Some people like to build things, just to build.  It occupies the mind, and 
keeps us off the streets and out of bars 
As promised, I'm posting some text articles here in HAM_TECH that would 
benifit the builder in all of us.
DTMF tone decoders are useful for all sorts of things, but for the Ham, it's 
a form of remote controlling of several devices (turning on lights, 
controlling repeaters, operating their HF rig from remote, etc.)
So, here's some text that I came accross, referencing DTMF frequencies
Enjoy.
=============DTMF.TXT=========================
RF> I have a Icom 2gat - i'm sure others have the ABCD keys on the
RF> keypad on thier radios too, but, what are they far?  My owners
RF> manual goes no further than to point to them in a picture.  Just curious.
All they really do is give you 4 more digits that can be dialed.
But one practical purpose for functions of a repeater controller
that can only be used over the air, and not by phone, since an
ordinary telephone does not have these keys.  The S-meter on a
repeater is one good example -- useful only when called for over
the air, and only says "S0" when called for over the phone.  So
just give it an access code that starts with A,B,C, or D, so that
the function cannot be called for over the phone.
The ABCD column generates a 1633 Hz tone, which is mixed with a
tone from the low group to produce a DTMF tone.
    ______________
   |              |
   |  1  2  3  A  | < Row = 697 Hz
   |              |
   |  4  5  6  B  | < Row = 770 Hz
   |              |
   |  7  8  9  C  | < Row = 852 Hz
   |              |
   |  *  0  #  D  | < Row = 941 Hz
   |______________|
      ^  ^  ^  ^
      |  |  |  +----< Column = 1633 Hz
      |  |  +-------< Column = 1477 Hz
      |  +----------< Column = 1336 Hz
      +-------------< Column = 1209 Hz
Other tones:
  Dial tone: 350 Hz + 440 Hz (Continuous)
  Line busy: 480 Hz + 620 Hz (1 second on, 1 second off)
  Fast busy: 480 Hz + 620 Hz (0.5 second on, 0.5 second off)
       Ring: 440 Hz + 480 Hz (2 seconds on, 4 seconds off)
All DTMF frequencies have a tolerance of +/- 1.5%.
Nominal level per frequency: -6 to -4 dBm.
Minimum level per frequency: Rows -10 dBm, Columns -8 dBm.
Level per frequency pair: +2 dBm.
Maximum difference in levels between frequencies: 4 dB.
Rise time: Each of the two frequencies of the signal should attain
  at least 90% of full amplitude within 5 ms, and preferably within
  3 ms for automatic dialers, from the time that the first frequency
  begins.
Pulsing rate: Minimum duration of two-frequency tone signal: 50 ms
  normally; 90 ms if transmitted by radio. Minimum interdigital time:
  45 ms.
Tone leak during signal off time should be less than -55 dBm.
Transient voltages: Peak transient voltages generated during tone
  signaling should be no greater than 12 dB above the zero-to-peak
  voltage of the composite two-frequency tone signal.
Extraneous frequency components: The total power of all extraneous
  frequencies accompanying the signal should be at least 20 dB below
  the signal power, in the voice band above 500 Hz.
On-Hook indication: Minimum DC resistance between tip and ring
  conductors of 30K ohms.
Off-Hook indication: Maximum DC resistance between tip and ring
  conductors of 200 ohms.
["dBm" refers to a power level, expressed in decibels, with respect
to a reference level of 1 milliwatt. For example, an actual power
level of 0.5 mW is equal to -3.01 dBm; and an actual power level of
2 mW is equal to +3.01 dBm.]
=========================================
73 = Best Regards                ka5thb@bigfoot.com
-Jeff KA5THB
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