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Undercolor/850105/Blue Skying It

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UnderColor, Volume 1, Number 5, February 20, 1985

  • Title: Blue Skying It
  • Author: William Barden
  • Synopsis: Things you ought to know
  • Page Scans: Link


Hackers are getting a lot of bad press these days. Our local television press has gotten much mileage out of a special report on computer hackers and how they’re using modems to break into all kinds of secure data processing systems, such as NORAD and the Acme Dry Cleaners Bulletin Board system. They’ve made it appear that any computer owner with a modem should be only slightly above John Dillinger on the most wanted list. Let me go on the record, however, as saying that l love hackers and hacking. But the old kind of computer hacking—the sudden insight into a real-world problem that could be computerized to amaze and amuse your friends and make you a hero to your spouse or magazine editor. Let me give you some examples of wild and crazy ideas, that, Von Neumann willing, I'm going to implement some day.

The Cheap Christmas Light Billboard Caper

My latest idea occurred when my early December flight had a near miss at Los Angeles airport with the Goodyear blimp. The collision was avoided only because the blimp pilot

had flashed a WATCH IT, COWBOY! message on the blimp's display as the 747 turned into its final leg on the approach. The display got me thinking. Why not make my own animated billboard driven by the CoCo? It could consist of LEDs or incandescent lamps. LEDs cost about 10 cents apiece, if you find the right electronics store. Incandescent lamps are about 60 cents apiece, normally, but I'll be darned—those small Christmas lights are only $2.00 per 50 lights—4 cents apiece!

If I got some pegboard, drilled out the holes so that I could press fit the small bulbs . . . How many would I need in a matrix . . . Let's see—64 by 24 would be 1536 lamps. I rushed down to the local drugstore and confiscated 35 strings at about $70. I spent the rest of the afternoon drilling out holes on a pegboard and then popping off the lights from the strands.

Then I got to thinking about the way I would drive the lights from the CoCo. I knew that I would not have to turn off each one at the same time, requiring 1536 discrete lines, transistor drivers, and so forth. My idea was to multiplex the lights the way LED displays are multiplexed. Multi-digit LED displays are turned on one digit at a time at a high current, and hence greater brightness than normal. A four-digit LED display, for example, turns on one digit for one-fourth of the time at four times the current. The eye integrates the brightness and comes up with an average brightness equivalent to a normal LED brightness. In doing this, you’ve saved three-quarters of the electronics that would be used for addressing all lights.

The same multiplexing could be done for the Christmas light incandescence. However, I soon learned that there were several problems I hadn’t thought of. One major stumbling block was the amount of current those little beasties take. Would you believe 130 milliamps at 2.5 volts? Putting it another way, to drive 1536 lamps directly from a 20 volt supply would require at least 200 amps, something comparable to an arc welder! Even with multiplexing the lights it would require, say, one-eighth of the current, or 25 amps.

Another "gotcha" I hadn’t considered was the inrush cur-

rent. It turns out that a cold lamp may requires as much as 10 times the current as a warm lamp, making the worst case power requirements 200 amps again, even with multiplexing! I could see screwdrivers and pliers arc-welded together as I turned on the display . . .

Another problem, which I knew could be surmounted, was how to fan out signals for the display, about 192 discrete outputs, even with multiplexing. I had envisioned a tree of 8255 PPIs driven by a general-purpose interface in the CoCo's ROM port. However, additional transistors or drivers were required to handle the high currents. Let’s see, 192 times one dollar’s worth of parts for each output . . . obviously more thought was required.

To sum it all up, my Christmas display, with Merry Christmas messages and animated toy shop scene, wasn’t operative for the '84 holiday season, but I have big plans for next year.

Feasible? Definitely—think current limiting resistors for the incandescence or LED lights and cheaper drivers.

The Line Printer Digitizer Fiasco

In the throes of finishing a Radio Shack book ("How to Use Your Radio Shack Printer"—out soon) I dearly wanted to implement an idea I had seen years before—a digitizer that operated from a printer. The basic idea in digitizing is that a photocell moves across the paper in much the same

fashion as the electron beam in a television picture tube. The light and dark areas of the paper reflect greater or smaller amounts of light which are converted by the photocell into voltage. The voltage can be fed into the CoCo’s joystick port and measured. (The joystick port really varies the resistance of two potentiometers, producing a voltage measured by the CoCo. The voltage measured is converted to joystick position value.)

A printer digitizer uses the basic printer mechanism to move a photocell mounted on the print head—after all, you can "scan" lines on many of the newer Radio Shack printers at 1/60th of an inch or less vertically, and move the print head horizontally under program control as little as 1/180th of an inch. Several people had tried it with other systems, but I had not seen any for the CoCo.

To get some ideas on the project, I called my expert on photo optics, Forrest Mims of Computers and Electronics and MITS fame. He had just completed some experiments using Radio Shack's cadmium sulfide photocell, and it worked fine for digitizing. However, the resolution was rather coarse and the CS photocell somewhat slow in response to changing light levels. He recommended phototransistors with a fiber optic light source. (Forrest loves anything with a fiber optic cable and was once seen trying to tap Pac Tel's massive San Francisco fiber optics routing with a mirror and photodiode.)

I had a better idea . . . why not use a laser I had bought from Edmund Scientific for another unfulfilled project—a laser burglar alarm system? The light source could be very precise and should produce excellent resolution for digitizing text or images. I envisioned the laser pointing in to a mirror on the print head mechanism and reflecting from the paper. I never did implement the idea, but I intend to. However, at Comdex in Las Vegas, I saw Apple's new digitizer; it looked suspiciously like a stripped down printer mechanism with a light source and photo detector ....

The Home Weather Station Disaster

Even though Southern California is not "weather active" (we had a thunderstorm a few years ago), a persistent blue sky project keeps raising its ugly head in my 2:00 a.m. CoCo

dreams—the CoCo could make a great home weather station. It’s cheap enough—you could leave it hooked up and accumulate windspeed, temperature readings, humidity, and other weather variables. As a matter of fact, I did partially implement this idea. Several years ago, I wrote an article for BYTE magazine in which I described a CoCo anemometer. The anemometer (windspeed device) was made out of doweling and plastic cups attached to PVC sprinkler pipe parts. A hole in the shaft interrupted an infrared light beam from a Radio Shack infrared diode. The light beam was detected by an infrared detector. When the anemometer shaft spun, a pulse was produced which could be read and measured by the CoCo joystick switch input. It worked great, until we had the Big Wind of '83 (15 miles an hour).

I thought the infrared diode and detector was an elegant approach to measuring shaft rotation speed until I happened to put together a Heathkit Wind Speed/Direction indicator.

Heath, eschewing sophistication, uses magnetic reed relays and magnets for their shaft rotation speed sensing!

My plan now is to build a complete home weather station using the CoCo. It would have magnetic reed relays to sense windspeed, an LM-334 temperature sensor, a National LX

series pressure transducer for barometric pressure, and a potentiometer direction indicator. All of these signals could feed into the CoCo joystick port, with a little additional electronics. Feasible? No question about it. If I can just find the time ....

And that’s what I mean about hacking. It’s a lot of fun, even though many of the projects only get finished in your mind. For more on hardware hacking, follow Dennis Kitsz, certainly, but also look at Forrest Mims' Engineer’s Notebook (Radio Shack), Bruce Artwick's Microcomputer Interfacing (Prentice Hall), and some of Don Lancaster's books (Howard Sams). Give me a call at 2 a.m. if you get the laser digitizer working . . . (end)