Easy Parabolic Mirror from a Trash Can Lid

Parabolic reflectors for solar applications are nice stuff, and making your own is a great project in itself. One of the easiest ways we have seen is that of [GREENPOWERSCIENCE], who uses nothing more than a trash can lid, mylar film, and tape. You need a way to make a partial vacuum though.

The idea is so simple that it´s almost like cheating. Cut a circle of mylar slightly larger than the lid, and tape it all around, taking care of stretching the mylar in the process. After you´re done with this, you end up with a nice flat mirror. Here´s where the vacuum is needed to force the film into parabolic shape. Extract the air from a little hole in the lid that was previously drilled, and tape it to prevent the loss of the vacuum. The atmospheric pressure on the mylar film will take care of the job, and magically you get a nearly-parabolic reflector ready for work.

In this other video, you can see the reflector in action burning stuff. One obvious problem with this technique is the loss of the vacuum after some time, about an hour according to the author. Here´s another way to make a more durable mirror also with mylar as the reflecting element, however the quality of the resulting mirror is not as good.

 

Pressure-formed Parabolic Mirror from a Mylar Blanket

Parabolic reflectors are pretty handy devices. Whether you’re building a microwave antenna or a long-distance directional microphone, suitable commercial dishes aren’t that hard to come by. But a big, shiny mirror for your solar death-ray needs is another matter, which is where this pressure-formed space blanket mirror might come in handy.

Pressure-forming was a great choice for [NighthawkInLight]’s mirror. We’ve covered pressure-formed plastic domes before, and this process is similar. A sheet of PVC with a recessed air fitting forms the platen. The metallized Mylar space blanket, stretched across a wooden frame to pull out the wrinkles and folds, is applied to a circle of epoxy on the platen. After curing, a few puffs with a bicycle tire pump forms the curve and stretches the film even smoother. [NighthawkInLight]’s first attempt at supporting the film with spray foam insulation was a bust, but the later attempt with fiberglass mesh worked great. A little edge support for the resulting shiny taco shell and the mirror was capable of the required degree of destructive potential.

We doubt this process can be optimized enough to produce astronomy-grade mirrors for visible light, but it still has a lot of potential applications. Maybe a fiberglass radio astronomy dish could be pressure-formed directly with a rig like this?

Foam and Antistatic bag Bring Vintage Compaq Keyboard back to life

After winning an online auction for an 1980s vintage Compaq Portable PC, [leadacid44] discovered why it only cost him $5USD – the keyboard was shot. Not willing to accept having forked out $45USD to ship a brick, he tore into the ancient machine and came up with a found-material solution to the wonky keyboard.

[leadacid44]’s very detailed writeup of the fix for his Compaq includes a thorough examination of the guts of the machine. He got it to boot to MS-DOS 5.0 off of a 20MB ISA hard drive card and began probing the keyboard problem. It turns out the Compaq keyboard has much in common with a modern touchscreen, in that it’s a capacitive keyboard. Unfortunately the foam disks used as springs under each key cap had degraded over the last 30 years, so [leadacid44] began a quest to replace them. After much experimentation and a few false starts, he created a sandwich of transparency film, closed-cell polyethylene foam, and a Mylar antistatic bag. Many discs were punched out with a leather punch and tediously placed in the body of each key switch, and the quick brown fox was soon jumping flawlessly over the lazy dog.

We’ve seen some fixes to these lovable luggables before, like this dumpster queen that became a Hackaday Retro submission. At least [leadacid44]s machine didn’t release the Magic Blue Smoke like that one did.

How To Laser Cut Mylar Solder Stencils

When you think about the difficulties of working with surface mount components, the first thing that often comes to mind is trying to solder those tiny little parts. Instead of soldering those parts by hand, you can actually apply solder paste to the pads and place all of the components on at once. You can then heat up the entire board so all of the parts are soldered simultaneously. It sounds so much easier! The only problem is you then need a solder stencil. You somehow have to get a thin sheet of material that has a perfectly sized hole where all of your solder pads are. It’s not exactly trivial to cut them out by hand.

[Juan] recently learned a new trick to make cutting solder stencils a less painful process. He uses a laser cutter to cut Mylar sheets into stencils. [Juan] appears to be using EagleCAD and Express PCB. Both tools are available for free to hobbyists. The first step in the process is to export the top and bottom cream layers from your CAD software.

The next step is to shrink the size of the solder pads just a little bit. This is to compensate for the inevitable melting that will be caused by the heat from the laser. Without this step, the pads will likely end up a little bit too big. If your CAD software exports the files as gerbers, [Juan] explains how to re-size the pads using ViewMate. If they are exported as DXF files, he explains how to scale them using AutoCAD. The re-sized file is then exported as a PDF.

[Juan’s] trick is to actually cut two pieces of 7mil Mylar at the same time. The laser must be calibrated to cut all the way through the top sheet, but only part way into the bottom piece. The laser ends up slightly melting the edges of the little cut out squares. These then get stuck to the bottom Mylar sheet. When you are all done cutting, you can simply pull the sheets apart and end up with one perfect solder stencil and one scrap piece. [Juan] used a Full Spectrum 120W laser cutter at Dallas Makerspace. If you happen to have this same machine, he actually included all of the laser settings on his site.