Ham Radio

The Bandersnatch -- CW Touch Paddle

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The "Bandersnatch" is a iambic paddle for use by a Ham Radio operator when operating CW (Morse Code). Unlike traditional iambic paddles, the Bandersnatch is not based on mechanical switches, but on a pair of capacitive touch plates. 

First, a bit of history...

While no longer required in order to earn an Amateur (Ham) Radio license, operating CW is still a valid and popular mode. Just as you need a microphone to talk over the radio, you need key in order to enter Morse Code. There are several different kinds of keys available, but most people are familiar with what's known as a Straight Key. A Straight Key is basically a rugged momentary switch that can be turned on and off accurately, and repeatedly on a daily basis. You've probably seen this kind of thing in movies where telegraphs are involved. Modern, mass produced straight keys can be bought for as low as $15 USD. Straight keys, however, have some drawbacks. Operating straight keys for long periods of time can be tiring on the hands, especially at higher speeds where the physical motion becomes a limitation.

In order to solve this problem, Vibroplex created a mechanical semi-automatic key. As opposed to a straight key where the lever moved up and down, the "Bug" as it affectionately became known, moved left to right. When you move the lever to the right, it operated like a straight key providing continuous contact. When you moved it to the left, however, the device made a series of short contacts -- dits -- without the need for additional motion from the operator. A series of weights allowed the speed of these dits to be controlled. The Bug is still sold today starting at $230, and has a dedicated following of hams. 

While some fully automatic keys exist, it was the introduction of the Electronic Keyer that led to the today's Iambic Paddle. Instead of a single up-down or left-right lever, an Iambic Paddle is two separate keys on the same base. A typical arrangement mimics the Bug. If you move the left lever, you get a series of dits. If you move the right, you get a series of dahs. This allows for much less motion from the operator, allowing higher speeds and longer comfortable operation. The speed is controlled by the circuitry inside the electronic keyer. Today, these keyers are typically built into ham radio equipment, and do not require a separate purchase.

A cheapskate in search of a good key

Iambic Paddles typically are in the middle ground of price. A makeshift model can be as low as $25, but paddles suitable for daily use typically start at $100. That's a lot for two switches! While it's not uncommon for ham radio operators to build keys out of nearly anything, building one for anything other than temporary operation requires a constellation of tools, materials, and skills. Granted, two contact switches could be used, and have on more than occasion, but it doesn't suit the aesthetics I was looking for in a key. Lacking readily available parts to build one, and tools to make one, I was at a loss until I ran across an unusual alternative.

The CW Touch Keyer is actually a series of electronic iambic key/keyer combinations that rely on the electrical properties of human fingers instead of mechanical motion. Instead of two physical switches, two metal touch plates are used. When you touch a plate, the flesh in your finger causes an increase in capacitance that can be registered with a small circuit. This, in turn, can be used to close a relay or solid-state switch that simulates the physical switch. This isn't a new idea. The series of keys/keyers offered by CW Touch Keyer, however, are one of the few if not the only commercial offering available today.

The most basic key, the Model P3, is available for only $22. Add two pieces of metal for touch plates, a case, and a battery, and you have an iambic paddle that'll last well beyond it's low investment price. 

Assembling the parts

It was early in December, and I had two weeks of vacation looming. I figured that if I ordered the kit early, it would arrive just in time for vacation, allowing me to have something productive on which to spend my time off. When I ordered the kit, I discovered that you could also buy a pair of gold-plated touch plates for an extra $16. While I could have used copper-clad PC board (PCB), cut pieces from stock I bought at the hardware store, or even old house keys, the cost and convience seemed more than worth it. Unfortunately, I soon found out that the supplier was out of the touch plates, pushing back the delivery of my order a month! Well, at least it gave me time to think. Originally, I wanted to build the key into as small of a case as possible. I was hoping I'd even be able to place the kit's PCB between the touch plates. As I thought about it, however, I realized that the touch plates were probably too sensitive for that sort of arraignment. It also left no room for a battery. The other problem was weight. Typically a CW key, straight, Bug, or iambic, has a bit of heft to prevent it from "walking" around the desk during operation. Assuming a plastic case, the PCB and touch plates wouldn't be enough to anchor it to a desk. While the circuit could be powered by a small 9V battery, I began to think I could several AAs. Not only did this provide more current capacity, but the added weight would help keep the key stationary. 

Finding a case proved more difficult. Radio Shack had stopped selling a cases with integrated battery holders. Sparkfun and Adafruit also no longer sold anything of the sort. In fact, none of the enclosures I found were suitable. This problem persisted even when the parts arrived in early January.

The first build

The first build of my touch key was little more than the parts I had at hand. I found a 9V battery connector, a 3.5mm stereo jack, wire, and a plastic bob that was originally part of a mobile phone car mount. It was slapped together, but that was the point. I wanted to get the circuit working. Only afterward, would I figure out how to make it look good. The touch circuit proved to be far more sensitive than I had expected. The close proximity of the wires or to mounting hardware such as screws would cause it to send dits and dahs. Obviously, I needed a better solution. For the moment, however, the key was a joy to operate. No force was required on the touch plates whatsoever. The gold-plating proved a luxurious touch as well. It not only made the key look good, it felt good to use as well. The gold-plating also increased longevity by being less reactive to skin and oils. 

The second build

Eventually, I decided to gamble on finding a case at Ax-Man, a local chain of surplus stores. A trip to Ax-Man rarely brings consistent victory, but you do have a higher than average chance of finding something you never expected for a very, very good price. There, I found a hollow metal enclosure that is primarily used as a heat sink for audio amplifiers. It was small, rugged, and beautiful. I picked up two different lengths given the low price. 

The small size of the case had me reconsider the use of a 9V battery. Nothing else had the voltage density in such a small package. When I finally got the cases home, however, it was obvious that wouldn't work. There simply wasn't enough room for a 9V without cutting the bottom of the case. Worse, the touch plates I had were too tall to fit. I could have cut them, but it would have possibly ruined the gold finish. I was stumped.

After letting the parts languish on my desk for another week inspiration stuck when I found the Pagasus Key by a Japanese CQ club. Unlike traditional iambic keys where the paddles where vertically mounted, the Pegasus had them horizontally mounted. While this breaks all convention, one of the YouTube videos demonstrating the Pegasus convinced me this was not only acceptable, but better! Younger hams like myself grew up using computer mice. I'm accustomed to using my pointer and middle finger on a regular basis for accurate control of a device, much more than the thumb. Changing the touch panels to a horizontal configuration not only was more comfortable, but it was more accurate, and could be used on the existing case without modification.

Once I settled on the touch plate configuration, I was able to confront the remaining issue of the power supply. There was enough room in the case for three AAs, but that left little room for a battery holder. Poking around the 'net revealed that short of using PCB mounted battery clips, AAs were not an option. I turned my attention to AAAs. Here, I met with some success. I was not able to find four cell AAA holders that would fit, although barely. When the next Saturday rolled around, I headed out early to look over my options. I found a four cell holder with a switch that might fit, and a 3 cell holder that certainly would. I also picked up a perfboard on which to mount the kit PCB and the touch plates. Once I got the parts home, I discovered that when the cover of the four cell holder was removed, the remaining holder would slide into the metal enclosure snugly. Shortly thereafter, I had the key working in it's attractive new home.

Lingering issues

The second build was not without problems. While the battery holder was in the case, you couldn't get at the power switch. Worse, there was no where on either opening of the case on which to mount the 3.5mm stereo jack. Instead, I had wires coming out of the front to the jack. This marred the otherwise great build. 

Again the parts sat on my desk, although not long. I can't entirely remember if it was a day or a few days, but it was by no means a week. The problem was that the circuit specified 5V or higher as input. That left no room in the back for a jack. The way the circuit was built, however, suggested that it could possibly be powered by 4.5V or less. I wasn't sure, and I had no idea how long the battery would last. A quick test showed that yes, it could be powered by three AAAs. When I discovered that, I sacrificed one battery slot for the jack. I removed one of the battery clips, and soldered a jumper to complete the holder's circuit. The now empty battery slot could then have a hole drilled in one side to mount the jack. 

The last issue was what to do about the open front. This is actually a remaining issue at this time. The problem is that the battery holder cannot be pulled through the case, it must be removed from one side. The limited space inside the case means that none of the wires can be that long, meaning the entire perfboard/PCB/touch plate assembly must come with it. To date, I've tried to solve the problem with a cap made of wood laminate, but it simply isn't attractive. The power switch is also inaccessible without removing the battery holder.

What's with the name?

The unusual horizontal configuration of the key gives it a rather bucktoothed appearance when viewed from above. While this could have suggested a more problematic name for the little gizmo, I found a much better name in what I was reading. Around the time I was working on the second build, the webcomic Digger by Ursula Vernon came into print. Pazi had bought the first several books, and encouraged me to read them. In book five, our unwilling wombat heroine encounters a two headed, horse-like creature called a "Bandersnatch". When I looked at the creature's two large front teeth, I thought of my key. Curiously, the wood cap on the front could be thought of as the "gums". I have yet to paint to little cartoon eyes on the key, but I may very well do that in the future.