The NanoBug III – Some Design Improvements
This bug is the third attempt to build a really small semi-automatic telegraph key whose size is in keeping with the really small QRP gear that lends itself to portable and outdoor operation. Though full-sized bugs are not well suited to portable operation (an intentional understatement), a small-sized replacement is also not of much value if its performance is not the equivalent of its larger counterparts. The first two bugs in the current NanoBug project worked well, but each had some drawbacks that encouraged further development. The major problem with the first attempt (NanoBug I) was that it was very sensitive to the levelness of the surface that it was sitting on. It also lacked a positive centering mechanism for the driving lever, which tended to overshoot the center at higher speeds, sometimes causing “bounce” in the characters being sent. The second bug in the series (NanoBug II) was not nearly as sensitive to position, but it still had some overshoot problems.
Design Challenges for the Third Attempt
The position sensitivity of NanoBug I was due to its short but heavy pendulum being mounted horizontally at one end; any tilting of the key allowed gravity to pull the pendulum out of position. The overshoot problems were due to the magnetic centering system – it was not very positive, because it was a compromise between the effort to move the paddle without upsetting the key and the force needed to bring the paddle back to the center. NanoBug II used a symmetrical vertical pendulum that greatly reduced the effects of orientation, and a Teflon ball detent system (along with a magnetic assist) provided a more positive centering. The very short vertical pendulum also required that its bearings have very low friction; any bearing drag provides a very quick damping, which limits the length of a string of dots. A third problem with the two predecessors was their mechanical complexity, which made them time-consuming to build.
NanoBug III approached the friction problem by using a significantly more massive pendulum, which allowed the inertia of the pendulum to be quite large in comparison to the bearing frictional drag. More attention was paid to keeping the pendulum symmetrical and balanced. A new lever system was designed, based on the “ball pivot” and “rocker plate” design used in some earlier paddles. This allowed a very positive “dead center” for the driving lever and greatly reduced the tendency for the key to “bounce.” And there was the third challenge – keeping it small but functional and less complex to build.
Here is the key that resulted, with its principal parts labeled –
The key is of the right-angle design, with the driving (operating) lever working against a vertical column that also supports the rear of the pendulum. This column contains the cylindrical holes for the pivot balls, which are attached to the lever (illustrated later), and it supports the top plate. The pendulum is suspended on ball bearings at its front and rear and carries two rare-earth magnets. At the top front is the follower magnet that responds to paddle movements, and at the lower rear is the magnet that activates the reed switch contained in the base of the key. The driver magnet (attached to the speed control screw) is borne on a lateral extension of the driving lever. When the paddle is pressed in the “dot” direction, this magnet moves toward the operator of the key and attracts the pendulum magnet to follow. As the driver magnet stops, the pendulum oscillates around this position. Its motion generates the series of dots, which continues as long as the paddle is pressed. The rate of dot generation is determined by the speed control screw – moving it towards the pendulum increases the magnetic attraction and speeds up the key. This is the only means of speed control, since the pendulum pivot position is fixed. Moving the paddle in the other direction allows for manual dashes, and the rest position stop prevents the pendulum from following this motion. The contact adjust screws limit the paddle travel, and the dot contact screw effectively controls the duration of the individual dashes.
Most of the key is made of type 360 brass, held together with stainless steel screws. Parts of the operating lever are made of 2024 T4 aluminum, in an effort to decrease the weight and prevent overshoot. The reed switch and wiring are contained within the base block, and the whole key is mounted on a brass plate. Brass parts were satin finished and protected with spray lacquer.
Connection to the keyed circuit is made via a 1/8” phone jack mounted in the base. Rather than providing lock nuts for the adjusting screws, coil spring tensioners were used to prevent drifting of the adjustments. The overall weight of the key is slightly under one pound (450 grams). Its base is a little over 2 inches square and the overall height is slightly under inches. Stick-on silicone rubber feet prevent “skating” even with vigorous keying. The speed range is from around ten to thirty words per minute – it can be “tweaked” for higher or lower speeds.
Stages of Construction and Assembly
The pictures that follow were made after the machining was done, and before the final finishing. Though the parts look messy, they show detail better than the finished and polished parts.
Details of the key base. (The cryptic numbers help me keep track of the photos.)
193. The base is a block of brass to which two cylindrical extensions are fastened with 4-40 screws. The phone jack is held in its hole with a setscrew. At the right edge of the base there is a hole drilled through the length of the base, although it is interrupted by the cutout for the bearing column. This will hold the reed switch tube and allow for its adjustment. 179. The lateral cylinders support another brass member that will hold the front pendulum bearing. This will also help hold the base plate. 180. The wiring channels are milled in the underside of the base block. 178. The contact pillars are mounted to the base with nylon insulators for the 2-56 screws that hold them. 186. This whole assembly is mounted on the brass plate, which also carries the rest position stop. The column that carries the bearing assemblies fits into the cutout in the base. 176. The last frame shows the base ready to receive the pendulum and the operating lever assembly. The coil spring will provide tension for the operating lever assembly.
Details of the operating lever assembly.
191. With the exception of the bearing balls (upper right) and the brass extension rods, all of the lever parts are made of aluminum for reduced mass. 183. The partly-assembled lever show the two brass balls mounted to its side. They will mate with the two holes in the support cylinder (184). These holes are reamed out 0.001” oversize so that the brass balls are a sliding fit. The coil spring (186 and 187) presses the lever against the flat side of the bearing column. Moving the fingerpiece in either direction allows the lever to pivot against a corner of the bearing column. This results in a very smooth action with a positive center position and very little “play.” Because the pivot point of the lever differs between dot and dash movements, the edges of the bearing column are modified to provide equivalent dot and dash force. Varying the coil spring compression changes the paddle tension. 188. The driver magnet and speed control screw are mounted on the lateral extension of the driving lever. Cutouts in the sides of the pendulum (next illustration) allow for clearance of the extension rods.
Assembling the Key.
There are three major subassemblies for the key. They are 1) the base and pendulum support, 2) the lever assembly, and 3) the pendulum. The pendulum rest stop mounts onto the base plate, while the top plate provides additional support to the upright structures.
171. The major subassemblies, ready to be put together. 176. The pendulum support consists of a ball bearing mounted on the bearing column and an upright support member at the far end of the key into which a pendulum-mounted bearing will fit. The column-mounted bearing fits into a hole on the rear face of the pendulum. The pendulum is shown mounted in 165, and the almost-finished key is in 160. Panel 156 shows the key ready for its initial testing before finishing and polishing.
Shown below are some views of the finished bug. Its overall footprint is slightly
smaller that of its predecessors, while its weight is greater. In operation, the key is stable and holds its adjustments well. It is quite insensitive to its position. In fact, it can work upside down or standing on its side.
This key has helped to test out some ideas that were suggested by problems encountered with the earlier attempts that are shown below for comparison.
73 de Rich, WB9LPU
© 2009 by Richard A. Meiss, Ph.D.
Aug. 8, 2009