.
     
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