ZL2PD CMOS Keyer
Two cheap CMOS ICs and a few transistors was
all that was required to rebuild my elderly TTL automatic morse keyer
into a low current lightweight battery powered iambic morse keyer.
No microprocessors were harmed in the
construction of this keyer!
over the past year or two, my elderly Accu-Keyer gave up the ghost and
died. I built it more than 30 years ago (!!) from the original TTL
published in the ARRL Handbook. It was an accurate copy of the
original, right down to the printed circuit board which I made using
nail polish as resist and ferric chloride etchant. As I was tearing the
old keyer apart, I instantly recognised the PCB production method I'd
used by the
quality of the resulting PCB. It was not very pretty, and I certainly
could not reuse the PCB simply by replacing the dead ICs with new CMOS
In those days, the Accu-Keyer was the most popular design. Most of
those keyers used TTL chips, and the current drain
worked out to be a few hundred milliamps. Running it off batteries was
hardly practical, and I never tried. But there were times when it would
have been useful.
In fact, I was never very happy with the
keyer. The power supply was my biggest gripe, and my own fault. I made
it using a rewound transformer. I was a poor high school student in
those days, without much money to spend on the hobby. To get the right
voltage for the power supply - I needed 5VDC - I found a transformer
somewhere, pulled it apart, and rewound the secondary by hand. The
problem was that the darned transformer used to hum like crazy because
I was unable to clamp up the laminations adequately. I guess I could
have dipped it in varnish or something, but I never got around to it.
The hum wasn't too bad when it was all enclosed inside the little box.
every time I turned it on, I would hear that hum, and I'd mutter to
myself about doing something about the transformer 'one of these
days'. But while the keyer was working and usable, I just couldn't be
bothered. There were too many other things of more interest to build. A
few months ago, however, I went to use the keyer again (You can tell
how much CW I do!) only to find the keyer had died. I checked out a
number of the chips, fixing my old TTL logic probe in the process (The
joint in 20+ years), only to find that almost all of the seven chips
used in the old design had up and died. I was prepared to replace one
or two, but the widespread deaths of ICs across the PCB made me forget
It was time to rebuild my keyer.
What is a Keyer?
or "Elbugs" as the Europeans call them, are used to send Morse code.
Most keyers use a two-paddle key for operator inputs. Pressing
either paddle towards the opposite paddle results in the keyer sending
a series of either 'dits' or 'dahs', depending on which paddle is
pressed. Unlike typical manual morse keys, keyers are designed to send
near-perfect code, with 'dahs' equal in length to three 'dits', and
with precise inter-symbol timing.
The really neat thing about the more advanced versions of these keyers,
called 'iambic' keyers, is that these allow you to insert a 'dit' in a
stream of 'dahs', or vice-versa, or send a string of 'dit-dah' or
'dah-dit' characters. To send an 'a' ('dit-dah') merely requires a
single squeeze of the paddles. Sending a 'c' ('dah-dit-dah-dit') take
no more effort - A single, slightly longer, squeeze does the
trick. The objective of these keyers is to reduce the effort of
sending Morse code manually. While computerized keyboard keyers also
exist, they are not as portable as these keyers, and certainly nowhere
near as popular.
Modern keyers are almost all microprocessor-based, and include memories
to hold standard messages. These provide storage for 'CQ' calling
messages, and routine sentences which regularly occur during a call
such as the operator's name, location and equipment details. Most also
add specialized auto-numbering messages for use in contests. Some older
TTL or CMOS designs also supported memories, but these designs were so
complex, with dozens of chips, that you'd have to have a lot of time,
and parts, on your hands to want to build one of those.
Choosing a Design
I had a
quick look around my books and magazines to see what I could come up
with by way of keyer designs. A common choice for much of the past
thirty years was the Curtis range of keyer chips. These came from a US
company of the same name. Those chips are obsolete, no longer
available, and so those designs were definitely out. (Of course,
nothing ever really dies. It turns out that the descendent of those
original Curtis chips is still available, the 8045, from MFJ.)
The rear view of the keyer shows the old AC power cord entry, now
empty, and the AC fuse, now unused, and the connectors for keyer output
(to the transmitter) and a standard phono connector for use with a
manual morse key
click with your mouse to get a closer view)
I could have rebuilt the keyer substituting TTL with CMOS devices,
using the same basic design, but I was never really happy with the
Accu-Keyer. It used a two-transistor dot clock which was switched on
and off by the keyer logic, and that led to slightly shortened leading
dits on occasions. My original keyer never exhibited that defect, but
others did. The more important problem was the dot oscillator itself.
It had the reputation of being hard to get going, a fact I recalled on
all three units that were built by a group of us back then. We ended up
scouting around the city to purchase exactly the right type of
transistor specified in the original article to get that oscillator
going properly. Very annoying.
Others attempting to build the Accu-Keyer in CMOS reported similar
problems in that area. And the idea of finding seven chips of the right
type in my
junkbox, (Yes, that was the number of ICs used in the Accu-Keyer) was
hardly likely. No. I wanted
simplicity, and I wanted to build the keyer right now.
I was briefly tempted to design a keyer using a microprocessor. Other
pages on this website demonstrate that I'm no stranger to these
devices, but I really was after something with a touch of the original
Something that was easy to build in an afternoon so I could get back to
other tasks. I just wasn't
keen on spending a couple of days writing software for a keyer I only
used from time to time.
Naturally, I could have purchased one of
those little ready-coded keyer
microprocessors which are available on the web. (For example, the K8
and K12 PIC-based chips from K1EL, VK1OD's PIK keyer chip, or the TiCK1
and TiCK4 chips from Kanga) But I wanted a quick fix, and I did not
want to have to wait a few weeks while an order went out and the chip
was delivered by snail-mail.
It was only after I had built the keyer
described here and used it for
a few weeks that, of course, I found a Czech website with an 8051-based
memory keyer complete with software which I could have copied in a
heartbeat. (Look for the latest version of the
Elbug ) Oh well. If I'd found that site a bit
sooner, I guess you wouldn't be reading this page now!
Anyway, what I did was spend an enjoyable hour or two thumbing through
my old files and
magazines looking for a simple CMOS design. I found a really simple one
in back copies of SPRAT, the UK-based QRP club newsletters. It featured
just two CMOS chips, and, yes, I had those in my parts box.
The design choice was made.
It turns out that this keyer design has a bit of a history. The first
reference I can find to it is a TTL-based design from '73' magazine in
the mid-60s. It seems this design was subsequently converted to CMOS by
Pierre, FE1MOG. His design was published in SPRAT, the UK QRP magazine,
back around 1971. This same circuit was republished in the recent RSGB
book 'Low Power Scrapbook' (page 195), along with all of the mistakes
of the original circuit(!) Coincidentally, and about the same time as I
came across the information on the web about the German 8051-based
keyer, I spotted a nearly identical keyer on the website of
Onno, PA2OHH. Onno's version used the /Q output of IC2a
(Pin 2) while the original circuit used the Q (pin 1) output of IC2a,
as mine does. The only impact of this is to reverse the dit and dah
input connections. Everything else functions identically to the
original circuit, at least as far as I can tell.
All of which goes to show that there is very
little in this life that is truly new and original.
The circuit diagram of the keyer is shown in
the box to the right. Just "right click" on it to see it at full
U1a and U1c form the dot
clock while Q1 and Q2 invert the input signals from the paddles.
The dit and dot paddles connect to the common ground (pin 2 on the
input connector) to send the respective symbols. Q6 and Q7 form a
simple audio oscillator with the tone frequency set by RV2. I actually
used a fixed 33k resistor here. This oscillator is turned on by Q4 and
Q5. SW3 allows the user to turn the sidetone off.
the way, this was the first schematic I drew using Basic Schematic, a
freeware program from Japan. I previously used CorelDraw exclusively.
Since it's so easy to use, and the results are more than acceptable, I
plan to use it for more circuits in future. I have to revise a few of
symbols in the symbol library to get them closer to standard schematic
symbols, but aside
from a few minor issues, I'm very happy with it. Automatic placement of
text around the component leaves a little to be desired too, but hey,
it's free, right?
Solving a Few Problems
is simple in life.
It turned out that the original design I had
located was shy of a few critical details. Some, of course, were
obvious. The original circuit diagram in SPRAT (and in the latest
RSGB-published 'Low Power Scrapbook') had
omitted a few critical items, like ground
connections on several pins on each of the ICs. Those were obvious, and
fixed in a
jiffy while building the unit.
Then I realised that the original design had another minor problem -
The keyer inputs (for the 'dit' and the 'dah' paddles) connected the
relevant 'dit' or 'dah' paddle inputs to the
supply voltage rail in the keyer to send each symbol. The normal
arrangement is to connect the relevant paddle input to ground whenever
the appropriate paddle is pressed. The original approach left a
risk of a short circuit if I was a bit careless with my metal-shrouded
keyer paddles. (I use a cheap and cheerful 'Galbraith' paddle, by the
way, bought many years ago from a local NZART branch. Sadly, they are
no longer available)
Solving that problem was easy. I added two transistors to invert each
line, and these can be seen on the circuit diagram.
Life, of course, continued to pose little problems. Next up: The keyer
had no sidetone oscillator. Most transceivers have built-in sidetone
oscillators which produce a tone simultaneously with the keyer output
signal. But I like to practice my sending a bit from time to time
without turning on the transceiver and letting the world hear my lousy
sending. So I needed to add a sidetone oscillator to the keyer.
Well, really, that was easily fixed too.
Another couple of transistors in my favourite audio oscillator
arrangement worked a treat. It drives a piezo speaker and draws only
1.5mA at full volume. You can find these little speakers inside many
toys or inside musical greeting cards. I seem to recover two or three
of these each year from broken items heading for the rubbish bin, and
they are just great for this sort of project.
Building the Keyer
in a hurry, and it probably shows in the photographs.
I used a scrap of printed circuit board - It wasn't even cut square -
and built it all in an afternoon. I built it 'Mahattan-style', which is
to say, the chips were soldered upside down on the scrap of unetched
circuit board, and
other components soldered in around them, using the legs of the ICs
like tagstrips of old. It actually looks a bit rough, and untidy, but
it's surprisingly robust. Saves making a PCB too.
I used a pair of AA sized batteries to run it since the CMOS devices
hardly draw any current. If I send continuously at top speed, the keyer
draws 30mA. I know that's not ultra-low current, but it's about 10% of
the current drawn by the old TTL design, so I think it's a low current
keyer. Besides, sending at normal speed (for me), at around 12 to 15
WPM (words per minute), the current drain averages about 10mA with the
sidetone turned on.
I glued the battery holder to the side of the box with hot glue. To
tidy it all up, I loomed the wiring using a few nylon cable ties. I had
some blue ones in my parts bin which added to the colour just a bit.
Wiring goes to the original connectors. There are a few holes left
unused in the box. That box was originally made from some thin
I recycled from old commercial VHF transceiver front panels. One unused
hole that can be seen is the original mains cable entry. I left the
rubber grommet on that hole just in case I ever want to build an AC
power supply into the keyer. I suspect it will stay unfilled this way
for at least another 30 years.
There is also a phono socket to connect a regular morse key. I didn't
wire this back in because when I use a straight key, I plug it directly
into the transceiver.
Maybe I'll wire that connector in one day. It will just connect
across the output socket, with a diode added to key the
The switches on the front panel (from right to left) allow you to turn
the power on, turn the sidetone on or off, and the last switch is
wired directly across the output jack. This allows me to continuously
key a transmitter for a few seconds. This is useful for tuning my older
transmitters and antenna tuners.
The knob on the top left hand side of the
front panel sets the sending speed of the keyer. It ranges from about 5
WPM to more than 50 WPM, as far as I can tell. It is much much faster
than I can manage. If that range is too wide for you, fiddle around
with the 470k fixed resistor, the 3M linear (not LOG!) variable
resistor and the 47nF timing capacitor. If I was making it again, I'd
use a 100nF capacitor, a 1M or 2M variable resistor, and a 470k fixed
The knob on the right was used to control the volume of the sidetone
oscillator in the original unit. The piezo speaker didn't need a volume
control. By the way, you can see the piezo speaker in the photo. It's the large black object on the lower
right hand edge of the PCB. It was recovered from an old cordless phone. The volume control
currently just fills a hole in the
front panel, and I left it there. But it does nothing.
run this keyer from 3V, it will work equally well from just above
2V to 12V. The piezo will get a bit loud at the higher voltages, so you
may want to adjust the audio level. Try adding a 1k to 10k resistor in
series with the piezo. Maybe that volume pot will come in handy one day
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