Galileo COSMAC active model

Last edited Nov 23 2016. updated April 26 2018 Edited by Herb Johnson, (c) Herb Johnson Contact Herb at, an email address is on that page.


[galileo model] [galileo model]

The purpose of this Web page, is to describe Lee Hart's work on a model of the Galileo spacecraft, which contains a COSMAC 1802 microprocessor board that displays light and sound. The original Galileo used several COSMAC processors. The model was built for exhibit at VCF-MW 9.0 in Sept 2014. On the right is the model after the exhibit, with Scan Platform details added after the exhibit.

This Web page is edited and published by Herb Johnson, with Lee Hart providing content. Contact information for those involved is provided. (photo courtesy of Josh Bensadon.)

Model on exhibit at VCF-MW 9.0, Sept 13-14 2014

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Lee Hart is holding his model on the right. On the left is an exhibit photo; both photos by Dave Ruske. Dave's photos of VCF-MW 9.0 and the exhibit are on flickr.

Lee posted this text with the Galileo model: "GALILEO - Used six 1802 microcomputers to explore Jupiter and its moons. This model has just one 1802. It is sending the Arecibo binary message. Program *and* data is just 256 bytes. Power consumption is only 0.25mA (plus LED current). :)"

In later comments posted on cosmacelf on Yahoo, Lee commented on the power consumption of his model: "The Membership Card in the Galileo satellite model draws 0.25ma at 4.5v at a clock speed of 50 KHz, running the program from a National NMC27C16-25 CMOS EPROM. During development, it drew 0.3ma at 4.5v running the same program from a Hitachi HM62256BLP-15 32k RAM."

"The LED that is blinking out the Arecibo message draws almost 100 times more power than the computer. Its series resistor is chosen to draw 20ma when on. But since it is off more than half the time, the average power consumption was about 8 ma. :-)" - Lee Hart

After the VCF 2014 exhibit, Lee said: "I used Google to locate online images of the Galileo to model it. There are hundreds of them, but mostly low-resolution or artist's conceptions. When I looked yesterday, *my* model appeared in the list of images, thanks to [this] Web page on it! Good work, Herb!" - Lee Hart

VCFMW or VCF MidWest hase been in mid-September, and located West of Chicago in Lombard, IL. It's part of the Fall Commodore Expo/ECCC, Check the VCFMW Web site for further details about the event, and links to photos and descriptions afterward.

Model on exhibit at VCF-MW 11.0, Sept 2016

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In Sept 2016, Lee Hart showed the Galileo model at the VCF-Midwest Festival 2016, near Chicago. Lee and several COSMAC 1802 owner-developers created an exhibit there, to mark the 40th anniversary of the COSMAC ELF. The ELF was a hand-wired COSMAC 1802 microprocessor kit, published in Popular Electronics in 1976. At VCF Midwest, they provided a hand-wired ELF kit as a prize. Lee has a construction article on his Web site. Photo from Dave Ruske used with permission.

model shown to Apollo, Skylab & Shuttle astronauts at Apollo High School

Nov 12 2014, Lee Hart: A local high school had a visit from *seven* NASA astronauts. Four were Apollo astronauts, 3 on Skylab, and 2 on the Space Shuttle. A rich "local boy made good" businessman, John Pederson, had paid to have them come to his old high school. The event, called Apollo at Apollo, was the vision of Apollo High School graduate Pederson, who said that no Apollo astronaut had ever re-visited St. Cloud, MN's Apollo High School since its 1970 opening, untill today. Lee Hart, as an audience member, brought his Galileo model craft to the event, where it caught the eye of Ed Gibson (Skylab 4). Lee thanked the astronaut for inspiring his career in engineering in the 1970's.

Here's a local-Web-page story about the event. If the link fails, use keywords in this description to find the story. - Herb Johnson

Initial progress, Mid August - Lee Hart

Here's Lee's progress with his "spacecraft" as a physical model of a real spacecraft:

Google comes up with hundreds of images of the Galileo spacecraft, many from NASA. But the Galileo model I bought is from High quality, very detailed, but hard to assemble and *very* fragile!

I enlarged the paper model by 2:1, and used the pieces as templates to cut each part out of thin metal, fiberglass, printed circuit board (PCB) material, and other stronger materials. The main body is mostly 0.030" PCB material, which I soldered together. Definitely a construction technique an Electrical Engineer would do!

Basically, I cut out dozens of little postage-stamp size bits of PCB material, and soldered them together to make the complex body shape. The body is about 2.5" across and 3.5" long, and vaguely cylindrical. The upper third is octagonal, the center third is shaped like a "+" sign, and the bottom third is cylindrical. Half a dozen booms, antennas, and other bits extend out the sides, top, and bottom in various directions. I'm building these now.

I also made some small modifications to put a Membership Card inside. The Membership Card and three AAA batteries fit inside. It's pretty easy to get at the board and batteries; you just pull off the (rotating) cylindrical section, and the board and battery holder slide out of card guides.

I haven't built the big high gain dish antenna yet. It's 9" in diameter and a very open "basket weave" sort of thing. It won't be any good as a light reflector, so I have a 10mm super-bright LED to put at its focus, aimed in the direction the dish is pointing. This LED produces a red spot about the size of your hand 6 feet away. This one is intended to hit the "receiver" phototransistor (if/when I get that far). But I'll probably have some wide-angle LED in series with it, that observers can see from any direction. - Lee Hart

Note from Herb: Lee was unfamiliar with the "Arecibo" program which was written a few years ago by a Membership Card owner. Herb tested the program by loading it into a Membership Card and running it in "write protected" RAM. That confirmed it could run in a ROM only 1802 card. Here's links to that prior work on running this Arecibo program, and where to get the program.


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(Aug 26th:) image 1 (left) is an end view of the upper hexagonal body. I cut a piece of 0.062" PCB material into an octagon 2.5" across. Then I cut eight pieces of 0.030" PCB material into 1" x 1.25" rectangles. I attached them with masking tape, and soldered them together.

image 2 (right) shows the octagon at the other end. I assembled the "+" shaped middle section onto it with more pieces of PCB material. The spaces between the legs of the "+" are where the spherical fuel and oxidyzer tanks go. I made them from 1.125" diameter styrafoam balls, painted with latex paint (not installed yet).
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Image 3 (left) is the Membership Card that slides into the hollow rectangular center of the body. I mounted a 4-cell AAA battery holder to the Membership Card with the two screw holes intended for the 25-pin D-connector. Only 3 of the 4 AAA cells will be used (4.5v). The 4th cell location was just a convenient place to put the two screws to mount it to the Membership Card.

Image 4 (right) shows the Membership Card slid into the card guides inside the body. It just barely fits! However, there are "nooks and crannies" due to the odd shape where other small parts can be placed (piezo buzzer, on/off switch, etc.)
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Image 5 (left) is the complete body, with the cylindrical bottom section added. It is made from a mint tin. The lid has the same rectangular hole in the center for the Membership Card. It is soldered to the bottom of the "+" shaped middle section.

The cylindrical tin is about 3/4" thick. It snaps onto the lid that's soldered to the body, covering the rest of the Membership Card. This section can be rotated, just like the real Galileo. :-) My dimensions were slightly off, so the at first the board did *not* quite fit inside! So I soldered the lid of a second mint tin to the top end, extending it by 1/8" to make it fit.

Image 6 (right) shows the two propulsion booms installed. These extend out from each side of the "+" center section. I bought some 1/8" square brass tubing at a hobby store, and used a belt sander to remove the two diagonal edges to create two "L" shaped pieces. These were used to build the trusses, which are soldered together. The diagonal bracing is #20 solid hookup wire.

Sheet brass was used to make the semi-circular thruster shields. It's (aug 27th) midnight... enough for now.

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Here are [Aug 27th's] work photos.

Image 7 (left) is a close-up of one of the RPMs (retro and propulsion modules). The main engine points down (a screw head). The little attitude jets are wires.

Image 8 (right) has the booms for the two RTG remote thorium power units. Each of these generated about 500 watts. I made the struts from 1/8" brass tubing. The booms are hinged, so they can be folded to more easily handle and transport the model. The flat top is a piece of PCB material; it simulates the heat shield on top of the 3-legged tower structure that I will be building next.
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Image 9 (left) is a close-up of the hinge. I used a piece of copper wire with a hook in the end to simulate the Galileo's deployment struts. Lift the hook, and the RTG booms fold down against the body.

The RTGs themselves are cylinders, which at this scale just happen to be exactly the same size as AAA batteries. If I had noticed this earlier, I would have made them battery holders. :-) [galileo model]

It's hard to find really good pictures of the Galileo. Apparently lots of changes were made along the way. There are also photos with various parts not deployed, or with solar shields blocking the view. I did find a site with some high-resolution CAD images. However, they were made by a hobbyist, who admits that many details are his own speculations and not entirely accurate.

(Aug 29th:) Image 10 (right) shows how I made the trusswork for the booms. I laid a piece of perfboard on a piece of styrafoam, and pushed 1.25" x #16 brads into the holes in the desired locations to make a bending jig. I then routed #20 bus wire between the brads. This is a quick way to make several identical pieces.

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Image 11 (left) shows the resulting trusswork on the two RTG booms for the thermoelectric generators.

Image 12 (right) shows how I made the RTG generators. Each one is a 2" piece of 3/8" dia. wood dowel. The cooling fins are bits of 0.025" aluminum. I made lengthwise slots in the dowel with a coping saw, and pressed the fins into them. The resulting assembly was dipped in black paint for both appearance and to glue it all together.

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Aug 29-30, 2:30am -- a long day today. I wanted to get the science boom finished, and it took longer than expected.

Image 13 (left) is the painted fuel and oxidizer tanks (red and white spheres), and the RTGs (thermoelectric generators) painted black.

Image 14 (right) shows the model with the science boom added. This is just the first section; the entire science boom has three sections and is over 16" long.

Like the two RTG booms, the science boom is hinged so it can be folded for storage. To fold it, remove the thumbscrew at the top. The thumbscrew and strut are the nutation damper on the actual spacecraft.

The length and ability to fold meant the science boom has to be fairly strong to support itself. It took more effort to work this out without changing the Galileo's design. Basically, the struts and hinge were made thicker and stronger than scale.

The little flaps that stick out the side are sun shields, to keep sunlight off instruments below them (which aren't on the boom yet).

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Sept 1-2: I got the long magnetometer boom done. But since its thickness is scaled up (for strength) it made the model side-heavy. So I went back and re-made the RTGs out of heavy copper to act as counterbalances. I also made them into battery holders for AAA cells.

Image 15 (left) is a closeup of the science boom. It is made with 1/8" L-angle brass, 0.010" sheet brass, and #20 bus wire for the truss.

Note: With all its brass and copper, the model has a definite "steampunk" look to it. I was going to paint it, but the colors of the real Galileo are pretty drab. So I might just leave it "naked". It's also hard to tell what Galileo really looked like, as much of it was hidden by thermal insulation. Illustrations also take liberties with the appearance, with the artist coloring it to make the best "picture" and provide more detail.

Sept 2: I got the Magnetometer boom built. This is the long thin one that extends out from the Science boom. Image 16 (right) shows the inner end, which slides into the Science boom and is held by a screw.
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Image 17 (left) is an overall view, showing the outer end of the Mag boom. I haven't attached the plasma wave antenna yet, which is a thin wire dipole that extends out 6.75" from each side of the end. I'll probably use something like wire wrap wire, so it won't present a poking hazard.

I made the Mag boom somewhat stronger, due to its length. A side effect is that this made the model off balance. To counteract this, I rebuilt the two RTGs to be heavier to act as counterbalances.

Image 18 (right) shows one of the new RTGs mounted to the model. Each one is made from a 2.25" piece of 1/2" copper tubing. I cut lengthwise slots for the fins with a Dremel mototool. This was tedious, as copper is very hard to cut -- it is so soft that it plugs up saw or file teeth. I wound up using an abrasive cutoff wheel.

Soldering the fins on was also a challenge. Copper conducts heat so well that I had to use a propane torch. Each fin had to be wedged in its slot with tiny shims to hold it in position, since there was no way to avoid soldering all of them at once.

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image 19 (left) shows that each RTG is also a AAA battery holder. It unscrews from the boom. The threads were another problem. 1/2" threads were too big for the RTG's diameter, and 7/16" too small for an AAA cell to fit through. The only thread size between the two is 15/32"-32 (the standard size used for toggle switches). So I soldered a toggle switch nut to the RTG. I cut off the bushing of a toggle switch, and soldered it to other end of the RTG boom.

To keep solder off the threads while soldering, I wrapped the bushing with teflon plumbing tape, then threaded the nut onto it. The teflon is thin enough to fit between the threads, didn't melt while soldering, and kept the threads free of solder.

Image 20 (right) is an overall view of the model [at this point].

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by Sept 10:Image 21 (left) is the horn-like Star Scanner. This is actually an optical telescope. The photo also shows the flange soldered to the top to mount the High Gain Antenna. The flange is a piece of 1/2" copper pipe, with a 1/16" copper washer soldered on top. Holes are drilled and threaded into the washer to attach the antenna.

Image 22 (right)is the High Gain Antenna. The original was an umbrella-like antenna made with thin ribs and wire mesh. Note: Galileo's HGA failed to fully open. My model shows the HGA as it would have looked if it fully opened.

On the model, the radial ribs are 0.043" steel piano wire, pinched between two PCB material disks with 3 screws and nuts. The outer rim is a piece of #20 bare copper wire. I salvaged pins from a female DB25 connector, and soldered them to the copper rim wire. When the steel wires are plugged into the pins, it naturally forms a nice symmetrical dish shape.

The mesh covering is very thin nylon cloth, like women's pantyhose material, but a metallic golden yellow. I got it at a fabric store. I cut out three pie-shaped sections, and glued them on with Elmer's Glue-All. This method is far from perfect, but the best I found after many other methods were tried and discarded. The glue dries slow, and doesn't stick all that well to steel or nylon. But it is water soluble, so fingerprints and drips can be washed off, and the fabric can be moved to reduce wrinkles.

I glued just the center, and waited until it dried. Then I started gluing the outer rim, one rib at a time. The mesh was wrapped around the outer wire, glued, and clamped until it dried. Then I glued each rib, one at a time, with a custom curved clamping setup. Polyethylene plastic bag material was used between the antenna and the clamp. Elmer's glue won't stick to polyethylene, so it peeled right off. In the photo, the masking tape shows the ribs that I still need to glue.

The central tower is hollow, to allow wires to pass through. The lower part is 3/8" dia. copper tubing. The central section was made from 0.010" brass, bent and soldered.

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Image 23 (left) shows that the top section is removable. Inside is another AAA battery holder. That makes three with the AAA cells inside each RTG. For now, they aren't wired and are just ballast and spare battery holders. When I get time, I'll wire them up in place of the 3-cell battery holder on top of the Membership Card. The battery holder is held on with a screw, so it can be removed to get at the wires on both ends of the battery older.

Image 24 (right) is the removable top section. It has a 10mm dia. super-bright red LED "transmitter" in the center, located where the Low Gain Antenna was on the Galileo. (It was used in place of the HGA, which failed to open). The top sunshade is a piece of 0.030" PCB material. The struts are made with #20 bare copper bus wire. A tiny round PCB holds the LED and two long 0.025" square pins. The pins extend out the bottom of the top section. They plug into two 1-pin Molex KK series sockets, one on each side of the AAA battery holder.

Operation with the COSMAC

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Image 25 (left) show the model as it stands today (Sept 10). For ease of handling, the RTGs, Magnetometer boom, lower cover, fuel tanks, and other odds and ends are not on in this photo.

Image 26 (right) is the 1802 Membership Card CPU board that will be the "brains" of the model. I programmed a 27C16 EPROM with the Arecibo program (only 256 bytes used!) The 4-cell AAA battery holder sits on top of it, with 3 AAA cells and an on/off switch in the 4th cell location. The 30-pin Membership Card I/O connector is cut in half. One half connects to the battery holder to provide power and ground.

The postage-stamp size perf board accesses the other half of the I/O connector. It is used to get power, ground, and the 1802's Q output to operate the LED transmitter. It also has a 2N3904 driver transistor and 100 ohm series resistor for the LED. This board unplugs, so the Membership Card can be removed from the model. You can see the circuit schematic in the photo.

I didn't want to make any modifications to the Membership Card, and wanted the battery holder and LED driver board to both simply unplug. But there is only one VDD pin on Membership Card I/O connector P1, and it has to go two places (battery holder and driver board). I solved this by connecting battery holder + to P1 pin 14 (VDD), P1 pin 13 (RUN), and P3 pin 2 with an extra 1-pin connector. This is /WE, which is not used since we have an EPROM. P1 pin 10 (/WE) now has VDD on it, to power the driver board. A description of the COSMAC's operation is discussed at the top of this Web page.

Power: The Membership Card only draws 0.25ma at 4.5v when running the program but with the LED off. The 100 ohm resistor limits the LED to 20ma with a 4.5v supply. Since the LED is on less than half the time when sending data, average supply current is under 10ma.

In the photo 26, everything is powered up and running. I caught the LED "on", so you can see it has a very tightly focused beam. It is easily visible in direct sunlight, and makes a clear spot on the wall 10 feet away in a well-lit room.

Scan Platform, added after VCFMW exhibit

Sept 19: I unpacked the model; it survived the trip there and back again intact. Somewhat like Dave Ruske's [exhibited] Membership Card, it ran for the entire two days of the show (plus testing time at home) on the same three AAA cells - used ones to begin with.

The bottom round cover [Lee's hand is holding the cover in this linked photo] is the de-spun section: The upper section rotated, while the de-spun section remained stationary. It had a smaller dish antenna, a steerable scan platform with a bunch of instruments, and the probe that was dropped into Jupiter's atmosphere. I didn't have time to finish it, so a blank cover was fitted at the show.
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Image 27 (left): Last night, I finished the small dish antenna. It was used to communicate with the Jupiter probe during its descent. The dish is made from the concave bottom of a spray paint can. The rectangular box that supports it is made from more PCB material. The dish is steerable in two axes; maybe someday I'll put little motors in the box to move it.

Image 28 (right) shows where it fits on the model. The struts on the other side are for the Scan Platform. It was also steerable in two axes, and has a rectangular box that could be motorized.

The last piece to make is the Jupiter probe itself. It's a cone-shaped capsule that dropped off the bottom.
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Sept 22: Image 29 (left) and 30 (right) are photos of the Scan Platform. This is a set of instruments mounted on the despun section and that could be directed and held pointed in any direction. There are more parts in this tiny little assembly than in any other part of the model.

The instruments are actually made from a collection of electronic connectors. Sharp eyes will spot an SMB, BNC, RG59/U, RCA, RG8/U-to-PL259 adapter, and a 9v battery snap. The L-shaped base is made from two pieces of PCB material, with a 5/16" wide strip of 0.010" brass wrapped around the edges and soldered in place.

The Scan Platform pivots up/down and left-right on two screws. Each screw threads into a corresponding nut soldered inside the little box at the hinge point. A tiny rubber o-ring on each screw adds friction, so the Scan Platform will stay in any position.

The sunshade over the top protects the instruments from the sun when they were in their parked position during the early part of the mission when the Galileo was near the sun (as low as the orbit of Venus). The rectangular shield protects the instruments from the exhaust plume of the Galileo's main rocket engine (in the center of this lower cover, under the Galileo's descent probe).

At this point, the descent probe and main engine nozzle still need to be built. After that, it's down to a few bits and pieces, like shields for the RTGs and some small insturments here and there.

Side Notes by Lee Hart

Why do this? This project is just my following through, on an idea that started several years ago. VCFMW just happens to be a good excuse to finally do it. I was looking for 1802 projects that demonstrate what it can do without the usual trivial things that only prove (to some people) how awful early computers were. And it's something to do on evenings, when I get tired of the usual day's work. Stress relief. It has usually moved right along smoothly. Though that hasn't always worked out, either.

A Construction detail: Last night was particularly frustrating. At one point, I had the science boom mostly finished but not attached to the model. As I concentrated on the model, I dropped a tool I was finished with off to the side, and out of the corner of my eye saw something "jump". At a glance, I didn't see anything wrong; so I continued. Later, I couldn't find the science boom! I did a meticulous search of whole area without finding it. I gave up and went to work on other parts.

Much later, I needed to trim some parts. As was my usual practice, I moved the waste basket underneath to catch the clippings. As I was working, I recalled that I had moved the waste basket like that just before the science boom disappeared. So I searched the wastebasket, and there is was! It had flipped off the bench into the waste basket. [Editor's note: Lee says he finished at 2:30AM; that may account for the mysterious "disappearance". - Herb]

Links and references

The COSMAC 1802 processor and Galileo

The Galileo spacecraft to Jupiter used multiple 1802's. Many early amateur and research spacecraft also used the 1802 because of its low power, high logic thresholds, and CMOS/ceramic resistance to radiation. It's likely the first microprocessor to go into space. Here's a history of the COSMAC in early spacecraft.

NASA Galileo Web site
Paper models from NASA
Galileo image

Arecibo program

Decades ago there was a radio "broadcast" into space, of a binary message about the Solar System. A Membership Card owner wrote a program that "blinks" that message on an LED. Here's a web page about Herb's build of a solar-powered "broadcasting" 1802 Membership Card. Lee is using the same card in his model. The Arecibo program was discussed in the cosmacelf Yahoo group in 2013 and 2014.

COSMAC 1802 and Membership Card

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The COSMAC 1802 microprocessor was first developed as a conceptual design in discrete (small scale) TTL logic in the early 1970's by Joseph Weisbecker and several of his RCA colleagues. I cover early COSMAC development on another Web page.

This model contains a COSMAC 1802 "Membership Card". Lee Hart developed the card in 2009, and sells a kit now in its 7th revision. The full kit includes a CPU board (photo on left), a front panel board (lights and switches, see right) and a cover panel. It's programmable in binary with LED's and toggle switches; and contains ROM and RAM. All parts are available in the kits. Assembled, this fits in an Altoids can! Web support for the 1802 Membership Card is at The card is sold by Lee Hart from a Membership Card Web sales page at Lee's site. Lee is a digital electrical engineer, an accomplished electric vehicle owner, power-train designer and developer, and operates the Sunrise EV2 Project.

This page is copyright Herb Johnson (c) 2018. Some content on this page, belongs to the authors so named, and is used with their permission. Contact Herb at, an email address is available on that page..

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