[sci.electronics] DTMF

jh@mit-amt.MEDIA.MIT.EDU (John Underkoffler) (07/17/87)

	Does anyone know who (if anybody) is making DTMF decoder
chips? Neither the TI nor the Motorola "Communications Databooks" as
recently as 1985 shows that any such chip exists (although there are
plenty of DTMF encoder chips available). Ideally, I'd want a chip
which accepts an analog input and produces either a digital hex output
(in the range 0-15)or a "row and column" output indicating the
position of the button on the imagined phone from which the signal
came.
	The point of all this is that I'd like to selectively activate
one of sixteen (or twelve, or whatever) devices by radio remote
control, and it seemed to me that one of the easiest ways to realize
this would be to follow the phone company's lead and transmit
voice-frequency DTMF tones (via disemboweled walkie-talkie); each
target unit could then be constantly receiving the same radio
frequency transmission; simple logic stuff inside the unit would then
read the output of the local DTMF decoder chip and decide if it should
activate the associated mechanism.
	Comments? Ideas? Insults?

				John "Wombat Stomper" Underkoffler
				alias "jh@media-lab.media.mit.edu"

larry@kitty.UUCP (Larry Lippman) (07/18/87)

In article <1377@mit-amt.MEDIA.MIT.EDU>, jh@mit-amt.MEDIA.MIT.EDU (John Underkoffler) writes:
> 	Does anyone know who (if anybody) is making DTMF decoder
> chips? Neither the TI nor the Motorola "Communications Databooks" as
> recently as 1985 shows that any such chip exists (although there are
> plenty of DTMF encoder chips available). Ideally, I'd want a chip
> which accepts an analog input and produces either a digital hex output
> (in the range 0-15)or a "row and column" output indicating the
> position of the button on the imagined phone from which the signal
> came.

	Try using Silicon Systems Inc. products such as their SSI201,
SSI202, SSI203 or SSI204.  These products all take an analog input and
provide a BCD digit output.  They all use 3.58 color burst crystals
for the reference frequency.  I have had good luck with SSI products.
	Or you can try Mitel products such as their MT8862 and MT8865
(used together, the former is the decoder and the latter is a filter),
or the integrated MT8870A.  The Mitel devices are better quality, but
also cost more money.
	In my opinion, you would be MUCH better off with the above devices
rather than attempting to build a discrete DTMF decoder using, say 567
PLL tone detectors.

<>  Larry Lippman @ Recognition Research Corp., Clarence, New York
<>  UUCP:  {allegra|ames|boulder|decvax|rocksanne|watmath}!sunybcs!kitty!larry
<>  VOICE: 716/688-1231        {hplabs|ihnp4|mtune|seismo|utzoo}!/
<>  FAX:   716/741-9635 {G1,G2,G3 modes}    "Have you hugged your cat today?" 

rpluth@pnet01.CTS.COM (Ron Pluth) (07/18/87)

   You can pick up a simple DTMF decoder chip at Radio Shack for $10.95.  It
outputs in straight binary for either the normal 12 digits or for all 16.  I
think it might also output in some other format, like Gray code, or something.
It takes, as has been mentioned, a cheap crystal, and I think it takes straigt
audio inputs.  Of course, you must be careful in interfacing it directly to a
telephone line, as ring voltages run up to ~150 VAC.

		                    Ron Pluth

UUCP: {akgua, hplabs!hp-sdd, sdcsvax, nosc}  | "We are all in the gutter,
      !crash!pnet01!rpluth                   |  but some of us are looking 
ARPA: crash!pnet01!rpluth@nosc.mil           |  at the stars."
INET: rpluth@pnet01.CTS.COM                  |              -Oscar Wilde
USPS: 13704 Celestial Rd.  Poway, CA  92064  |

ron@topaz.rutgers.edu (Ron Natalie) (07/20/87)

Silicon Systems SSI 201 Integrated DTMF Reciever.  I bought mine
for $5.00 including the crystal and a 22 pin socket.

Outputs in binary (except that Zero is coded 10)
Uses 12V supply and a Color Burst (3.579545 MHz) Crystal.

The application notes even point out how to add pusle dial
detection to the chip.

-Ron

hanley@cmcl2.NYU.EDU (John Hanley) (07/22/87)

In-Reply-To: <1377@mit-amt.MEDIA.MIT.EDU>


Using audio sounds good, especially since for debugging purposes you can just
listen to a radio receiver.  However, especially since you don't have them
readily at hand, I'm not convinced that DTMF chips are the way to go.
You can easily get VCO's to hum at any frequency you want by tweaking
discrete resistors, and further you can build a frequency detector with
a single 747 dual op amp: one side accepts audio input and acts as a
bandpass filter, the other side acts as a comparator and just cleans
up the analog so it's TTL on the output (or it could act as a Schmitt
trigger, which still requires but a single op amp).  This might be simpler
than DTMF because now you've only got one frequency to worry about at a
time instead of two.  Of course, I'm blindly assuming that you've got enough
bandwidth and steep enough filters for all these frequencies to give each
other enough breathing room.  If need be you can make the filters steeper by
adding another stage or two, and the whole thing would still fit on a single
(quad) chip.  Of course, if some DTMF chips drop into your lap, by all means
use them, but I tend to think in terms of what I can build _now_.  What?!?
You don't have a drawer full of 741's?  Shame on you!

If for some reason you're not real big on op amps, you can use a 567 tone
decoder to do the same thing, but I'm guessing you don't have a whole lot
of those lying around or you'd be using those instead of posting to the net.

P.S.: I almost recommended the use of 555 timers to generate your tones but
      then caught myself because I think you want sinusoidal tones to
      transmit.  I did this because I recall reading something on modems
      that mentioned that mod/demod is done because the phone system has
      low bandwidth and is optimized for the human voice, so sine waves
      make it through the phone system _much_ better than square waves.
      Why is this?  I understand that the rapid hi-lo transitions count
      as high frequency transitions and thus will be attenuated, but it
      seems to me that the worst case should be that you put a square wave
      in and get the corners lopped off so the output looks kind of
      sinusoidalish.  But since an X volt p-p square wave carries more
      power than an X volt p-p sine wave, you should get a better S/N ratio
      at the output using square simply because you applied more signal.
      Why doesn't this work?

                   --John Hanley,
 /  /   ____ __  __  System Programmer, Manhattan College [ ..cmcl2!mc3b2!jh ]
/__/ /__ /  /-< /-/  Researcher, NYU Ultracomputer Labs   [  Hanley@NYU.arpa ]

"The Ultracomputer: to boldly go in log N time where no N processors have
 gone before."



Addendum:  Bandpass active filter  (40 db/decade, I think -- it's 2nd order)

                                   R                 R
                           +----/\/\/\/-----+-----/\/\/\/-----+
                           |                |                 |
                         ----- C          ----- C           ----- C
                         -----            -----             -----
                           |                |                 |
                           +----------------+-----------------+
                           |                |    R/12         |
                  Rin      |   |\           +--/\/\/\/--GND   |
audio in  o-----/\/\/\/----+---| -\___________________________+------o Vout
                             +-| +/
                             | |/
                             |
                            GND

where Rin is whatever input impedance you would like to present to your
RF reciever circuitry, and RC determines the detection frequency:
 f = sqrt(3) / ( 2 pi R C )

rep@genrad.UUCP (Pete Peterson) (07/22/87)

In article <17966@cmcl2.NYU.EDU> hanley@nyu.arpa (John Hanley) writes:
#>      But since an X volt p-p square wave carries more
#>      power than an X volt p-p sine wave, you should get a better S/N ratio
#>      at the output using square simply because you applied more signal.
#>      Why doesn't this work?
#>

It does work, provided that the 3f, 5f, etc harmonics of the square-wave
don't cause you any problems.  The square wave has a fundamental frequency
content which is about 1.27 times the sine wave giving about 1.6 times the
power.  Of course the square wave has twice the power of the sine wave, but
the power in the harmonics doesn't help your S/N ratio.


#>
#>Addendum:  Bandpass active filter
#>
#>                                   R                 R
#>                           +----/\/\/\/-----+-----/\/\/\/-----+
#>                           |                |                 |
#>                         ----- C          ----- C           ----- C
#>                         -----            -----             -----
#>                           |                |                 |
#>                           +----------------+-----------------+
#>                           |                |    R/12         |
#>                  Rin      |   |\           +--/\/\/\/--GND   |
#>audio in  o-----/\/\/\/----+---| -\___________________________+------o Vout
#>                             +-| +/
#>                             | |/
#>                             |
#>                            GND
#>

Simpler circuit with roughly equivalent behavior and many fewer parts:

                   Rin
audio in  o-----/\/\/\/------+------o Vout
                             | 
                             |
                            GND


Unless you have a pile of op amps and R's and C's which are burning a hole in
your junk box (or other parts storage facility) and you are extremely
poverty stricken and take great delight in building and tuning multiple
bandpass filters, the integrated DTMF encoders/decoders are definitely the way
to go.

	pete peterson
	{decvax,linus,wjh12,mit-eddie,masscomp}!genrad!rep

larry@kitty.UUCP (Larry Lippman) (07/22/87)

In article <1319@genrad.UUCP>, rep@genrad.UUCP (Pete Peterson) writes:
> Unless you have a pile of op amps and R's and C's which are burning a hole in
> your junk box (or other parts storage facility) and you are extremely
> poverty stricken and take great delight in building and tuning multiple
> bandpass filters, the integrated DTMF encoders/decoders are definitely the way
> to go.

	Right on!  Besides, most of the integrated receivers have far
superior selectivity (read spurious signal rejection) than any receivers
built with discrete tone detectors.  Not only can integrated receivers
implement more filter poles than is convenient for discrete construction,
but some of the newer integrated receivers use switched-capacitor filters.

<>  Larry Lippman @ Recognition Research Corp., Clarence, New York
<>  UUCP:  {allegra|ames|boulder|decvax|rocksanne|watmath}!sunybcs!kitty!larry
<>  VOICE: 716/688-1231        {hplabs|ihnp4|mtune|seismo|utzoo}!/
<>  FAX:   716/741-9635 {G1,G2,G3 modes}    "Have you hugged your cat today?" 

phd@speech1.cs.cmu.edu (Paul Dietz) (07/22/87)

In article <17966@cmcl2.NYU.EDU> hanley@nyu.arpa (John Hanley) writes:
>Addendum:  Bandpass active filter  (40 db/decade, I think -- it's 2nd order)
>
>                                   R                 R
>                           +----/\/\/\/-----+-----/\/\/\/-----+
>                           |                |                 |
>                         ----- C          ----- C           ----- C
>                         -----            -----             -----
>                           |                |                 |
>                           +----------------+-----------------+
>                           |                |    R/12         |
>                  Rin      |   |\           +--/\/\/\/--GND   |
>audio in  o-----/\/\/\/----+---| -\___________________________+------o Vout
>                             +-| +/
>                             | |/
>                             |
>                            GND
>
>where Rin is whatever input impedance you would like to present to your
>RF reciever circuitry, and RC determines the detection frequency:
> f = sqrt(3) / ( 2 pi R C )

I don't think this is what you meant. None of the RC's (except Rin) are
doing anything!

Paul H. Dietz
Carnegie Mellon University

asm@utcsri.UUCP (07/30/87)

In article <1867@kitty.UUCP> larry@kitty.UUCP (Larry Lippman) writes:
>In article <1319@genrad.UUCP>, rep@genrad.UUCP (Pete Peterson) writes:
>> Unless you have a pile of op amps and R's and C's which are burning a hole in
>> your junk box (or other parts storage facility) and you are extremely
>> poverty stricken and take great delight in building and tuning multiple
>> bandpass filters, the integrated DTMF encoders/decoders are definitely the way
>> to go.
>
>	Right on!  Besides, most of the integrated receivers have far
>superior selectivity (read spurious signal rejection) than any receivers
>built with discrete tone detectors.  Not only can integrated receivers
>implement more filter poles than is convenient for discrete construction,
>but some of the newer integrated receivers use switched-capacitor filters.

	You can get a DTMF receiver for less than it will take to put
one together using op-amps etc. Just about everyone makes a dozen
chips to do this these days.

	For the fun of hacking though, nothing beats putting the stuff together
from scratch. It can be done using two bandpass filters and two phase-locked
loops (better than doing 8 filters!)

		anees munshi


	

pff@thumper.bellcore.com (Peter Ferris) (01/26/89)

Greetings,
Excuse me if this is something that's been asked 583 times already, but I'm
attempting to build a DTMF (Touch-Tone) --> RS-232 converter.  I've seen the
units in the magazines for $90, but it seems to me that's about $75 too much!
Does anybody have any experience in this area?

My specific questions are:

1)  I guess the choices seem to be the SSI-202 (Radio Shack = c.$13.00)  or the
SSI-204 IC's.  Who makes these (Samsung?), and what is the difference between
them?

2) How should I (or SHOULD I) control baud rate; eg; dipswitch and/or software?
Or would it be simpler / cheaper / "better"  to decide on a "fixed" baud rate
(would 300 baud be fast enough for a very rapid burst?)?

3) I need to decode all <16> characters (0-9, A-D, # & *).

4) The circuit should be somewhat "forgiving" in that the DTMF will be decoded
from a variety of audio sources, ie; scanner, handie-talkie, vcr, and other
audio sources - but the audio could have static or other artifacts in it.

Permit me to add that this project is what we call a "G-Job" - it's for my own
personal pleasure / experimentation.  If anyone has any suggestions, comments
or schematics on this rainy day project I'd sure appreciate hearing from you.  
Best bet is Internet E-mail.

Many Thanks in Advance,

Pete Ferris, N5KBD
pff@thumper.bellcore.com

byron@pyr.gatech.EDU (Byron A Jeff) (01/26/89)

-Greetings,
-Excuse me if this is something that's been asked 583 times already, but I'm
-attempting to build a DTMF (Touch-Tone) --> RS-232 converter.  I've seen the
-units in the magazines for $90, but it seems to me that's about $75 too much!
-Does anybody have any experience in this area?

I helped build a DTMF decoder for a school project last quarter. I also
currently have a homebuilt 68010 computer with a rs232 port. 

Price should be between $20-$25 depending on what you use.

-
-My specific questions are:
-
-1)  I guess the choices seem to be the SSI-202 (Radio Shack = c.$13.00)  or the
-SSI-204 IC's.  Who makes these (Samsung?), and what is the difference between
-them?

Silicon Systems Incorporated. Tustin, Ca. (714) 731-7110.

I happen to have their data book in my office. Let's see... Description...

SSI-202 - The SSI 202 (and 203) are complete DTMF receivers detecting a
selectable group of 12 or 16 standard digits. No front end prefiltering
is needed. The only externally required components are an inexpensive
3.58-MHz telvision "colorburst" crystal (available from Radio Shack
among others - Ed.) and a bias resistor... Both are CMOS, use 5V and
packaged in a 18 pin DIP

SSI-204 - 14 Bit version of the 202. Uses same external components.
It's missing some of the more esoteric pins like Inhibit last row decoding,
Binary coded 2 of 8, Clear Data available, early detection. Real
big loss ;-)

Both have 3 state outputs and a data available output for direct connection
to a microcomputer.

-
-2) How should I (or SHOULD I) control baud rate; eg; dipswitch and/or software?
-Or would it be simpler / cheaper / "better"  to decide on a "fixed" baud rate
-(would 300 baud be fast enough for a very rapid burst?)?
Well... Let's see

(300 b/s) * (1 char / 10 bits) = 30 char / s 
So 1 char every 33 ms. Ok.

Now according to the data sheet both the time to detect the tone and then
the following pause is 40 ms apiece. So with 80 ms to do 1 digit 300 bps
should be plenty fast enough.

-
-3) I need to decode all <16> characters (0-9, A-D, # & *).
No problem. Both decoders do the job.

-
-4) The circuit should be somewhat "forgiving" in that the DTMF will be decoded
-from a variety of audio sources, ie; scanner, handie-talkie, vcr, and other
-audio sources - but the audio could have static or other artifacts in it.

The data sheet claims to be able to detect with S/N of 12db. That's really
really noisy. They warn that if you have signal above 28kHz it can alias
back into the audio range and that it needs to be filtered.

-
-Permit me to add that this project is what we call a "G-Job" - it's for my own
-personal pleasure / experimentation.  If anyone has any suggestions, comments
-or schematics on this rainy day project I'd sure appreciate hearing from you.  
-Best bet is Internet E-mail.

Two ideas immediatly spring to mind.

No micro
--------
                      ____-------              ------  -----            
              |------|____|     |              |    |  |MAX|             
 audio in ----| DTMF |____|EPROM|-8 data lines-|UART|--|232|--ascii serial out
              |------|____|     |              |    |  |   |             
                 |        -------              ------  -----            
                 |                                |                            
                 |-Data valid from DTMF to UART send                

Rather quick and dirty. Whenever the DTMF decoder receives a digit the
EPROM changes it to ascii, the data available of the decoder does a
write to the UART which sends the converted value out the serial port.
By using a MAX 232 tranceiver you can do the whole circuit with 5V supply.

Micro
-----
Replace the EPROM and UART with a EPROM microcontroller like a 68701
or an 8051. Of course then you can program it on the fly with different
baud rates and different conversions by sending data back out the serial
port. Or put an encoder in the circuit for two way communication. Or
any number of rather nifty things.

Another part you may want to look at is the SSI 2090C. All the 
functionality of the 202 plus an encoder and a simplistic ring
detection system.

-
-Many Thanks in Advance,

Sure enough. Let me know if anything comes of it.

-
-Pete Ferris, N5KBD
-pff@thumper.bellcore.com

BAJ

-- 
Another random extraction from the mental bit stream of...
Byron A. Jeff
Georgia Tech, Atlanta GA 30332
Internet:	byron@pyr.gatech.edu  uucp:	...!gatech!pyr!byron

scotts@bu-cs.BU.EDU (Scott Statton) (01/26/89)

Two points about the SSI 20x series of decoders.

The DTMF detector works VERY nicely -- we put it through our "torture"
test and it passed with flying colors.  What, you may ask, is the
torture test?  Pick the ickiest Inter-Exchange Carrier [we chose one
famous for telegrams, not so famous for phone service].  Call a RASU
on the opposite coast [This was done in Boston, so RASU was in
Mountain View, CA] and call back in to the decoder under test.  Send
DTMF at very high rate [12 digits per second] and count total number
of errors in a 5 mintue test.  As I recall, the chip weighed in at 7
mis-decodes.

Other point:  They also make a very nice MF decoder called the
SSI-207.  Unfortunately, they're about $70 in small quantities.  It
appears not everybody uses MF as much as DTMF.  Great for those
semi-secure application machines.  Try & bust me for Blue-Boxing -- my
computer uses it for remote control applications.  [Or, I'll just use
the test-frame at work]

  scotts@buit.bu.edu ... aka N1GAK ...

 "The full-time phone hacker who spends an unholy amount of time with
his Dan-Ray CTSS-4000"

larry@kitty.UUCP (Larry Lippman) (01/26/89)

In article <1441@thumper.bellcore.com>, pff@thumper.bellcore.com (Peter Ferris) writes:
> Excuse me if this is something that's been asked 583 times already, but I'm
> attempting to build a DTMF (Touch-Tone) --> RS-232 converter.  I've seen the
> units in the magazines for $90, but it seems to me that's about $75 too much!

> Does anybody have any experience in this area?

	You rang? :-)

> 1)  I guess the choices seem to be SSI-202 (Radio Shack = c.$13.00)  or the
> SSI-204 IC's.  Who makes these (Samsung?), and what is the difference between
> them?

	They are both made by Silicon Systems.  Both of these IC's are almost
identical, but the SSI-202 is a somewhat better quality device with a few
more control lines (which may be unnecessary for your application, though).

> 2) How should I (or SHOULD I) control baud rate; eg; dipswitch software?
> Or would it be simpler / cheaper / "better"  to decide on a "fixed" baud rate
> (would 300 baud be fast enough for a very rapid burst?)?

	Well, if *I* were going to build this device, I would use an 8571
(or 8031 with external eprom) single-chip microprocessor with built-in
uart; the programming is trivial and can be done in less that 50 lines of
assembly language.  The microprocessor could provide two-way serial
communication to set options via software command.  You won't have any
convenient software method of setting options without some intelligence.

	However, you could probably get away with a simple uart with
internal baud rate generator for hardware "programmed" operation.  You
can use the valid tone detect pair output (DV pin) to strobe the uart
transmitter.

	For fixed baud rate, 300 baud is fine.  No DTMF system will EVER
dial that fast; typical autodialing speed is 10 digits/sec.


> 3) I need to decode all <16> characters (0-9, A-D, # & *).

	No problem.

> 4) The circuit should be somewhat "forgiving" in that the DTMF will be decoded
> from a variety of audio sources, ie; scanner, handie-talkie, vcr, and other
> audio sources - but the audio could have static or other artifacts in it.

	That's a tough call.  One basically gets what one pays for, and
these ciruits are not THAT forgiving - but they may be fine for your
application.  If you want forgiving circuits, you will have to spent more
$$$ and go to a quality device like those made by Tel-Tone or Mitel.
Tel-Tone is on the Net, so maybe someone will give you more specific
advice about their products.

<>  Larry Lippman @ Recognition Research Corp., Clarence, New York
<>  UUCP:  {allegra|ames|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry
<>  VOICE: 716/688-1231          {att|hplabs|mtune|utzoo|uunet}!/
<>  FAX:   716/741-9635 {G1,G2,G3 modes}   "Have you hugged your cat today?" 

sl@van-bc.UUCP (pri=-10 Stuart Lynne) (01/26/89)

In article <7139@pyr.gatech.EDU> byron@pyr.UUCP (Byron A Jeff) writes:
>-attempting to build a DTMF (Touch-Tone) --> RS-232 converter.  I've seen the

>I helped build a DTMF decoder for a school project last quarter. I also
>-SSI-204 IC's.  Who makes these (Samsung?), and what is the difference between

Now that we can decode DTMF, how hard would it be to add some functionality
to product a dial tone. 

For example the device should go offhook when "DTR" is raised. A dial tone is 
produced when RTS goes high and DTMF recogition starts.

Dial tone should disappear after first digit typed, or when RTS is dropped
(whichever comes first).

It would also be nice to be able to plug this in between a modem and phone
line, so the ability to make modem think the phone is ringing after you have
gathered data from person would be nice. (It could also be between modem and
computer serially so you wouldn't need a second serial port for it). 

-- 
Stuart.Lynne@wimsey.bc.ca {ubc-cs,uunet}!van-bc!sl     Vancouver,BC,604-937-7532

jpd@usl-pc.usl.edu (DugalJP) (01/27/89)

Pete, I recently purchased the Radio Shack DTMF decoder chip.  Looks
very easy to interface.  Although you asked about RS-232 interfacing,
I think it would be easy to interface it to a PC's parallel port.  Phil
Karn posted some time back on rec.ham-radio about how to modify a
parallel IO card for 8-bit input.  I'm not sure a stock card would have
enough input lines .... I think you'd need 5 (4 data plus strobe).

-- James

-- 
-- James Dugal,	N5KNX		USENET: ...!{dalsqnt,killer}!usl!jpd
Associate Director		Internet: jpd@usl.edu
Computing Center		US Mail: PO Box 42770  Lafayette, LA  70504
University of Southwestern LA.	Tel. 318-231-6417	U.S.A.

gbell@pnet12.cts.com (Greg Bell) (01/27/89)

 
     I've built DTMF decoding circuits for a variety of applications.  The
SSI-202 chip seems to work well.  In fact, I've got one hooked up to a
microcontroller project right now!  By the way, SSI stands for Silicon Systems
Inc.   You might also want to look into Teletone's DTMF decoders.  
 
     For Teletone's number, call 1-800-555-1212 (800 information).
 
 
     As for interfacing the output of one of these to RS-232, you need a UART.
 This is a chip that will take the 4 bit output of the decoder and convert it
into serial information.  There are several of these available.  General
Instruments makes one called the AY-3-1015.
 
     Both of these chips are extremely easy to use.  However, you might have a
problem with the noise levels of the signals you are trying to feed in.  Seems
to me like the success or failure of your project might lie in how good the
input amplifier circuit is.  The app. engineers at Teletone might be able to
help you out with this...
 
 
    Greg Bell_________________________________________________________
      Hardware hacker          |
      Electronics hobbyist     | UUCP:  uunet!serene!pnet12!gbell
      EE major at UC San Diego |

gilley@goofy.apple.com (Tom Gilley) (01/27/89)

In article <1441@thumper.bellcore.com> pff@thumper.bellcore.com (Peter 
Ferris) writes:

>  I'm attempting to build a DTMF (Touch-Tone) --> RS-232 converter.


Hints:

The Radio Shack (on sale today) DTMF decoder will give you four parallel 
bits which you can connect to the parallel input side of a simple UART.  
The AY-3-1015D UART for TTL or the IM6402 UART for CMOS are easy to use.


Tom Gilley

keith@m5.UUCP (Keith J. McQueen) (01/28/89)

Myself along with a group from our local Ham Radio ARES group
has designed a DTMF decoder circuit that we use for DTMF signalling,
remote control and paging.  One of the many features it incorporates
is a CMOS level ASCII serial output (RS-232 but 5v levels) of all
tones recieved.  The project uses a microprocessor for control, and
draws about 12 microamps when no audio is present.  Costs about $50
to build.

This could be what you are looking for.