hollands@hale.UUCP (Chris C. Hollands) (04/25/91)
Hello. I'm new to "comp.dcom.telecom".
I need a kind person's help to design a circuit. The circuit is for a
device to be used on my office telephone line. I know of no
commercially available product that can do what I need this circuit to
do. Here is a (probably too verbose) explanation of what I need help
with.
The purpose of the device is to be able to remotely use the three-way
conference call feature offered by the telephone central office.
The single-line telephone in my office has three-way conference
capability. Somebody can call me at work, then I can put that caller
on hold so I can dial a third party, and then I can bring the original
caller back on the line. The company I work for also lets me access a
private long-haul network that it uses to carry its business long
distance calls at economical bulk rates. Often I need to call overseas
to Europe or southeast Asia to conduct business, but the time
difference sometimes makes it inconvenient to call during normal
office hours. Rather than having to drive to my office late at night
or in the wee hours of the morning to use that phone, I would like be
able to call from my home phone and use the device (attached to the
line in my office) to make the economical long distance call.
The circuit should operate like this:
1. It will detect an incoming call and go "off hook," similar to the
way a computer modem answers a call (minus the carrier tone).
(From this point on, the circuit (device) should "beep" if five
minutes elapse without detecting a DTMF tone. It should go "on hook"
(hang up) several seconds after the beep unless it hears a DTMF tone.
Any DTMF tone except "*" should cause this timer to reset for another
five minutes. The purpose of the timer is to make the device hang up
in the event the caller was cut off. Any time it detects a "*" tone,
it should immediately hang up.
2. After answering the call, the device should accept a four-digit
security code (DTMF tones) and emit two beeps, indicating to the
caller that the security code was correct. If the caller enters an
incorrect security code, the device should abruptly hang up and "wait"
at least one minute before being able to answer a subsequent call.
(The security code will be manually set by concealed thumbwheels or
dip switches inside the device. The purpose of waiting a minute after
an incorrect code is to discourage someone repeatedly calling to try
to learn the code.)
3. If the security code was correct, then after the two beeps the
device will accept and store up to 20 DTMF digits, the content of
which is the third party telephone number, terminated with a "#" tone
meaning "done." (Don't store the "#" tone.)
4. The device will then perform the electronic equivalent of a "hook
flash" (approximately 200 to 500 milliseconds duration), which has the
effect of putting the original caller temporarily on hold. The hook
flash duration should be manually adjustable to allow for telephone
central office compatibility.
5. The device will then wait approximately one second (or detect dial
tone), and then transmit the stored telephone number as DTMF tones at
a normal dialing pace (the way a computer modem dials).
6. When the string of digits has been transmitted, the device will
then wait approximately two seconds for the telephone central office
to begin processing the call, and then hook flash again to bring the
original caller online, thus establishing a three-way conference call.
7. Thereafter, the device will "listen" for a DTMF "*" tone,
indicating the end of the conference call, and then hang up, reset
itself, and wait for the next call. In this case there is no need to
"wait" a minute, as it would have done for an incorrect security code.
(As previously mentioned, during the conference call the device will
produce a warning "beep" every five minutes. To continue the
conversation, the original caller must press any digit or "#". This
should reset the timer and allow the conversation to continue another
five minutes. However, when both parties finish the conversation, the
original caller should press "*" before hanging up, to tell the device
to immediately hang up itself.)
Also, for the sake of power outage, the device should default to an
"on hook" mode.
Well, this conceptually simple device is pretty far beyond my
experience level. I think it requires a single-chip computer with a
bit of programming, perhaps some relays, a DTMF decoder and encoder, a
power supply, etc. Thanks in advance for your help.
Please reply privately to "cholland@nosc.mil" or post here.
Note: I saw Larry Casterline's email about just such a device and I am
trying to contact him directly. However, I would still like to build
the device I described above. Thanks.
Chris Hollands Chula Vista, CA cholland@nosc.mil, hollands@hale.uucp
HALE TELECOMMUNICATIONS - Public Access Node, San Diego
619/660-6734Jon Sreekanth <jon_sree@world.std.com> (05/02/91)
In article <telecom11.308.6@eecs.nwu.edu> hollands@hale.UUCP (Chris C. Hollands) writes: > The purpose of the device is to be able to remotely use the three-way > conference call feature offered by the telephone central office. > Well, this conceptually simple device is pretty far beyond my > experience level. I think it requires a single-chip computer with a > bit of programming, perhaps some relays, a DTMF decoder and encoder, a > power supply, etc. Thanks in advance for your help. > Note: I saw Larry Casterline's email about just such a device and I am > trying to contact him directly. However, I would still like to build > the device I described above. Thanks. If you've already contacted Larry, that's one good off-the-shelf solution. If you don't mind programming, you can do most of what you wanted with your own modem. Assuming you have a modem in a PC in your office, you have most of the pieces. I think most modems don't have DTMF detectors, but they can generate DTMF. Hayes compatible modems have standard, interrupt driven responses to telephone line activity and to program commands. I don't know the bit locations and such, but it should be possible to do the following: Program a TSR (terminate and stay resident), and leave your pc at work powered up. The modem will sense an incoming ring, pick up the phone, and then hang up. Then it dials your home number (that gets you the security you need), waits, flashes the switchhook, gets another dial tone, and makes the second call. Getting it to hang up after the call is over is a little messier, but once you and the other party hang up, there should be a dial tone after a few seconds, and the modem can sense that to hang up. If you have a fax card based on the Yamaha chip, the chip has a DTMF decoder, so that's a complete solution. I don't know about Rockwell and Sendfax(tm) type chips (anyone know ?). Programming all this stuff might take more time than it's worth, of course. Regards, Jon Sreekanth Assabet Valley Microsystems Fax and PC products 346 Lincoln St #722, Marlboro, MA 01752 508-562-0722 jon_sree@world.std.com