TD4 DTMF Decoder/Controller

TD4 DTMF Decoder/Controller
HAMTRONICS® TD-4 SELECTIVE CALLING UNIT
AND ECONOMY DTMF CONTROLLER
ASSEMBLY, INSTALLATION, OPERATION, AND MAINTENANCE
TABLE OF CONTENTS
GENERAL DESCRIPTION. .............................................................1
PC BOARD HANDLING PRECAUTION. ...........................................1
CONSTRUCTION. ..........................................................................1
INITIAL TESTING. .........................................................................1
PROGRAMMING............................................................................1
How To Program.......................................................................1
Programming Options................................................................2
INSTALLATION. ............................................................................2
Mounting. .................................................................................2
Power......................................................................................2
Tone Inputs. .............................................................................3
Local Switches. ........................................................................3
Latching Output Transistor Circuit. .............................................3
Momentary Outputs. ..................................................................3
LED Indicators. ........................................................................3
Selective Calling Applications.....................................................3
Emergency Alert Decoder Application. .......................................4
OPERATION. ................................................................................5
General....................................................................................5
Operation as DTMF Controller....................................................5
Operation as Selective Calling Adapter. ......................................5
CUSTOMIZING. .............................................................................5
THEORY OF OPERATION. .............................................................5
TROUBLESHOOTING.....................................................................5
General....................................................................................5
Power Supply Problems. ...........................................................6
Erroneous Logic Operation........................................................6
Tone Level................................................................................6
Tone Quality.............................................................................6
Tone Frequency. ......................................................................6
Tone Level Twist. ......................................................................6
PARTS LIST..................................................................................8
SCHEMATIC AND COMPONENT LOCATION DIAGRAMS ............. 8,9
GENERAL DESCRIPTION.
The TD-4 Selective Calling Module is
an economy touch-tone decoder with
one latching output. This versatile module is primarily designed to mute the
speaker of a receiver or transceiver until
someone calls by sending a 4-digit dtmf
signal, thus making it unnecessary to
listen to all the activity on a channel
just so someone can call you once in
awhile. When the proper dtmf digits are
received, the speaker is unmuted and
an LED is illuminated to let you know a
call was received. When you are done
listening, a pushbutton switch (not
supplied) is pressed to reset the latch
and mute the speaker until another call
is received. The latch may also be reset
by another dtmf command.
The TD-4 also may be used as a
simple dtmf decoder/controller to turn
on an autopatch or any other single device which requires a simple ground to
activate it. It may be used with the AP-3
Autopatch module, for instance, but it
would not provide toll call restriction
and tone muting as the more elaborate
TD-2 DTMF Decoder/Controller Module
does.
The 4-digit dtmf address is easily set
in the field with wire jumpers. The 2¾
inch square pc board is easily packaged
for custom installations, and it operates
on 7-14Vdc at only 10-15mA. Low
power consumption is made possible by
the exclusive use of CMOS ic's.
The TD-4 uses a central-office quality dtmf decoder ic with built-in antifalsing circuits. It automatically accepts
any audio source from 100mV to 2Vp-p.
No alignment is required.
The latch output is an open collector
npn transistor, which can ground any
dc circuit up to 15Vdc and sink up to 50
mA. This can be used, in the case of a
selective calling application, to ground
an audio line to mute it or operate a
small relay in series with an external
speaker on a receiver. In the case of
controlling another module, like an
autopatch, usually you can use the
output transistor directly to ground a
control circuit on the target module.
PC BOARD HANDLING
PRECAUTION.
Be careful whenever you handle the
module or any of the ic's. Even though
static damage occurs most easily before
ic's are installed in their sockets,
damage can still occur to the ic's in a
completed module if a static discharge
occurs at any part of the board during
handling. Although wrist straps are not
absolutely necessary just to handle the
completed board, you should make it a
habit of discharging your hand to a
grounded object before touching a
CMOS module.
CONSTRUCTION.
The pc board is double-sided with
plated-through holes. Because it is
more difficult to unsolder from this type
of board, be sure parts are oriented
properly and of the correct value before
soldering. Pc traces are close together,
so use a fine soldering iron tip.
During construction, orient the
board right side up as shown in the
component location diagram. The top
side as shown has the pin number
©1999 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark.
Revised: 1/31/00
- Page 1 -
markings. Also refer to the parts list and
schematic diagram during construction.
L The CMOS ic's used in this kit are
all static sensitive; so handle them with
care. A grounded wrist strap should be
worn whenever you pick up an ic. Do
not open the packages containing the ic's
until you are ready to install them, and
then handle them only with suitable static
control measures.
a. Install ic sockets oriented with
notches as shown, and solder.
b. Install led's DS1 and DS2 on the
board, observing polarity. The anode
(positive) lead is longer.
L Small LED's are heat sensitive.
Keep the LED's up off the board about
1/4 inch and solder quickly, applying
minimum heat to avoid damage.
c. Install electrolytic capacitors,
observing polarity. Longer lead is positive.
d. Install Q1 and U5, observing
orientation.
e. Install
diodes
CR1-CR3,
observing polarity. Vertically mounted
parts are illustrated with a circle indicating the body of the part, and the diode
symbol next to CR2 on the diagram
indicates the polarity.
End of diode
body with band is cathode and should be
toward board when oriented as shown.
f. Install resistors.
g. Install the remaining parts other
than the dip ic's, according to the parts
list.
h. Check over construction to be
sure all parts are installed in proper
places and with proper polarity and that
solder connections are good. Look for
things like cold solder joints and solder
splashes. Programming wires will be
installed later.
i. Using static protection described
earlier, carefully unpack the dip ic's and
install them in the sockets. Be sure to
orient them as shown, according to
notch on the end of each ic. Be careful
to engage all the pins in the socket;
don't let any leads bend over.
j. Refer to the PROGRAMMING section below to determine jumper wires
which must be installed. Wired and
tested units are factory-wired with
jumpers as shown in the detail drawing
above the component location diagram.
INITIAL TESTING.
Although it is not necessary, you
may wish to test the tone decoder por-
tion of the board before you proceed with
programming. To do so, connect a power
supply ground to the ground trace at
one of the four corners of the board, and
connect +10 to +15 Vdc to the power
input at pad E3. Connect a source of
touch tones to audio input pad E1, with
a level between 100 mV and 2V p-p. The
outputs
at
the
three
rows
of
programming pads noted as "decoder
outputs" in the component location
diagram should have a CMOS logic level
(lo close to 0 or hi close to +5V) with the
hi corresponding to reception of a valid
touch tone digit. You can test all 16
decoder outputs to verify that they
respond to corresponding dtmf digit
commands.
PROGRAMMING.
How To Program.
Regardless of what the TD-4 is to be
used for, it must be programmed to
respond to a particular set of dtmf
commands. Normally, this will be a four
digit command to turn on the output (set
the latch) and a similar four digit
command to turn off the output (reset).
The first three digits are the same for
both commands and the fourth digit
determines whether it turns on or off.
There is an option to use fewer than
four digit commands. This is covered in
the next section in order to keep the
following discussion simple.
Programming is done by soldering
short jumper wires between the desired
"decoder outputs" and the "latch inputs".
This is easy to visualize if you compare
the schematic diagram with the
component location diagram. Take a
minute now, and do that before reading
further.
Now, look at the three rows of
decoder outputs surrounding the U2
"16-line decoder" chip in the component
location diagram. Those 16 output pads
provide connections to the outputs
corresponding to the 16 possible digits
on a full dtmf pad. The five "latch input"
pads, in the second row below U2, are
the five inputs which need to be
programmed. A jumper wire from each of
these five pads is required to be
connected to one of the "decoder
outputs" above.
Following is an example of the codes
established to set and reset the latch, as
used in the example illustration shown
above the component location diagram.
In this example, we used the code "123[
" to turn on (set) the latch and "123#" to
turn off (reset) the latch. As you see, the
same first three digits are used to turn it
on and off. Only the fourth digit is
different.
Example Programming Codes
DIGIT
1
2
3
4S
SET CODE
2
1
3
[
RESET CODE
2
1
3
4R
#
Although you can use any digits you
want, there are a few guidelines to make
it easier.
First, you should consider the first
three digits as your personal code,
unique to your application. These digits
should prevent the circuit from responding to anyone else's codes or
random dtmf tones the receiver might
hear on the air. These are the same
three digits in both the on and off
commands.
Second, the same digit should not be
used for two consecutive latch inputs
because both would be triggered at once.
Use a different digit for each latch input.
The last digit of each command
should be something logical to indicate
on or off, something easy to remember.
One common practice in many dtmf
operated devices is to use "[" for "on"
and "#" for "off". This is why we chose
the example codes as we did. They are
also digits which are not used in telephone numbers; so it prevents your unit
from responding to a series of dtmf digits
used as part of a phone number someone
is dialing for an autopatch.
Finally, remember that the "set" and
"reset" functions are named relative to
the active state of the output transistor,
i.e., "set" makes the output transistor
turn on and "reset" makes it turn off.
When planning your programming, take
into account how the output transistor is
to be used.
In a selective calling
application, for instance, setting the
latch will normally mute the receiver
and resetting it will allow you to hear
the receiver. In a case such as this, you
will want to program the "reset" function
(not "set") to allow the receiver to be
unmuted with a dtmf command; so don't
let that confuse you.
Following is a blank table you can
fill in with the codes you choose. The
dashes prevent entering a digit where
©1999 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark.
Revised: 1/31/00
- Page 2 -
inappropriate for each command, the
fourth set digit being different from the
fourth reset digit. The X's on the reset
line indicate that the same digit is used
for reset as was already programmed for
the set function; so it is not necessary to
enter anything for the first three digits of
the reset command.
My Programming Codes
DIGIT
1
2
3
4S
X
X
X


SET CODE
RESET CODE
4R
Once you have planned the codes to
be used, you can solder jumper wires on
the board. Depending on the location of
the decoder output pads needed for any
given jumper, the length of the jumper
can be determined.
A convenient
length for jumpers that must span the
width of ic U2 is 2 inches. For jumpers
which don't need to cross the ic, shorter
jumpers may be used, about 1 inch long.
The ends of the jumper wires should be
stripped 1/8 inch, inserted into the
pads, and soldered under the board.
Check the location of each jumper before
it is soldered.
Programming Options.
Normally, four digits are used in
each command; however, modifications
can be done to allow operation with
fewer digits. Here are the options.
To use three digits instead of four for
the set and reset commands, connect
latch inputs 1 and 2 both to the same
decoder output pin. For example, if you
want to use the codes "13[" and "13#",
connect latch inputs 1 and 2 to the
decoder output for the digit "1". That
way, the first digit will trigger the first
two latch inputs simultaneously. Latch
input 3 will be jumpered to decoder
output "3", latch input 4S will be
jumpered to decoder output "[", and
latch input 4R will be jumpered to decoder output "#".
To use only 2 digits instead of 3, a
similar trick is used. The first three
latch inputs are all connected together
to the decoder output for the first digit.
The 4S and 4R latch inputs are each
connected to their corresponding decoder output digits.
To use a dtmf command to set the
latch and use only a local pushbutton
switch to reset the latch, as might be
done in a selective calling application,
jumper latch input 4R to ground. This
must be done because it is not a good
practice to leave cmos gate inputs unterminated. Because of their high input
impedance, the voltage at the input gate
would float up and down, causing
erratic operation.
To use only one digit commands,
unplug U3 so it is out of the circuit.
Jumper the decoder output digits that
you will use for set and reset directly to
E6 and E5 at the left edge of the board.
INSTALLATION.
L Be careful whenever you handle
the module. Even though static damage
occurs most easily before ic's are
installed in their sockets, damage can
still occur to the ic's in a completed
module if a static discharge occurs at any
part of the board during handling.
Although wrist straps are not absolutely
necessary just to handle the completed
board, you should make it a habit of
discharging your hand to a grounded
object before touching a cmos module.
Mounting.
The board should be mounted to
chassis with 4-40 standoffs about 3/8 in.
long in the four corners of the board.
Placement of the board is not critical,
but the board should not be mounted in
a strong rf field. In a repeater, the
transmitter already is shielded; so
usually, you don't need to do anything
special.
Power.
The unit is designed to operate on +7
to +14Vdc at about 10-15 mA. The low
power consumption is due to the CMOS
circuitry. A voltage regulator on the
board takes care of voltage variations
within the range specified, but be sure
you use filtered dc power and don't
allow any spikes or reverse polarity to be
applied. The positive
power supply connection should be made to
pad E3 on the TD-4
board.
Be sure to get a
good dc and signal
ground through the
RESET
mounting hardware. (If
you cannot get ground
SET
connection
through
the hardware, connect
a ground wire from
Figure
your power supply to pad E2 on the
board.)
Tone Inputs.
The range of audio tone levels which
the tone decoder ic will accept is 100
mV to 2V peak-peak. Audio can be
applied from any source, including
radio receivers, and telephone lines
(with some sort of interface such as an
autopatch board). The audio source
must be referenced to ground.
Check to be sure that your source is
compatible. If not, some adjustment will
have to be made. If you cannot alter the
level from your source to within this
range, you may be able to change the
sensitivity of the tone decoder to some
extent. See the CUSTOMIZING section
later in the manual.
If you interface with our AP-3
Autopatch Board, you don't need to be
concerned about compatibility.
The
tones for the TD-4 should come from E8
on the Autopatch Board.
If you are using the TD-4 as a selective calling unit, see the section on
Selective Calling Applications later in
this manual.
Local Switches.
If you want to use a pushbutton
switch to allow manually setting or resetting the latch, wire it as shown in
Figure 1. For example, in a selective
calling application, it makes sense to
use a dtmf command to trigger the
"reset" input to open the squelch so you
can hear the message the person calling
you transmits. However, you probably
will want to close the squelch by setting
the latch in the TD-4 manually with a
little pushbutton switch installed on the
case you mount the TD-4 in. You might
also want to be able to open the squelch
manually to monitor; so it also makes
TD-4 PC BOARD
E1
E2
E3
E4 +5V OUTPUT
E5 - LATCH RESET INPUT
E6 - LATCH SET INPUT
E7
1.
Wiring for Manual Set/Reset
©1999 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark.
Switches
Revised: 1/31/00
- Page 3 -
sense to have a second pushbutton
switch for the "set" function, even
though that function would be done by
dtmf command most of the time.
Latching Output Transistor Circuit.
The output transistor Q1 can be used
to drive solid state circuits directly, and
it can drive small relays to switch power
loads. The output transistor can sink
loads up to +15V and 50 mA. An
example of using the TD-4 to turn on a
dc circuit directly would be using the
output transistor to ground a control
terminal on our AP-3 Autopatch module
to activate the patch.
L If you drive an inductive load, like
a relay coil, be sure to connect a diode
with reverse polarity across the load to
absorb any inductive spikes which could
damage the transistor.
If you are making a selective calling
device, refer to Selective Calling Applications, which follows.
Momentary Outputs.
In some applications, you may need
just a momentary logic output and not a
latching output. Outputs at various
points in the logic circuits can be used
externally, providing not too much load
current is drawn. These are cmos ic's;
so the available output current is only
about 500µA. This is sufficient to turn
on a small signal transistor if you put a
resistor in series with the output voltage
going to the base of the transistor. The
resistor should be 47K or greater.
One option available is to use the
decoder outputs for individual dtmf
digits directly from the output of 16-line
decoder U2. These are accessible on
pads surrounding U2, as shown in the
component location diagram.
Another option is to use the logic
result of the four-digit logic circuits,
which are provided by U3 (see schematic). In this case, the "set" and "reset"
outputs of U3A and U3B are available on
pads E6 and E5 at the left edge of the
board.
LED Indicators.
There are two led's on the board,
which indicate the status of the various
circuits. DS1 is illuminated whenever a
valid dtmf signal is received. DS2 indicates when the output circuit is turned
on, i.e., output transistor Q1 is conducting. Although these were included
primarily for use in testing, it is possible
to remove them from the board and
carefully extend them to a front panel
with hookup wire.
Selective Calling Applications.
Since many TD-4 owners will want
to use it for selective calling (paging)
operation, we cover all the details here
in one place. Many of these items have
been discussed earlier, but we want to
review them so you have a good idea of
everything that is needed in one
comprehensive discussion.
The first thing to do in setting up a
selective calling adapter for your receiver is to decide how the receiver
audio will be muted so you know if the
muted situation corresponds to transistor Q1 conducting (turned on) or nonconducting. This establishes the sense
of the set and reset functions. Normally,
it is easiest to use the output transistor
to short some point in the audio path to
ground when you want to mute the
receiver.
Another way to mute the
speaker is to use a small relay to interrupt one of the speaker wires, but this
should be a last resort.
Figure 2 shows a typical installation.
The output transistor is used to ground
the wiper of the volume control in the
receiver to mute it until the dtmf code is
received to unmute the audio. (We are
not referring here to your receiver's
mute circuit but just our added muting
circuitry, which effectively does the
same thing.)
In order to mute the audio, it is
necessary to find a point in the audio
path which has some resistance in series
with it so the ground can work, in effect,
as the bottom half of a voltage divider;
the top half is the resistance in the audio
path, and the bottom half is the almostzero resistance of the turned on
transistor.
In this case, we were able to make
the assumption that we would never run
RECEIVER
DISCRIM.
DE-EMPHASIS
NETWORK
ADDED
RESISTOR
(SEE TEXT)
AUDIO OUTPUT
AMPLIFIER
TD-4 PC BOARD
VOLUME
CONTROL
E1 - AUDIO INPUT
E2 - GROUND
E3 +12V POWER INPUT
+12Vdc
RESET
SET
E4 +5V OUTPUT
E5 - LATCH RESET INPUT
E6 - LATCH SET INPUT
E7 - OUTPUT TRANSISTOR
Figure 2. Wiring for Selective Calling Applications
©1999 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark.
Revised: 1/31/00
- Page 4 -
the volume control over about ¾ throttle;
so there will always be some resistance
in the top half of the volume control to
work against. This is important because
if you short the point you use to feed the
dtmf tones to the input of the TD-4
board, once you mute the receiver, there
will never be any audio (dtmf tones)
applied to the TD-4 to allow the
appropriate code to unmute the receiver
again.
If you want to be able to run the
volume control wide open sometimes, or
you cannot short the wiper of the volume
control to do the muting, then it may be
necessary to add some fixed resistance
in series with the audio to the volume
control to work against, as shown if
Figure 2. The resistor value should be at
least half the value of the volume
control.
It may also be possible to mute the
audio by using the output transistor of
the TD-4 to ground some dc control
point in your squelch circuit. To do so,
however, requires that you have a good
knowledge of how the squelch circuit
works, and it requires that this method
does not squelch the audio needed to
provide dtmf tones to the TD-4 input
circuit.
The audio source for E1 of the TD-4
board must be at a level of at least 100
mV p-p with dtmf tones present. This
can be taken from any point in the receiver after the detector. In the case of
an fm receiver, it must also be after the
de-emphasis circuit so that the high and
low tones will be at about equal levels
and with minimum distortion. Do not
take audio from a "discriminator" output
of the receiver if it is unfiltered or not
de-emphasized audio.
L Note that the dtmf decoder ic in the
TD-4 has anti-falsing features which
prevent any response if the audio tones
are distorted or have excessive twist
(large difference in level between high
and low tone of a pair.)
It is important to have a good ground
connection between the receiver and
the TD-4 to minimize hum and noise
and provide good quieting when the
receiver is muted. Connect a ground
wire between E2 on the TD-4 and the
ground plane on the receiver board near
the audio circuits.
It is also important to keep the audio
and ground leads between the receiver
and TD-4 relatively short to avoid noise
and hum. Normally, 12 inches should
be OK. Remember, you are connecting
directly into the low level audio stages of
the receiver.
DC power for the TD-4 can be any
regulated and filtered dc power source
of at least 7Vdc but not over 14Vdc. The
TD-4 has an on-board 5V regulator. You
should be able to steal the power from
the receiver, since only 10-15mA is
needed.
The manual "set" and "reset" switches were described earlier. You probably
want to install both on the panel of your
enclosure, one to mute the audio until
the required dtmf digits are received and
one to unmute it when you want to
listen.
If you want to do the extra work, it
would be nice to extend the led's to the
panel of the enclosure as described earlier.
Emergency Alert Decoder
Application.
Recently, some amateurs have
established an unofficial standard for
alerting listeners to emergency traffic. A
number of clubs have endorsed this
standard, which calls for a dtmf 0 (zero
digit) to be sent for six seconds to open
monitors. This is referred to as a "LiTZ"
system, or "long tone zero" system, as it
is referred to in a Dec 1992 73-Magazine
article.
It is easy to modify the latch input
circuits on the TD-4 to perform this
function, ie, mute the receiver until a
dtmf zero has been received for six
continuous seconds.
Refer to the
schematic diagram. To make the TD-4
respond this way, first remove diodes
CR1-CR3, which makes each "and" gate
respond slowly, in about 2 seconds,
instead of instantly, as happens with the
diodes in place. Then, tie all four inputs
to the "0" decoder output pad; these
being latch inputs 1, 2, 3, and either 4S
or 4R, depending on which is used to
unmute the receiver. If the other fourth
latch input (4S or 4R) is not used, be
sure to tie it to ground to prevent erratic
operation.
The way the circuit works, when
wired this way, is in a bucket brigade
fashion. When a zero is received, first
C5 charges slowly, then U3-D turns on,
charging C6 slowly, and finally U3-C
turns on, charging C7 slowly. After all
three capacitors are charged in se-
quence that way, U3-A or U3-B turns on
to unmute the receiver.
OPERATION.
General.
Operation is fairly simple. A control
sequence normally consists of four
digits, with the first three being a
common "key" used with both commands, and the fourth digit being a
function identifier: "set" or "reset". As
soon as the fourth digit is received, the
selected function is performed. Normally, these dtmf commands are sent
over the air with a touch tone microphone on a transceiver.
For security, the entire four-digit
command must be sent with no more
than a few seconds between digits. Each
digit remains active in memory in the
TD-4 logic circuits for only a few
seconds and then it is cleared.
Operation as DTMF Controller.
The output transistor in the latch
circuit can sink up to 50 mA to ground
in any dc control circuit up to 15V. This
could, for instance, ground the control
terminal to turn on an autopatch
module.
Any small relay with a coil resistance over 250 ohms also can be used.
Reed relays are ideal. Be sure to connect a reverse diode across the relay coil
to protect the transistor from inductive
spikes.
Operation as Selective Calling
Adapter.
When used as a paging device to
mute a receiver until someone wants to
talk to you, the TD-4 normally is latched
into the "set" state with a pushbutton
switch on the enclosure.
Then, if
someone wants to call you, he/she
sends the appropriate four-digit command to "reset" the latch in the TD-4,
which unmutes the receiver audio. You
could also use a manual "reset" pushbutton switch on the enclosure to turn
on the audio locally so you can monitor
or use the transceiver in a normal
manner.
You can extend the led's and mount
them on the enclosure so you can see
when any valid digits are received for
testing and also see what state the latch
is in. This is very handy. If you come
into the shack and find the led indicates
someone called you when you were
©1999 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark.
Revised: 1/31/00
- Page 5 -
away, you can give a call to inquire if
someone was looking for you.
CUSTOMIZING.
There are several parameters which
are preset to what is considered normal,
but you may wish to change them if
settings don't do what you want.
The first is the duration of tone
presence necessary to be recognized as
a valid digit. This is set, along with
minimum time required between digits,
by R3/C2. The present setting is what is
considered normal in the telephone
industry, namely 40-50 mSec. The only
reason you might wish to change it so
you can dial faster (but with possible
falsing) or slower (for more protection).
However, we recommend you simply use
it as is.
Another parameter you can play with
is the amount of time each digit is stored
in memory, which affects how long a
user has to complete the full four-digit
sequence. This is controlled by the r/c
time constant in input gate circuit of
each "and" gate in U3. For instance,
when the first digit is received at latch
input pad "1", the voltage charges C5
quickly through CR1. C5 is slowly
discharged through R4 when the sender
release the first digit on his/her dtmf
pad. When the voltage falls far enough
that U3-D no longer responds, it is too
late to enter the second digit, which is
applied to the other input of the "and"
gate. To lengthen the time constant,
increase the value of R4, and vice versa.
The gain of the input op-amp in U1
can be changed within reason to allow
various ranges of audio levels to be accepted. The gain is set by the ratio of
R2/R1.
THEORY OF OPERATION.
Refer to schematic diagram. The
input to tone decoder chip U1 is processed through a dial tone filter and a
band split filter using switched capacitor
technology. The signal is then analyzed
by two zero-crossing detectors and a
digital detection algorithm to determine
if and which tones are present. Then,
the resulting logic signals are processed
in a code converter and latch circuit to
provide four binary outputs (called Q1Q4) with encoded information about
dtmf digits which may have been
received.
A valid digit strobe (StD) signal indicates when any valid digit tones are
received. This valid digit signal is used
to gate U2 and to illuminate DS1
through U4 inverters when tones are
received. The whole circuit is run on a
clock controlled by color burst crystal
Y1. The ratio of R2/R1 sets the gain of
the op-amp input circuit. R3/C2 sets
the length of time necessary for presence of valid tones and the interval between tones to prevent erratic operation.
U2 decodes the hexadecimal information from U1 to provide 16 CMOS
output lines corresponding to the digits
on a DTMF pad. Programming is done
by soldering jumper wires from decoder
output pads at the output of U2 to
adjacent latch input pads at the input of
U3.
The output of the TD-4 is a smallsignal npn transistor, which can ground
an external circuit when turned on by
latching flip-flop U4-C/U4-B. When the
latch is set, U4-B applies a positive
voltage through R8 to the base of Q1 and
U4-C grounds DS2 through current
limiter R7.
The latch can be set and reset in two
ways. First, an external pushbutton
switch can be used to complete a circuit
from +5Vdc (at E4) to apply a logic hi
directly to the set or reset input of the
latch. The latch can also be set or reset
by the four-digit dtmf decoder logic of
U3.
To set the latch, U-4A must be activated; and to reset the latch, U3-B must
be activated. To be activated, these two
"and" gates must have both their input
gates hi (positive). One input of each
"and" gate is activated by the fourth digit
of the dtmf command on lines "4S" and
"4R". The other input of each "and" gate
is activated by U3-C, which turns on
only if the first three dtmf digits are
received correctly. After the first digit is
received, C5 is charged positive through
CR1, and the charge slowly discharges
through R4 after the first digit is
released. If the second digit is received
at U3-12
before
C5
discharges
completely, then U3-D turns on and
charges C6 in the same fashion. C6 then
holds the charge for a length of time,
even if C5 has fully discharged. If the
third digit is received at U3-9 before C6
discharges completely, then U3-C
charges C7. If the fourth digit is then
applied to U3-1 or U3-5 before C7
discharges completely, the set or reset
signal is applied to the latch to complete
the command.
TROUBLESHOOTING.
General.
Tracking down trouble is fairly
straightforward. The Theory of Operation
section describes the signal path and
what each circuit does. A table at the
left side of the schematic diagram near
U1 gives typical dc voltages for those
pins which are analog functions. All
other voltages represent digital circuits,
which have either a CMOS hi (near +5V)
or lo (near ground) as marked on the
schematic by the little pulse symbols
indicating the active state. The only
circuit with a different voltage is the
latch output transistor, which has about
+0.7V at the base when turned on. The
collector is at ground when conducting
and open circuited when off.
Power Supply Problems.
The operating voltage of the unit is
+7 to +14Vdc. Current drain normally is
about 10-15mA, depending on whether
led's are lit, since they draw more
current than the other circuits. If your
TD-4 is drawing excessive current, look
for shorts on the B+ line and perhaps ic's
plugged in backwards.
Voltage regulator U5 normally limits
current drain to 100 mA, even if the
output line is shorted to ground. If your
TD-4 draws about 100 mA of current
and U5 is hot, check for a short on the
+5V line somewhere. You may even
want to unplug the ic's, one at a time, to
see if one of them is the cause.
If you find an ic has been damaged
during operation or testing, check the
power supply which operates the TD-4 to
see if there are any transients. These
could be generated by a relay or some
other inductive device operating on the
same supply line. Any such inductive
devices must have a diode connected
across their coil with polarity reversed so
it absorbs any inductive kickback the
coil generates when the operating
voltage is turned off.
L Remember that the ic's are static
sensitive. You don't want to further
damage the board while troubleshooting.
A ground wrist strap should be worn
when handling the ic's.
Check the latch flip-flop to see if it is
responding and supplying the required
©1999 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark.
Revised: 1/31/00
- Page 6 -
current to turn on the output transistor
and "ON" led. The latch can be set and
reset manually by jumpering +5Vdc from
pad E4 to pads E6 and E5.
Erroneous Logic Operation.
If an ic isn't responding properly,
also check to see that all of its pins are
plugged into the socket properly and
that one or two pins are not bent under
or outside the socket.
Leaving an input of one of the cmos
ic's unterminated can cause erratic and
unpredictable operation. Be sure any
unused latch inputs are grounded.
Tone Level.
A logical troubleshooting procedure
would be to start by checking the audio
source to be sure valid tones of proper
level are being fed into the TD-4. The
range of acceptable levels is 100 mV to
2V peak-peak.
The input circuit of dtmf decoder
chip U1 is an op amp, between input
pins 1 & 2 and output pin 3. The level of
audio at pin 3, which is what is applied
to the detector circuits, must be at least
90 mV p-p (both tones at once). You can
check this level with a scope (not a
voltmeter) to be sure your installation
provides at least this level. If not, you
may change the place you derive the
dtmf audio for the TD-4 in your receiver
or decrease the value of R1 to get more
gain in the op amp circuit.
Tone Quality.
The anti-falsing circuits in U1 prevent valid digit recognition if there are
certain things wrong with the audio.
There may not be any strong tones
present other than the two required for
the dtmf digit. Having too strong a
subaudible tone, if used, can be a problem, although normal deviation levels of
about 300 Hz are acceptable for
subaudible tones.
There may not be excessive distortion of the tones. This may be caused by
over deviation in the transmitter or
being off frequency, either of which
DTMF Frequencies Vs Digits
Freq (Hz)
1209
1336
1477
1633
697
1
2
3
A
770
4
5
6
B
852
7
8
9
C
941
[
0
#
D
causes distortion by having modulation
sidebands splash over the edge of the
filter in the receiver.
Tone Frequency.
The tones must be on-frequency
within 1% to 2% or they will not be recognized. The following table gives frequencies of touch tones. You should
check your tone pad if some of the digits
don't respond, to see if the pad is
sending tones on frequency.
This
usually may be done by pressing two
adjacent buttons at once on the dtmf
pad, which allows only one tone of the
pair to be sent.
If the U1 chip is not decoding touch
tones, also check to see if its oscillator is
running and at the proper frequency.
You may need to use a 10:1 scope probe
for the frequency counter input to keep
from loading the oscillator circuit. The
next step would be to check the
hexidecimal signals from U1 to U2 and
check for a valid digit signal from pin 15
of U1. (The valid digit led should light).
The next thing to check is the inputs
to the U3 logic circuits. Is each time
delay circuit operating properly when
the digit is applied? Does the capacitor
charge quickly and discharge slowly
when the appropriate dtmf digit is sent.
Check to see that each "and" gate output
goes hi when the required inputs are
applied.
Tone Level Twist.
If falsing occurs with some commands or the unit fails to respond, you
should check the twist of the incoming
tones in addition to their frequencies.
Twist is the relationship between the
level of tones in the high group to the
level of tones in the low group. There
should be no more than 10 dB difference
between the two tone levels in any digit.
That is about a 3:1 difference in voltage.
Various factors influence the twist of
the tones, including the tone pad at the
transmitter, coupling capacitor values in
your system, receiver de-emphasis,
transmitter pre-emphasis, and how hard
you drive the tones at the transmitter.
One problem with some transmitter
setups is that a ham will set his tone
level too high, trying to get full 5 kHz
deviation. The level is actually set so
high that it goes into limiting. This may
cause the high and low tones to be
transmitted at the same level instead of
having the desired pre-emphasis. It may
also add distortion to the tones. Then
the receiver de-emphasis at the other
end causes the low tone to be at a higher
level than the high tone because the preemphasis at the transmitter was wrong.
You should encourage system users to
be conservative in setting tone levels at
their transmitters.
Another cause of bad twist on touch
tones can inadvertently occur if a subaudible tone decoder is used in the receiving system. The high-pass filters
supplied on sub-audible tone decoder
boards, usually connected in series with
the audio in the receiver to get rid of
buzz from sub-audible tones, can
severely degrade the levels of lower
frequency touch tones as well. If you
have such a board installed in your receiver audio path, you might want to
check its effect on touch tone twist. If a
problem, you may want to take your
touch tones from a point in the receiver
unaffected by the high-pass filter or even
just not use the filter in your receiver.
Generally, the required level of
deviation to make a sub-audible tone
system work is very low, about 0.3 kHz
or less, and some people run the level
much higher than needed, which causes
the buzz. Running the proper level may
allow the filter to be removed with no
great
problem.
©1999 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark.
Revised: 1/31/00
- Page 7 -
PARTS LIST.
Ref Desig
C1
C2
C3
C4
C5-C7
CR1-CR3
DS1-DS2
Q1
R1-R2
R3
R4-R6
R7
R8
U1
U2
U3
U4
U5
Y1
Description
(marking)
.01 µf disc (103)
0.15µf mylar (red)
1 µf electrolytic
47 µf electrolytic
1 µf electrolytic
1N4148 switching diode
T1 red LED
2N3904
100K
510K
1 meg
1.2K
10K
G8870 dtmf decoder
4514 hex to 16-line
decoder
4081 quad and gate
4001 quad nor gate
78L05 5V regulator
3.5795 MHz color burst xtal
0
8
C2
B
9
*
MTG
#
C
A
13
24
U2 4514
1
12
7
6
5
4
3
1
2
D
DECODER OUTPUTS
LATCH INPUTS
4R
4S
3
2
1
Example shows 213* to turn on and 213# to turn off.
Figure 4. Typical Programming Jumpers.
MTG
0
R2
1
18
8
B
MTG
#
R3
C2
9
*
C
A
R1
13
24
C1
Y1
U2 4514
U1
G8870
AUDIO INPUT
9
E1
10
12
1
_
GROUND ( -12V)
E2
C3
+
+12V POWER INPUT
E3
U5
78L05
7
1
14
6
5
4
3
1
2
DS1
DS2
4R
4S
3
2
1
CR1
+5V OUTPUT
LATCH RESET INPUT
E4
_
C4 +
1
LATCH INPUTS
R4
14
+ C5
R7
E5
DECODER OUTPUTS
D
_
U4
LATCH SET INPUT
OUTPUT
4001
E6
7
CR2
8
OUTPUT=ON LED
R5
Q1
_
E7
U3
4081
C7 +
R8
7
MTG
8
+ C6
_
MTG
R6 CR3
(ANODE OF LED
HAS LONG LEAD;
ANODE IS THE TOP LEAD,
WITH ARROW HEAD)
VALID DIGIT LED
Figure 5. TD-4 Component Location
©1999 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark.
Revised: 1/31/00
- Page 8 -
+5 VDC
PROGRAMING PADS
9
4
E1
AUDIO
INPUT
Vref
1
+IN
2
- IN
C1
Vdd
R2
Q4
3
U1 Pins 1, 2, 3, & 4 = 2.5Vdc
Y1
3.5759
U1 Pins 16 & 17 =
5Vdc with DTMF Tones Present
0Vdc without DTMF Tones
8
SC2
5
6 Vss
9
C2
E3
+12Vdc
U5
78L05
+
E2
_
23
3
U4-A
INV
_
4
18
17
19
INHIBIT
14
4001
12
U4-D
INV 13
U2
4514
16-LINE
DECODER
Vss
13
16
15
11
1
2
3
4
5
DECODER
OUTPUTS
6
7
8
9
0
Q
#
A
B
C
D
4001
PROGRAM BY
JUMPERING
THE 5 PADS BELOW
TO DESIRED DIGIT
OUTPUTS ABOVE
+5 VDC
+
C3
5
20
1
12
DS1
VALID DIGIT
8
STROBE
22 D4
21
D3
3
D2
2
D1
+5 VDC
11
Vdd
7
R3
2
U1
G8870
DECODER
10
6
14
Q3 13
12
Q2
11
Q1
StD 15
Gs
SC1
7
1
10
TOE
St/Gt 17
Est 16
R1
DC Voltages at Analog Ckts:
24
18
C4
GND
PROGRAMING PADS
R4
R5
13
R6
SET
3
8
2
10
U3-A
1
4081
+
CR3
U3-C
9
4081
11
+
CR2
U3-D
4081
12
1
+
CR1
_
C5
_
C6
_
C7
2
RESET
4
LATCH
INPUTS
3
6
4S
U3-B
5
4081
4R
+5 VDC
LATCH
E6
SET
8
U4-C
INV
EXTERNAL
SET AND RESET
SWITCHES
(IF DESIRED)
E4
9
+5 VDC
10
R7
DS2
ON
4001
SWITCH
2N3904
E5
5
RESET
6
U4-B
INV
4
R8
Q1
4001
E7
OUTPUT
Figure 6. TD-4 Schematic Diagram.
©1999 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark.
Revised: 1/31/00
- Page 9 -
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertising