🚀 go-pugleaf

John Goggan <34II5MT@CMUVM.CSV.CMICH.EDU> writes:
: This comes up now and then and I'm always interested, but no one ever
: seems to come up with a definate answer.  I'm looking for a schematic
: for a touch-tone decoder circuit.  Something that simply takes audio
: input and lights an appropriate LED for 0-9,#,*,A-D when it "hears"
: it on the audio line.  I've done this project with a bunch (12-16)
: seperate tone decoders and then used AND gates, but as everyone knows,
: it's not a very good design and it is very sensitive to false readings.
:[...]

12 to 16 decoders is quite a lot.  Why not use a DTMF Receiver (not
really called a Decoder even though that's what it does) and you'll
get to solve the tone mystery with less parts?  Most DTMF Receivers read
the tone(s), and sends the data out in 4-bits (hexadecimal) (can also
be interfaced with a micro- controller/processor).

Anyway, if you're interested, check the databooks from various
manufacturers such as National Semi, Motorola, Mitel, AT&T, et al.
Those books should have the word(s) - "communications" or "tele-
communications" on their covers (not the Linear databook for sure).
(The keyword inside, as mentioned, is "DTMF Receiver".)

I've got a bunch of Motorola MC145436P DTMF Receivers laying around
on my table right now (we're prototyping a 12-port phone switch &
voice mail system (analog & digital)).  The datasheet for that chip
says:  +5V supply required, detects all 16 standard digits, requires
a 3.579545 MHz crystal, 14 pins, etc.  It's also pin compatible with
the SSI-204 (Siliconix?)

Anyway, since you wish to hook up some LEDs to indicate which key was
pressed, you'll also need a 4-to-16 Decoder/Demultiplexer, two octal
Line Drivers, and some resistors.  Probably have to include a voltage
divider somewhere in the front end of your circuit if necessary.
(Sorry, but I'm not familiar with the board that you mentioned later in
your article (the one that vi deleted; heh-heh, blame the software:-)))


Hope this helps a little,

-phil