©1999 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised: 2/4/03
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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 dig-
its, by R4/C3. The present setting is
what is considered normal in the tele-
phone industry, namely 40-50 mSec.
You may wish to change it so you can
dial faster (but with possible falsing)
or slower (for more protection).
Another parameter you can play
with is the five second timer setting
for the window after reception of the
"key" digits during which other tones
will be accepted as function indica-
tors. This is set by the R8/C11 time
constant. You could shorten the time
experimentally to find a time which
requires someone to really send the
four digits quickly and allow no addi-
tional time to do other commands. Or
you could slow it down so that it
would allow a series of commands to
be sent with one digit each after the
initial "key" digits are sent.
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 R3/R2.
SOME ADDITIONAL TRICKS
YOU MAY BE INTERESTED
IN FOR USE WITH THE TD-2.
SELECTIVE CALLING UNIT.
The following diagram illustrates
how you can use a TD-2 module as a
selective calling unit with the addition
of only a few parts. This allows you to
monitor an fm radio channel for calls
directed only to you and mute the
speaker the rest of the time.
The receiver audio is routed to the
TD-2 input and also via relay K1 con-
tacts, which mute the relay unless the
"B latch" function is activated at the
TD-2. The TD-2 requires an audio in-
put level of 100 mV to 2 V p-p for
proper operation. If you run the re-
ceiver audio at a low to moderate level
(less than 1/10th watt), you can con-
nect the TD-2 input at pad 1 directly
to the speaker output of the receiver.
If you plan to crank the speaker level
up fairly high, you may exceed the al-
lowable level into the TD-2 and cause
blocking of tones. In such a case, ei-
ther build a simple L-attenuator with
about 1000 ohms in series and 100
ohms in shunt from pad 1 to ground
or connect the TD-2 to the discrimina-
tor in the receiver after the de-
emphasis network.
Since the default condition of the
"B latch" in the TD-2 at power up is
"off", the speaker relay will operate
only after the required tone sequence
is received for the "B" function. The
speaker may be turned off again either
by momentarily breaking the B+ to the
TD-2, as shown in the diagram with
the push-button normally-closed
switch, or by sending the proper tone
sequence to turn off the "B latch" in
the TD-2, which of course can be done
by the sending station remotely.
The output transistor in the latch
circuit can switch up to 50 mA. Any
small relay with a coil resistance over
250 ohms can be used. Reed relays
are ideal. Although 13.6 Vdc is shown
in the diagram for relay power, any
voltage up to 15 Vdc can be used if
compatible with your relay. Be sure
to connect a reverse diode across the
relay coil to protect the transistor
from inductive spikes. If you like, an
LED can be connected across the re-
lay coil too and located on your front
panel to indicate when the audio cir-
cuit is open.
DEFEATING THE TOLL CALL
RESTRICTOR.
If you plan not to use the toll call
restrictor, the easiest thing to do is
break the traces from the "0" and "1"
lines to pins 8 and 9 of U3-C. Since it
is never good practice to leave a
CMOS gate input floating, pins 8 and
9 should be connected to ground.
If you want to be able to turn the
toll call restrictor on and off with one
of the latching functions of the TD-2,
break the ground trace between pins 7
and 8 of U4 and connect a 1K resistor
from pin 8 to the +5 V bus at U4 pin
5. You can turn the toll call restrictor
on by grounding pin 8 of U4 with any
of the extra latch outputs you may
have available. U4 pin 8, the "set" in-
put of second stage of the digit coun-
ter in the toll call restrict circuit, will
block operation of the toll call re-
strictor when pin 8 is ungrounded be-
cause the resistor applies a hi unless
the controlling latch grounds pin 8.
With the "set" input constantly high,
the counter output remains high and
U3-B thinks you have already passed
the first digit in the phone number.
PARTS LIST.
Ref # Value (marking)
C1
.001 uf (102, 1nM, or 1nK)
C2
.01 uf disc (103)
C3
0.15 uf mylar (red)
C4-C7 2.2 uF electrolytic
C8-C10 .01 uf disc (103)
C11
4.7 uF electrolytic
CR1-CR3 1N4148
diode
DS1-DS6 Miniature
red
led
L1 Ferrite
bead
Q1-Q5 2N3904 or 2N4124
R1 22K
R2 330K
R3 100K
R4 510K
R5 Not
used
R6 1
meg
R7 10K
R8 1
meg
R9 10K
R10 1
meg
R11-R12 10K
R13 1
meg
R14 10K
U1
GTE/Mitel G8870 Tone
Decoder
U2
4514B Hex - 16 Line
Decoder
U3
4001B Quad Nor Gate
U4-U6 4013B Dual D Flip-Flop
U7
4081B Quad And Gate
U8
4013B Dual D Flip-Flop
U9
4081B Quad And Gate
U10
4013B Dual D Flip-Flop
U11
4081B Quad And Gate
U12 78L05
Regulator
Y1
3.59 MHz Color Burst
Crystal