Designs and Implementation

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Designs and Implementation

• Ring Detector circuit Design

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• Description
• - The above circuit can be directly
  connected to the phone line via the TIP and RING
  wires (green and red), or it can beconnected in
  parallel with a telephone.
• - When the phone rings, capacitor C2 will
  charge until the break over voltage of the diac
  diode is reached turning on the transistor Q1. As
  Q1 turns on and off, when a ring signal is
  detected, a series of pulses will be send to the
  voltage regulator.
• - The output results in a 5 volt square wave
  every time a pulse is detected, an LED also
  flashes to indicate that the ring tone is being
  detected.

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• Below is a picture of the ring Detector circuit
  soldered

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• DTMF (Dual Tone Multiple Frequency)
• DTMF diagram
  DTMF chip pin layout

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• We know that the tone controlled system is
  implemented using a feature of commercial phones
  called Dual Tone Multiple Frequency (DTMF). In
  this Feature, the tones generated by the keys on
  a telephone or mobile are actually a combination
  of 2 frequencies a high group and low group
  frequency that are sent simultaneously. There are
  eight standard frequencies and the yare
  distributed as we see in the next slide

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Standard DTMF Distribution
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• Description of the Frequency tones
• Standard and commercial telephones or mobiles
  generally include digits 0 to 9 with the
  characters and . Each number or character on a
  keypad is given a set of low and high frequency.
  Theses frequencies are common in all phones and
  it can be used to input commands. Since
  telephones are frequently used as input sources,
  decoding such tones is fairly common. That leads
  us to think of implementing and designing
  band-pass filters, one for each frequency.
  Band-pass filters allow only a certain range of
  frequencies to pass through, and block all
  frequencies lower or higher that the designated
  range.
• Since we know that implementing 8 band-pass
  filters will be time-consuming we decided to
  purchase a DTMF chip (MT 8870 D ) which fulfilled
  the goal we need.

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• Description of the DTMF MT 8870 D function
• The MT8870D/MT8870D-1 monolithic DTMF
  receiver offers
• - Small size
• - Low power consumption
• - High performance
• It consists of a band-split filter section, which
  separates the high and low group tones
  frequencies.
• Following the filter section is a decoder
  employing digital counting techniques to
  determine the frequencies of the incoming tones
  and to verify that they correspond to standard
  DTMF frequencies.