1  Rebuilding Paddles and Joysticks

The easiest way to learn how to build controllers for your home computer is by rebuilding a commercial paddle or joystick. This is a useful training exercise and, in many cases, a necessary project because commercial paddles, which may cost up to $60, are not built very substantially, and they often don't stand up to the rigorous workouts they get.
     Commercial paddles can fail for several reasons: wires can be broken inside the cables, connectors can be bent or crushed, the potentiometer wipers can fail to make contact, or the pushbuttons can break mechanically. Virtually all of these failures are due to the breakage of weak parts that any competent designer would have realized were inadequate for their intended use. This chapter will concentrate on teaching you to fix this kind of controller failure. Fortunately, most of these breakdowns are easily diagnosed and repaired. Doing so will provide a good lesson in basic electronics.

TYPES OF CONTROLS

We have an Apple II Plus Computer; most of the controls in this book were designed and tested for that machine. We will attempt, however, to point out the changes in the designs that are necessary to adapt them for other computers whenever we have the appropriate information. Most home computers have similar electronic circuits for their game controls.
     Controls come in two general types: digital and analog. A digital control (like the Atari paddle) consists of a group of pushbuttons. An analog control, made up of potentiometers that can be adjusted uniformly over a range, is much more versatile.
     Most of the projects in this book will feature analog controls, but we will throw in a couple of projects for digital controllers just for good measure. The various digital paddles are almost identical, but there are two distinct types of analog controls. They differ in the cost of their components and in their electrical wiring, and we will point out these differences.

A WIZARD FOUND THESE PADDLES AND RESURRECTED THEM

Strange as it may seem, it is cheaper, and the final product is better, if you rebuild paddles rather than buy new ones. Let's see what it would take to reconstruct a beat-up pair of Apple paddles and end up with units that are better than the originals. Let's assume that there are broken wires in the cables, that the plug has a missing pin, and that one of the pushbuttons and both of the potentiometers are broken.
     The heart of the paddle is the potentiometer (pot), the electrical component that is located beneath the knob and controls the adjustment range. (See the Electronics Tutorial in Chapter 14 for more complete explanations of the different electronic components used in the projects.) The maximum resistance of the pot is measured in ohms. The Apple, unfortunately, uses a paddle pot with a value of 150K ohms (K indicates thousands).
     Pots of this value are often hard to locate. Other computer manufacturers use much more common values, like 100K ohms and 1 meg-ohm, for their paddle pots. In order to rebuild the paddles you will need to buy two new pots. (The information in the section on correction capacitors demonstrates that it is possible, though somewhat complicated, to use lower values for the paddle pot.)
     You can obtain decent pots from mail-order electronics houses or from local electronics supply stores that sell to the public (look for ads for the latter in your local Yellow Pages). Radio Shack pots are of such poor manufacture that we cannot recommend them. Pots advertised as Mil Spec (built to military specification) are usually excellent, but the 150K value is rare.
     A good pot will be completely sealed, will feel very smooth mechanically, and will be linear, i.e. a graph of how far the knob is turned versus the resulting change in resistance will be a straight line. Most good pots will have quarter-inch round stems. If you want to use the knobs from the Apple paddle, you will have to file the shaft flat on one side.
     The next item on our agenda is the paddle pushbutton, often referred to as the FIRE button. A good pushbutton should make a click that can be heard and felt. The button should be about 3/8-inch in diameter so that using it doesn't tire your finger. A good quality pushbutton switch will cost up to $3. Many of the better ones are slightly larger than the factory originals, so you may have to enlarge or move the hole.
     Since the index finger can control the FIRE button faster and more precisely than the thumb, you may want to move the switch to the back of the paddle where it can be pressed with the index finger when the paddle is held in either hand.
     The cable is also a critical component of the paddle. The wires can be small, but the cable must be mechanically sound and quite flexible. This project requires two cables (3-wire, 3-conductor), each about five feet long. We have found that telephone modular cable with four fine, multistranded wires is good for building paddles. You can purchase satisfactory telephone cable from Radio Shack. You can also weave several individual wires into a cable with a Boy Scout rope-making machine or pull #26 wires inside aquarium tubing. Both of these procedures have worked for us in various projects.
     Replacing the plug is another important part of rebuilding the paddle. On the Apple, the plug is a standard DIP (Dual Inline Package) plug. It can easily be made from a device called a header, or component carrier, at a cost of about $1.25. A header looks something like a standard chip, but has a row of tiny forks to which the individual wires are soldered. Whenever you are soldering on this device, plug it into a loose socket so that the pins will be held straight and will not loosen in the plastic.

MECHANICAL REBUILDING

You will note in figure 1-1 that several mechanical changes were made to the paddle case. First, the new pots were mounted and, if necessary, filed flat to accept the knob. The new switches were mounted in the original holes; you can relocate them if you want to. Paper labels clearly identifying the paddle number were put on the front of the case and covered with transparent tape. A line was drawn on the knob with a felt-tip marker to indicate the amount of turn.
     To give the paddle a solid feel, weights were glued into the bottom half of the case. We used lead wheel weights that had fallen off automobile wheels; they were scavenged on bicycling trips. Fishing weights would also serve the purpose. Such weights can be installed with either epoxy or silicone sealant. We covered the bottom of the paddle with felt, attaching it with rubber cement, to further improve the feel and insure that the paddle will not scratch furniture. In addition, it may be necessary to enlarge the notch for the cable, particularly if you use telephone cable.

ELECTRICAL WIRING

Figure 1-2 shows the standard schematic for an Apple paddle. You may want to refer to page 100 of the Apple II Reference Manual for more details. Note that two resistors from the original paddle, with a value of 570 ohms each, are mounted in the game connector. You may use resistors of any value from 570 to 1000 ohms, rated at 1/4 watt and 5% tolerance.
     You should do the soldering with a small pencil-type iron of from 25 to 42 watts and resin-core soldering. The use of acid-core solder on electronic equipment will destroy it forever and always-no resurrection is possible.

CHECK AND DOUBLE CHECK

The professional procedure for making up a circuit from the schematic requires two photocopies of the schematic drawing and a colored pencil. As you solder the connection, neatly color in each connector and wire on the first copy of the schematic. When everything is colored in, you are finished.
     The second copy is used for the test. When your work is complete, take the fresh copy of the drawing and, with a multimeter, carefully check each line for continuity, coloring it in on the drawing. You will often find that you have missed soldering a wire or two in the circuit.

SOLDERING

You may want to unbolt the potentiometer and the pushbutton to make soldering easier. Cut the cables to length and very carefully strip back the outer insulation. Be especially careful not to nick the wires. A small pair of wire strippers, the type that look like pliers and have an adjustment bolt, are best for this job.
     If you use telephone modular cable, you will find that you have four wires, one more than is necessary. The best use for this extra wire is to double up and use two wires for the line from pin 1 (the +5 volt power supply), which goes to one side of the pot and one side of the switch. This will reduce the chance of the pushbutton affecting the pot reading.
     You will note that figure 1-1 also shows a jumper between the unused leg of the pot and the center terminal. This is considered good electronic practice and helps performance somewhat when the pot begins to wear. If the pushbutton has three terminals, be sure to use the pair marked C (common) and N.O. (normally open).
     To attach the header, strip back the insulation from the cable and expose the wires, trim them neatly to the length required, and tin about 1/8-inch of bare wire on each with solder. Plug the header into an empty socket and locate the mark for pin 1. The cables usually are fed in from the pin 8 end to make them easy to plug into the Apple. Now you can fit the wires into the tiny forks, holding them with a pair of long-nose pliers, and solder them in place.
     Clip off the excess wire with a small pair of diagonal cutters. Place the two pull-down resistors into the header, shortening and bending their leads to fit the forks precisely. Hold the wires with long-nose pliers, not your fingers, while you are soldering.
     If you have a multimeter, you can now check out your work without the risk of plugging it into the computer. Put it on a low ohms scale and measure for continuity between the pins on the connector and the appropriate points indicated in figure 1-2. In addition, measure from pin l, the +5 supply, to pin 8, the ground, to insure that impedance is greater than 50 ohms, and that it remains greater than 50 ohms for all settings of the pot and all pushbutton combinations. It is a good idea to have a friend check your work for you. In any case, checking it three times usually insures correctness.

THE SMOKE TEST

Turn off your computer. Never attach or remove anything from a computer with the power on. Check again for your #1 pin, properly plug it into the paddle connector, and turn the computer back on. If the computer behaves irregularly, turn it off immediately, unplug the paddle, and recheck all your work. An example of irregular behavior: if the disk routine starts up over and over again, it indicates a short to the +5 pin.
     If nothing untoward happens, you can run the Controller Checkout program in chapter 15 to check out the functions of the paddle and pushbuttons. If all is well, turn off your computer and remove the paddle connector.

FINISHING UP THE JOB

Your next task is to install the bottoms on the paddles and build up a strain relief for the cables out of silicone sealant. This material, used for bathroom caulking, is available at most hardware stores. The clear sealant is best for electronics work since it is the least messy. (This material is quite irritating if it gets on your skin or in your eyes, so be careful using it.) You may have to apply two coats of sealant to get a neat result. Allow each coat to dry overnight.

A FINAL TOUCH

One of the most common problems with Apple paddles is bent pins in the connector. This isn't a failure in the design; computer users simply leave them lying around unprotected. Figure 1-3 shows a protective foam block for the connector that should be used whenever the paddle isn't attached to the computer. The best material for this is the stiff but flexible white foam that is used to pack delicate electronic equipment. This foam is easily worked with a pair of scissors. A rubber band holds the foam block in place. A loose socket can also be used to protect the pins.

JOYSTICKS

Most of the procedures we use in reconstructing a paddle can also be used for fixing a joystick. You can obviously replace the connector and the cable. In this case, the cable will require more conductors: four conductors if there is one pushbutton, five if there are two, and six if correction capacitors are required. A double run of the modular telephone cable with stranded wires works much better than the commonly used ribbon cable. There is usually plenty of room in the joystick case to install new pushbuttons; it is just a matter of matching holes.
     The joystick element containing the two pots and the mechanical linkage is more difficult to replace. It is almost impossible to find replacements for just the pots, and the commonly available replacements for the entire element are not very good. You may also have to use pot values other than those originally intended and add correction caps (see the explanation of correction caps below). Joystick elements with centering springs and tabs are usually better made than those without.
     One of the special problems that occurs with joysticks is failure to zero. When this occurs, a pot will read a small positive number-instead of zero-even though the pot is in its extreme low position. Failure to zero is usually caused by poor mechanical construction of the joystick element. There is a procedure to correct this problem, described in the section on Zeroing Joystick Elements below, but it is difficult to complete successfully.
     Be sure to provide proper strain relief for the cable where it exits from the box. The wires of ribbon cables often break at this point.

CORRECTION CAPACITORS

One of the most common ways in which home computers read input from paddles uses a timer circuit. You can tell if this is the procedure used by your computer by counting the wires from the paddle pot back to the computer. If there are two wires (the +5 and pot wires), then the pot is wired as a variable resistor and is used in a timer. If there are three wires (the +5, ground, and pot wires), then the pot is used as a variable voltage device, or true potentiometer. The Apple, the Commodore VIC-20, and many other computers use the timing circuit. Some Radio Shack models use true potentiometers.

     The timing circuit can be adjusted for pot values lower than the original values. This adjustment can be made by adding small capacitors within the paddle and thus requires no modification of the host computer. Figure 1-4 shows how the correction caps can be added to a standard controller circuit.
     The Correction Cap Calculation program in chapter 15 will assist you in calculating the correction cap values for the Apple II. The program works by taking the maximum value of the pot and multipying it by the value of a small cap inside the computer to form a constant. You then divide that constant by the new maximum pot value and subtract the original cap value. This gives you the correction cap value required.
     For the Apple the original maximum pot value is 150K ohms. The original cap is a .022 microfarad. If you have a different computer you can probably get the maximum pot value by opening a paddle and either reading the value off the pot or measuring it with a multimeter. You can usually find the cap on your computer's schematic by tracing the wires from the paddle port back into the machine. These values can then be placed in line 18 of the program.
     The correction caps are soldered from the game control pin to the ground. If they are put inside the paddle, you must bring a ground wire out to the paddle. Alternatively, they can be mounted on a small printed circuit board a few inches down the cable from the connector and encased in packing foam.
     It is difficult to obtain the exact values desired, so you are probably just as well off to buy an inexpensive selection of caps and use trial and error.
     To test your work, run the Controller Checkout program again. If the reading reaches 255 before the pot is turned to its maximum, the cap is too big. If the reading never reaches 255, the correction cap is too small. Several small caps in parallel may be needed to get the correct value. The Correction Cap Calculation program contains the limitations of this procedure.

ZEROING JOYSTICK ELEMENTS

A common failure of joysticks, particularly cheap ones, is not to go to zero. This can be corrected, but the procedure is tricky, and it is possible to ruin the joystick element. If you decide to attempt the correction, first unsolder and disassemble the joystick element and remove the potentiometer. The metal back must then be removed from the pot by straightening the small metal tabs. Do it carefully, because these tabs will not bend many times before they break. In removing the back be careful not to lose any of the internal parts.

     Now the pot element should look like figure 1-5. Note that there is a small active area of resistive element in the middle and two large inactive areas on either side. The problem is that the inactive areas have too much resistance to read as zero. Look closely at the limits of the wiper track, which leaves a mark, in the active area.
     The trick is to reduce the resistance of the inactive areas. This can be done by painting over them with sterling silver pigmented paint, available at electronics stores, or with a homemade paint made from clear nail polish and lock graphite. Needless to say, the expensive silver paint is better.
     Carefully wipe clean the inactive areas. Make up a mixture of nail polish and graphite into a paste on a smooth surface, then paint the inactive area of the pot element, covering the terminal end and continuing until you just touch the end of the wiper track. Be very careful that the paint goes nowhere else, particularly not where it might touch the metal case or the metal feelers for the central terminal. It may take two coats for complete coverage. Allow the paint to dry thoroughly.
     Now reassemble the pot and the joystick element. Resolder the wires according to the schematic. Run the checkout program. If correction caps were used you may have to adjust their values.