sceiv

output and charged up C I). If the second
digit of the DTMF entry is also correct
(matches U2), the output of 4073/section A
will go logic high and charge up C2. At the
same time. since 4017 will move the logic
high 10 its D2 output. the 40731section B is
now ready to decode the third DTMF digit
via U3_
If a logic high is received from U3 within
one second. a logic high will be presented
from 4073/section B and charge up the two
following circuits. The first is a 45-second
de lay formed by 4073/section C (just a
buffe r), the output of which will drive Iransister
Q I and activate the external speaker
until time-our. That is how the DTMF
squelch becomes unmuted by detecting the
station 10. The previously mentioned 4081
dual input gates are connected as additional
remo te control command decoders. Once
station 10 plus one more matching DTMF
di gi t is rece ived (programmed by
FI ,F2,F3.F4), the output of the corresponding
4081 will tum logic high while external
devices can be tr iggered via the ou tput
points (a.b.c.dj.
Additional Cont rol Features
One special section is provided by the hex
inverters. Sections A and F of the 4049 are
connected as a toggle nip-nap. If a logic
high is received on the [on] input. the output
will stay logic high until another logic high
is received on the [ofT] input. This is just an
example of how to construct a self-locking
on/off control circuit. You can connect output
(alia [on], and output (b] to [off). in
order to program the [on] command as [sralion
ID + FI] while the [off) command is
[ station ID + F2). More contro l can be
obtained in the same manner just by adding
similar circuits.
If any error occurs in the command sending
sequence, the counter will prevent further
decod ing immediately until the selfreset
function executes. So. any command
must be entered correctly within three seconds
after a start. Do not hesitate more than
one second between any two digits or the
decode will fail due to time our. With such
digit-by-digit checking. repeal numbers can
be used consecutively in the station ID
(except that digits FI .F2.F3.F4 should not
be the same as U3 or the 4th digit for that
command will be ignored). The command
circuit will fire after the reception of the station
ID since the whole fou r-digit command
is already received. even though the user has
not yet sent his fourth DTMF tone!
Since only four units in the 4049 hex
inverter are in use. the two spare units are
connected in series (on the PCB) as a spare
no n-invert buffer. This can be used for
future expansion or making the LED monitor
become a high impedance logic probe.
This simple device is very useful for diagnostic
work at the constru ction stage. As
shown in the ci rcuit diagram. the LED is
connected to show DTMF detection. while
the 4049 buffer is left vacant . Users can
determine their own needs and change the
connection accordingly.
24 73AmateurRadioToday·June.1992
DTMF character
o
1
2
3
4
5
6
7
8
9
o •
• ABC
Table J. Decoded BCD numeric data format.
Tune-Up and Testing
Thc only precaution to take when working
with construction of this type is to be careful
with static discharge on CMOS ICs. All the
les should be kept inside their ami-static
packaging until the last moment before they
are transferred to the PC board. To avoid
unnecessary damage, first so lder all the
jumpers on the component side. Then put on
all the Ie sockets. This sequence is important
since some jumpers are hidden under the
sockets. They will no longer be accessible
after the sockets are in position. Next, put on
all the resistors. diodes. capacitors. the crystal.
Q I and the LED. Finally. put on the 7805
regulator.
Connect OC power to the -vcc .input and
measure voltage for the pins at all the IC
sockets. You should sec +5 volts ONLY on
the Vee supply pins. If +5 volts appears on
any other pins you probabaly have a short
circuit somewhere. Check out those shorts
before an IC is toasted because of careless-
MT8870
MCl4073 or4073
MC14081 or 4081
MC1401 7or 4017
MCl4049 OR 4049
MC1451.
OutpUl data line
OC B A
0000
000 1
00 10
00 1 I
0100
010 1
0110
01 I 1
1000
1001
1 0 1 0
1 0 1 1
1 1 0 0
1 1 0 I
1 1 1 0
1 I 1 I
Parts List
Mitel DTMFdecoder
Triple three input AND gate
Quaddualinput ANDgale
Decimal counter
Hex inverter
BCD to HEX decoder
7805 5-YOIt regulator> 50 rnA
Q1 VN10KM Power FET
Rl l Mohm II. wall.
R2 1M ohm 1/4 wall.
R3 2Mohm 114 wall.
R4 1Mohm 1'" wall.
AS 1Mohm 1/. wall.
R6 101:*. ohm lJ.4 wall
R7 500 ohm IJ.4 watt
R8 l Oll ohm IJ.4 watt
R9 lOkohm I"' wall.
Rl0 lOOk ohm II. wall.
Rll l OOk orvn II. wall
R12 500k ohm II. wall.
R13 lkohm II. watt
Cl Illf 'J(1V tantalum
C2 I Ilf 'J(1V tantak.m
C3 33 JlF I OV tantakJrn
C4 1 IIF JOY tantalum
C5 4.7 JlF 16Vtantalum
C6 0.1 JlF 16Veleetrolytic
C7 0.1 JlF 3OV tanla.lum
01-09 1N914 Of equlvalenl switching diode
LEO klw powerconsumptiontype
Ie sooets aa pln xt , 18 pinxt , 16pinsa . 14 pin x2
Etc hed and dri lled PC board is available at $12 plus
postage; lhe unassernbleO' kit is $41 plus postage. Order
1rom: Patr'ck Wong VE3AGW. 10 HalderCr.• Markham,
Ootario, L3fI 'lEa, canada.
ness. then check the conductance of all the
ground pins are actually grounded. Make
sure the return path is also good.
Remove OC power from the board. Now
you can insert the ICs onto the PC board and
test them out one by one. First insert
MT8870, then power on the board. Connect
the RCV input to the speaker output of your
receiver. Assume thai the LED has been connected
to the DTMF strobe line as suggested.
Send DTMF tones into this receiver and
increase the volume control of the receiver
from silence until the DTMF LED on the PC
board lights up. The LED should turn off
once the DTMF tone is gone. MT8870s have
a very wide dynamic range for the input
DTMF level, so even very weak audio is sufficient.
Don't overdrive thc circuit; distorted
DTMF is no good for decoding.
After you have verified that the chip is
able to decode DTMF. enter all 16 tones and
make sure everyone makes the LED Flicker,
Next, use the voltmeter to check out the fi ve
data lines at the expansion port. SID should
nag logic high (+5V). j ust like the LED
does. The ac tivity of th e other data line
should respond exactly as shown in Table I.
Remove DC power an d insert the
MC145 14.4049 into the ci rcuit Turn the
power on again and enter all 16 DTMF
tones. Make sure a corresponding logic high
can be seen on the correct pins at the program
board.
Re move power again and insert 401 7.
4081, 4073 onto the PC board. Complete the
necessary programming for U I. U2. U3, FI,
n .F3. F4. then tum the powcr back on. This
time. hook the voltmeter to pin 3 of 4017.
then send in one DTM F tone. This pin
should be logic high on idle. go logic low
immediately after DTMF received. and
restore logic high automatically within three
seconds. Check out the inverter or D5.C5
and R5. if failure occurs. Also, make sure the
logic high stage moves from pin 3 to pin 2.
thcn to p in 4 o f 4017 wit h consec uti ve
DTMF entries.
Wait fo r three seconds (to ensure 4017's
self-reset). Connect the voltmeter to pin 6 of
4073. Send in the DTMF tone for the station
ID (U I U2 U3) and make sure a logic high
nickers once on the voltmeter. If this is successful
and the external speaker is connected.
you can hear the monitor speaker switch on.
lt will stay on until time-out occurs on R3
and C3 (within 45 seconds).
Move the voltmeter to the output of 4081
(pin 3.4.10.11) and check out thcir activity
on sendi ng in commands ID+FI.ID+F2.
ID+F3 and ID+F4. If you have connected
output [a) to (S] and output (b] to [R), by
performing the above rest on ID+FI and
ID+F2 you can also cause an on/off activity
on output [LK].
The decoder is considered tested out after
all of the above has been eomplcted.
Endless Possibilities
The above is only a start fo r your entering
the world of DTMF control. By using these
DTMF switches. you can create unlimited
applications.
•