Late last year (2011) our local radio club announced
that they were planning to add Echo Link to the club repeater. While I applauded this move, it did raise a small problem for
me personally. My primary radio at home is an Icom IC-706 MkIIG which does not come stock with a DTMF keypad. While there
are DTMF mics available for this particular model, I decided to take a shot at rolling my own. I remembered an article from
quite a while back by a ham who converted a Radio Shack Tone Dialer (an arcane device from years back when most phones were
still pulse dialed). He converted the device so that the DTMF tones could be transmitted directly into the mic circuit of
the radio instead of the original mode which played the tones through a small speaker that was held up to the transmitting
end of a telephone handset. I looked and looked for that article but could not find it anywhere. I did find a ton of web sites
that proposed to teach one how to avoid paying tolls to the phone company using “Red Boxes” and “Silver
Boxes” and other such devices. I also came across many data sheets from various integrated circuit manufacturers, some
of which had “typical applications” somewhere near the back of said data sheets. Most of these applications were
aimed squarely at the “land line” industry but some did have clues as to how one would go about injecting the
tones into the mic + line to the radio. They also provided good guidance as to what it took to power the chip and how to interface
the keypad to the circuit.
After all this research, I started looking for
present day sources for DTMF chips only to discover that almost all the chips made for the present uses are surface mount
chips. Now I have nothing against surface mount technology, indeed all of my radios are full of them, but I was going to try
to assemble this project and my soldering technique is more suited to wires and discrete components than capacitors the size
of a flea. Some say my technique is more appropriate to stained glass, but my wife says that would be insulting to stained
glass workers. BTW, she is not only a ham (KJ4YMX) but also a darn fine stained glass artist. Just when I was about to concede
defeat and order a DTMF mic from far across the oceans, a fellow ham, Chuck, W4CLL, offered to send me a DTMF mic from his
“junque box”. It had no cord and Chuck had no idea if it was working. It did have a discrete DTMF chip and he
figured that that would be a start.
The IC in the old mic turned out to be an AMI
S2559E. This is an almost perfect choice because it does have provision for a full 16 key keypad (four rows and four columns)
and because it can be powered directly from 6 to 12 volts. If one wants to get fancy it even has a pin that will pull the
PTT signal down to ground anytime a key is pushed and it will hold it there for about 1.5 seconds, plenty of time to hit the
next key in the string. This pin does appear to need extra circuitry to be operational so I chose not to use that feature,
just to keep the circuit as simple as possible.
When I started to plan the actual construction
of the DTMF Keypad, I wanted to keep things as simple and easy as possible. After all, I was going to build it and that was
vital. Anything more than a few components and my eyes cross. The resulting circuit fully met my criteria. Other than the
IC itself there are only four components:
A keypad: Matrix type, I initially used the one
from the donor microphone and then switched to the Jameco 16 key keypad (Part # 169245 for $9.95) A similar keypad is
available from Jameco and All Electronics that appears to have the right configuration for the TCM5089N chip mentioned below.
It has 9 pins instead of 8 and thus appears to have the ground connection required for the TCM5089N IC to operate properly.
I cannot verify this as I have not worked with that part myself. It will require new labels for some of the switches
since the 4th column is not marked ABCD nor are the * and # keys properly IDed.
A 1K trimmer Radio Shack part # 271-280
A 0.10uF capacitor, non-polarized Radio Shack
part # 272-135
A 3.579---- Color Burst Crystal (again from the
donor mic, but these are quite common) Mouser Part # 520-HCU357-17DNX Jameco Part # 14533
A SPDT switch to select column 3 or column 4
(More on that later) Radio Shack Part # 275-613 (Not needed if using a 16 key keypad.)
Of course there are other parts to buy, a 16
pin socket for the chip (RS# 276-1998),a small PCB from Radio Shack (RS#276-159), connectors for the radio and the hand mic
to plug into, some kind of enclosure to house everything, but those are common to every project in some way.
An alternate DTMF IC (TCM5089N) is available from Jameco for $4.95 (Jameco Part # 32803) Please note that this
chip will apparently not work with a matrix style keypad. Thus a different type keypad will be required to make use of this
The IC (S2559E) is a 16 pin dip style chip. This
chip DOES work with a matrix style keypad Starting from pin 1 (see the diagram) they are:
1. VDD (6 to 12 volts DC)
2. XMIT (no connection here)
3. C1 (this connects to
column 1 [1, 4, 7. *] on the keypad).
4. C2 (ditto for column 2 (2,
5, 8, 0)
5. C3 (again ditto for column
3 (3, 6, 9, #)
6. VSS (we call this ground)
7. OSCIN (one side of the 3.579
crystal connects here)
8. OSCOUT (the other side of
Now we go up the other side of the chip
9. C4 (this column covers A B
C and D)
10. MUTE (no connection here for our project)
11. R 4 (this connects to row *, 0, #, D)
12. R3 (connects to 7, 8, 9, C)
13. R2 (connects to 4, 5, 6, B)
14. R1 (connects to 1, 2, 3, A)
15. MDSL (In some uses this allows the option
of single tones. We ground this pin to remove the option.
16. TONE (when a key is pressed, the DTMF
tones come out here)
Pin 16 has to see some kind of load for the chip
to work properly. For that reason we add two components here. The output of Pin 16 goes to a 1K trimmer potentiometer of which
the opposite end is grounded. The wiper of the pot connects to the 0.10 uF capacitor which then feeds into the Mic + line
of the radio. Pin 6 on the 706
Pins 6 and 15 are grounded to the chassis ground
of the radio. On my 706 this is pin 7 of the RJ-45 mic connector. Pin one of the radio mic connector provides 8 VDC to the
microphone and to Pin 1 of the IC.
At this point in the project, the only thing
left to do is to connect R1-R4 and C1- C4 to the appropriate pins of the keypad. Each key controls an SPST momentary contact
switch which connects the keys row to its column, thus triggering the generation of the proper two tones.
this point perhaps a bit of background is needed. Many areas of electronics have depended on DTMF tones to operate many things.
When you dial a phone you use DTMF. When you set the burglar alarm on your house, you use DTMF. When you want to call up Echo
Link or IRLP on the local repeater, you use DTMF. DTMF means the use of two different musical notes or “tones”
played simultaneously to indicate that a particular button in a grid has been pushed. Each column in a grid plays a given
tone, different for each column. Each row plays a different tone as well. When a key is pressed, both the row tone and the
column tone are generated. Thus only one key is on that row and that column. The device that decodes those tones thus knows
that that particular key has been pressed. The tones standardized are:
R1 667 Hz
R2 770 Hz
R3 852 Hz
R4 941 Hz
C1 1209 Hz
C2 1336 Hz
C3 1477 Hz
C4 1633 Hz
Thus, if the decoder at the other end of the
system hears 667 and 1209 together it knows that the 1 key has been pressed. If it hears 941 and 1633 it knows the D key is
in action. You may note that the row tones are all lower than the column tones. Some DTMF chips actually output the two tones
separately. These are then combined outside the chip by the added circuitry.
Back to the project at hand. Having built the
DTMF encoder, we have to connect it to the microphone circuit of the radio in such a way that the tones are transmitted via
the radio to the device we are attempting to control, whether that be a computer that will link the local repeater with another
repeater far away or just a widget that will turn the lights on at home. The three connections are the source of power, usually
the same wire that feeds power to the electret mic we talk through, the ground to the radio chassis, and the mic + line that
will feed both our voice and/or the DTMF tones to the transmitter section of the radio. In the case of the Icom 706 series
of radios these are pin 1 for power, pin 6 for mic + and pin 7 for ground. By using a short length of either microphone 8
conductor cable or CAT-5 cable with an RJ-45 on one end we can connect to the radio via the mic jack. If we add another RJ-45
jack at the other end of the enclosure we have a place to plug in the hand mic or desk mic we are using. In between the two
we can connect the DTMF unit and its keypad so that we can key the PTT and send DTMF tones when needed. I did not connect
the chips Pin 2 (XMIT). I also decided against, at least for now, on adding a side tone monitor to the DTMF unit. The
side tone monitor would allow me to hear the tones as I was pressing the keys. A nice feature to be sure, but the complexity
of adding that option was left for a later project.
I mentioned before that there is a SPDT switch
in the list of components. The keypad that I originally had in hand for this project was a 3 x 4 keypad. That is identical
to the normal touch tone phone 1 through 0 plus * and #. The full 4 x 4 keypad adds the letters A through D as the fourth
column. In order to access this 4th column in the "proof of concept" prototype I resorted to the same trick
as that other article long ago. I placed a switch that fed either the column 4 pin or the column 3 pin to the column 3 series
of keys on the keypad. By flipping the toggle switch one way the device is a normal touch-tone phone keypad. Flipped the other
way the 3, 6, 9, and # are converted to A, B, C, and D. Having found a 4 x 4 keypad from Jameco in CA it was
simple to remove the switch and connect the wire from C4 directly to the fourth column on the keypad.
In testing, using a simplex frequency, in part
because the local repeater strips the tones off the outbound repeater transmission, and in part to avoid interference to anyone
else’s use of the repeater frequency, the tones came through clean and clear. I simply turned the output power on the
706 down to its lowest level and put my handheld across the room on the same frequency. I could hear the tone through the
HT just fine and feedback was not a problem as long as the hand mic I was using on the 706 was not too close to the HT. I
did not even have to adjust the trimmer pot to adjust the volume of the tones transmitted. If necessary, a simple adjustment
of said pot would bring the level of the DTMF tones into proper specification.
The final step is to provide a suitable enclosure
for the device. It needs to be small enough to sit on the desk of the operating position without taking up too much space.
It needs to place the keypad at an ergonomic position for the operators hand and it needs to accommodate the jack to plug
in the microphone the operator will be using. Harry Seyfert, WD9FYF, a fellow ham in Kankakee Illinois answered
my query on the IC-706 Yahoo Group with an offer of a housing that was built by Western Electric for a PBX switchboard. It
is pefect for the purpose. The pictures at the top of the page show the finished unit. A stub of Cat 5 with an RJ-45
male plug connects to the radio. I have experienced no RFI issues so far from or to the DTMF keypad. The audio quality of
the HM-103 is unchanged with the DTMF encoder in the circuit.
I have tried to describe a way of adding a DTMF
Keypad to radios that lack that feature. The cost in my case was minimal, since the donor mic was free and the parts that
had to be purchased rather than taken from my growing collection of components were few and inexpensive. To purchase a useable
DTMF IC new may take a little effort. They are available from overseas vendors in the Far East and are also available at
Jameco (www.jameco.com) Jameco also has the 16 key keypad and the necessary color burst crystal. You can find the part numbers for
Jameco in the components list above. Often an older DTMF mic for a long dead radio can be cannibalized as I did
to get the chip and the crystal it requires. Nasco Electronics Wholesalers in St.
Petersburg Florida may have the S2559E chip or a similar but not identical TCM5089N chip that has a similar pinout. If you use a different IC than the S2559E, make sure you read and
follow the information in the data sheet for the chip you actually use. Often there are slight differences such as voltage
tolerance or what to do with unused pins. These slight differences can have major and catastrophic effects on the chip if
not heeded. Do it yourself ham radio requires an ability to think outside the box with an open mind. You might be surprised
to find that the materials you need are all around you. They just don’t say “Ham Radio” on them before you
put them to use.
One of the main reasons for taking on this project
was that it required more effort than just soldering a couple of discrete components together. The research and parts hunting
resulted in several long conversations on our Section Technical Net and without the help of Chuck, W4CLL and Harry, WD9FYF,
the search might still be going on. Here again, as I have mentioned in past articles, one must compare the cost of buying
the device (a DTMF mic in this case) versus the cost in human terms of tracking down the elusive parts and the time spent
with soldering iron in hand. I actually did purchase a DTMF equipped mic during the course of this project from a store in
Hong Kong. The mic, an Icom 118N works very well and is pure "plug and play". I use that mic for the 706 that is mounted
in my SUV. The cost was about $30+/-. I am using the home-brewed unit at my home station. I sometimes
choose to build some accessories rather than buy just because of the satisfaction I feel when I can say “I built