11 Desk Switched Outlet Box
On the Apple and most other home computers the ON/OFF switch is not conveniently located and frequently wears out. This switch usually causes more problems than any other single piece of hardware in the system. The switch itself is difficult to replace because it is built into the power supply box. It would also be handy to automatically turn on other devices like the monitor and the printer at the same time that the computer is switched on. There are commercial switched outlets that serve this purpose, but they are priced much too high ($30-$50) in relation to the materials they contain. Besides, the switches in these units are usually located beside the outlets and thus are no easier to reach than the original Apple switch.
This chapter covers the construction of a desk switched outlet box in which the ON/OFF switch is placed in an attractive box beside the computer and the outlets are in back of and below the desk. Special add-on features, including a power-on light, a voltage spike suppressor, and an electromagnetic interference filter are also outlined. You can install only those features that you decide you need.
The materials for this project are readily available. Most can be found at any hardware store that has a home electric department and the rest can be found at Radio Shack. A few simple household tools including a screwdriver, long-nose pliers, and a soldering iron are required. Construction involves primarily the assembly of ready-made parts.
CONSTRUCTION OF THE OUTLET BOX
The outlet box is a standard 4-inch square conduit box with a front plate for two duplex outlets, a cover (if the front plate does not include one), and two cable clamps. If you want to include some of the special features you will need a larger box. Local electrical supply houses usually carry larger multiple conduit boxes or single outlet boxes that bolt together side by side to house any number of outlets.
We pop riveted a small sheet metal tab to the back of the box so that we could hang it under the desk with two panhead screws. If you plan to place the box on the back of your desk you can attach plastic feet or cover the bottom with felt.
To install the cable clamps, knock out two of the metal slugs, place the clamps through the hole, and tighten the nut inside the box. The clamping screws should face the front of the box. A bit of Loctite or Super Glue on the nut threads will secure the nut.
The box for the ON/OFF switch should be decorative. You can, of course, use any metal or plastic box, but most electronic and electrical boxes look out of place on a desk. Our prototype switch box has sloping hardwood sides and a sheet metal top and bottom, as shown in figure 11-1. Making the sloping sides is a good exercise in the use of a radial arm or table saw. If you don't have a shop manual that demonstrates how to set the angles you will have to do some trial and error work on scrap wood. The corners are held together with wood glue and finishing nails driven into predrilled holes. After the four sides were glued together, the top was sloped and the bottom flattened with hand tools. You will have to file out a notch in the back for the cable. Sand the hardwood pieces and finish them with polyurethane varnish.
Cut the top and bottom plates from aluminum or steel. Drill and file the top to fit the switch and the lamp. Drill and counter-sink holes in the bottom plate for six #6 flathead screws. Attach the top to the box with silicone sealant or epoxy so that no screws will show. Finish the top plate with an orbital sander to a. uniform matte finish and then varnish the entire box.
For the electrical work you will need wire strippers, long-nose pliers, wire cutters, and a medium-size standard screwdriver. This project requires a soldering iron of about 50 watts to tin the wires and solder the connections on the switch and lamp. A 50 watt iron is larger than the size called for in the other controller projects, but it is a standard iron used for many household jobs.
The electrical wiring is shown in figure 11-2. Note that wire colors are indicated. These colors are an important part of the National Electrical Code and must be followed scrupulously. The Electronics Tutorial in chapter 14 contains a detailed explanation of the correct procedures for wiring this unit and other AC appliances. You should review the section on AC Codes and Wiring Practices before starting this project. As before, coloring in two photocopies of the wiring diagram (figure 11-2) is the best way to keep track of and check your work.
The easiest way to obtain the cable is to purchase an extension cord long enough to make both cables. A 12-foot cord is usually sufficient. Cut off the outlet end and about four feet of cable to make cable #2. Number 143-conductor cable is best for almost all home and office computer systems.
You will have to join several wires together in four places, as shown in figure 11-2. This is most easily done with wire nuts; yellow ones are the correct size for this project if you use #14 wire. The proper installation of wire nuts is covered in the discussion of AC Codes in the Electronics Tutorial.
The varistor shown in figure 11-2 removes voltage spikes from the incoming line. It has two bare solid-wire leads. You should cover all but the last 1/2-inch of each of these leads with insulation stripped from scrap wire to insure that the leads don't touch the metal box.
You can attach green wire pigtails about six inches long to the green screws on the two outlets. Attach another pigtail to the box with either a green screw or a green metal clip. You can group these pigtails and the green wires from cable #1 together with a yellow wire nut.
Make the connections between the outlets with scrap wire of the correct color and size. Alternatively, you can purchase a short piece of 3-wire #12 Rolex cable (used for the wiring in houses) and remove the outer cover to provide three separate wires of the correct size and insulation color.
Choose a switch that looks good on the desk box. A simple ON/OFF switch, single-pole single-throw (SPST), with 10-amp current capacity will be suitable for all but the largest computer systems. If the switch you purchase has more sections (double-pole single-throw or double-pole double-throw), you can parallel the SPST sections to increase the current capacity. Some of the better switches have a 110-volt AC light built into the switch.
When you have completed all the connections and checked your work, screw the box together and tighten the cable clamps. Plug the unit into a 3-wire house outlet with nothing plugged into the new outlet box. If the circuit breaker doesn't blow, throw the desk switch. The lamp on the switch box should turn on and off with the switch. If it stays lighted all the time, it is connected to the wrong side of the switch.
Now plug a light load like a reading lamp into the outlet box and try the switch again. If you encounter any problem (a blown circuit breaker or a switch that works backwards), unplug the unit, recheck all the wiring, and make corrections. When the unit works properly with a household lamp you can plug your computer into the outlet.
There are several other features you may want to add to your unit. If you are concerned about lightning strikes or other severe electrical surges, you can add two additional varistors. One should be installed from the black wire to the green and the other from the white wire to the green wire beside the first varistor.
If electrical interference is affecting your television set or your neighbor's when your computer is on, you might add an EMI filter like Radio Shack's #273-100. This will prevent electrical noise from leaving the computer by way of the AC line. Since most such noise is radiated directly into the air, this probably won't help your television reception much. If you add the filter you will also need a larger box.
If you decide you want a fuse and fuse holder you will, again, need a slightly larger outlet box. Alternatively, small pushbutton-style circuit breakers can be purchased through large mail-order electronics houses. A rating of 10 amps at 110 volts would be suitable for most personal computer systems.
If unauthorized people have been using your system or turning off the computer when you have intentionally left it on to run a long program you may want to add a key switch to the desk box. Mount the key switch (for example, Radio Shack #49-523) in the switch box either in series (lock OFF) or parallel (lock ON) with the main switch. Since the key switch usually has a lower amp rating, use the main switch routinely and key only when you need the lock feature.
We are confident that you will use the Desk Switched Outlet Box more than any other device in this series of designs. We have built at least a half dozen of them. They make great gifts for anyone who uses a computer, either at home or at work.
Desk Switched Outlet Box
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