You will find two files, one called DTMF_1.GIF and DTMF_2.GIF.  The file
DTMF_1.GIF is very simple, and will work with any type scanner or amateur
radio.  This circuit does not load down the speaker output so you can 
parallel a speaker across it if you like.

The 8870 takes a valid DTMF tone and generates a 4 bit binary word, the 
combinations are as followed:

      DTMF TONE                        BINARY 

        "1"                             0001
        "2"                             0010
        "3"                             0011       
        "4"                             0100
        "5"                             0101
        "6"                             0110
        "7"                             0111
        "8"                             1000
        "9"                             1001
        "0"                             1010
        "*"                             1011
        "#"                             1100
        "A"                             1110
        "B"                             1111
        "C"                             0000
        "D"

The four bit binary word is transferred to the status bits on the computer's
Parallel port.  The status signals are the usual signals a printer would 
generate if connected, for example, paper out.

The 8870 does a "DSP" on the incoming tone, using the color burst crystal
as a clocking source, effectively masks out the noise and validating the 
DTMF tone.

The decoded output of the 8870 is passes through a buffer chip, a 74HCT573.
This chip was chosen because it is a low power chip from the HCT family
and it is available at the local Radio Shack.  This chip can drive a cable 
up to ten feet long.

The complete circuit draws an average of about 10 milliampere, so a 9V 
battery would last a long time.

The circuit also has an LED which lights when a DTMF tone is received, I
tested the circuit with my scanner, and with the Squelch completely off,
the circuit did not response to the white noise.  

The resistor are 1/4 watts and the capacitors have 50v ratings.


The second file is called DTMF_2.GIF and it is for differential signals such
as LandLines.  It can be used with a scanner or amateur radio and it does
not matter which way you connect it up to the radio.

The FCC has rules about connecting thing up to PhoneLines, and your tele-
phone company won't like it very much if it messes things up at the station.
Check with your telephone company about getting a validated isolation device
before you connect it up permanently.  For testing purposes you can connect
it across the tip and ring lines, I believe they are the Blue and Red wires.

The parts are basically the same,  However you must use 300V rated capacitors
for C1 and C2.  This is because the ring signal is usually around 150 Volts.
The 5.1V zeners are there for lightening protection, make sure you place them 
after R1 and R2.  If the circuit is on-line all the time then you may want 
to have them, otherwise you can leave them out.