Building a Scaled-Down Space Invaders Game With Raspberry Pi

Some months ago I finished an original Space Invaders restoration project, but as you can see on the picture on the left, it was a little too big for my daughter.

So the other day I stumbled across a cheap 5.5 inch CRT black and white TV and I decided to make a “mini-me” version for her.

There are many Space Invaders scale models out there, including what is supposedly the world’s tiniest version, but … they use LCDs. To make an accurate reproduction you need a CRT.

Also, one of the coolest things about SI is that, instead of looking directly into the monitor, it uses a semi-translucent mirror with a backdrop of the moon illuminated by a backlight to create the illusion of depth.

Sounds a little complicated, but let’s just say that the resulting effect is unique and really cool.

WARNING: CRTs are nasty, dangerous devices even at 5.5 inches … I’ve taken a lot of shocks in my life, but this one can kill you. So if you’re going to make this project, first learn how to discharge a CRT. Do not do improvise here or it will be your last act.

So lets start with the cabinet.

I’m going to use 5.5mm MDF in place of the original 3/4″ panels so that will dictate the scale. Since I have the original machine it was easy to get the angles and measurements, but I was surprised to find a pretty accurate model in the SketchUp cloud repository that saved a lot of time. I remodeled over it, corrected the angles, and ended with something really close to original.

You can get the file with all the parts used to make this model here: http://www.thingiverse.com/thing:82583

Then this needs to go to the laser cutter so the next step is to “unfold” and flatten out the model. Most was cut in 5.5mm MDF. I used 3mm for small parts. It took two 60x40cm sheets of 5.5mm, and one 40x40cm of 3mm.

Next we need to glue the parts. It is important to not to go too fast here. We need only the basic structure, so it’s easy to mount the electronics. The front and roof will be glued at the very end.

Then it was time to dress up the cabinet. I discarded the spray paint and stencils (for the next one!) and instead I printed decals. The trickiest one was the bezel. I printed a translucent decal so I could stick it onto the acrylic. Then I masked and spray-painted with white the opaque areas. The result was very nice.

The side art fit perfectly and the overall result was better than expected (not as “accurate” as if I’d used stencils, but it works).

Now for the coin door and speaker cover. I decided to 3D-print them so I started again with SketchUp:

The prints turned out amazing well. I used my Ultimaker 3D printer and white PLA. PLA doesn’t have the warping that usually makes ABS parts less accurate. If you look at the hinge detail you’ll see how well it printed.

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For the bezel illumination I used standard LEDs and some resistors. This will be powered with the 12V TV supply so you need to do some calculations. Since the LEDs create a very narrow spot of light, I used some cotton pads to diffuse them and the result was very good.

One important detail of SI is the black light that illuminates the backdrop of the moon (and the galaxy beyond) that goes behind the “one-way” mirror.

For the moon I cut the outline in the laser cutter and then printed the art on a water-slide decal. Another important detail is to add fluorescent paint so it reacts with the backlight. I used fluorescent orange with an airbrush to get an even finish. For the black light I was thinking of using UV LEDs, but incredibly enough I found this device to check counterfeited bills. It has a small UV tube that fits perfectly and makes for a much more accurate reproduction of the real game.

Then I 3D-printed a new backlight support similar to the one in the original cabinet. Here is the support and the end result:

As I mentioned earlier one of the distinctive things about SI is that it uses a “one-way” mirror to reflect the actual monitor. It’s a really neat feature. The one-way mirror is widely used for office windows (or interrogation rooms) so it was easy to get.

The control panel was a little bit tricky since I wanted to use the Raspberry Pi’s GPIO pins. I used an Arduino proto board to create the board. It’s really simple, just a few resistors to pull up the pins and protect the Pi. You can get info on how to do it here. Then it’s all about running the service on the Pi so MAME can recognize the inputs. (Be careful to not short-circuit or send the wrong voltage to the PI GPIOs or you will fry it).

For the control panel, I cut a 1.5mm aluminum plate with the CNC router and combined it with water-slide decal. When the decal dried out it looked like the original silk-screen version.

The brain of the machine is a Raspberry Pi. You need to install MAME and configure it so it directly launches SI. Also, if you’re going to use the GPIO, you need to run a service so PiMame detects those inputs. For the output, you use the composite video so you can connect it directly to your TV in. Same thing with audio, from Pi audio out to the TV audio in. That’s all you need.

That’s mostly it. Besides this, the last trick was to power everything. I used the TV’s 12V supply to power the bezel LEDS. The Pi uses a hacked iPad power supply glued to one of the internal walls and the UV tube gets direct 110V. At the end, it all fits inside the cabinet so you have just one power cord coming out.

So here is the end result. A Raspberry Pi, scaled version of the classic Space Invaders.

Tiburcio de la Carcova is an entrepreneur based in Santiago, Chile, and founder of Santiago’s Mini Maker Faire.