It is a problem as old as the Internet. You want to access your computer remotely, but it is behind a router that randomly gets different IP addresses. Or maybe it is your laptop and it winds up in different locations with, again, different IP addresses. There are many ways to solve this problem and some of them are better than others.
A lot of routers can report their IP address to a dynamic DNS server. That used to be great, but now it seems like many of them hound you to upgrade or constantly renew so you can see their ads. Some of them disappear, too. If your router vendor supplies one, that might be a good choice, until you change routers, of course. OpenWRT supports many such services and there are many lists of common services.
However, if you have a single public accessible computer, for example a Web server or even a cloud instance, and you are running your own DNS server, you really don’t need one of those services. I’m going to show you how I do it with an accessible Linux server running Bind. This is a common setup, but if you have a different system you might have to adapt a bit.
There are many ways to set up dynamic DNS if you are willing to have a great deal of structure on both sides. Most of these depend on setting up a secret key to allow for DNS updates and some sort of script that calls nsupdate or having the DHCP server do it. The problem is, I have a lot of client computers and many are set up differently. I wanted a system where the only thing needed on the client side was ssh. All the infrastructure remains on the DNS server.
[Kryzer Channel] takes making a DIY RC car to a whole new level with this prop-driven electric car that is made almost entirely out of cardboard (YouTube video, also embedded below.) By attaching an electric motor with a push prop to the back of the car, [Kryzer] avoids the need for any kind of drive system or gearing. Steering works normally thanks to some scratch-built linkages, but the brake solution is especially clever.
Braking is done by having a stocky servo push a reinforced stub downward, out of a hole in the center of the car. This provides friction against the road surface. After all, on an RC car a functional brake is simply not optional. Cutting the throttle and coasting to a stop works for a plane, but just won’t do for a car.
Layers of corrugated cardboard and hot glue make up the bulk of the car body, and some of the assembly techniques shown off are really slick and make the video really worth a watch. For example, the construction of the wheels (starting around 2:24) demonstrates making them almost entirely out of cardboard, saturated with CA glue for reinforcement, with a power drill acting as a makeshift lathe for trimming everything down. A section of rubber inner tube provides the tire surface and a piece of hard plastic makes a durable hub. Wraps of thread saturated in CA glue, shown here, is another technique that shows up in several places and is used in lieu of any sort of fasteners.
Schlieren imaging is a technique for viewing the density of transparent fluids using a camera and some clever optical setups. Density of a fluid like air might change based on the composition of the air itself with various gasses, or it may vary as a result of a sound or pressure wave. It might sound like you would need a complicated and/or expensive setup in order to view such things, but with a few common things you can have your own Schlieren setup as [elad] demonstrates.
His setup relies on a cell phone, attached to a selfie stick, with a spherical mirror at the other end. The selfie stick makes adjusting the distance from the camera to the mirror easy, as a specific distance from the camera is required as a function of focal length. For cell phone cameras, it’s best to find this distance through experimentation using a small LED as the point source. Once it’s calibrated and working, a circular field of view is displayed on the phone which allows the viewer to see any change in density in front of the mirror.
The only downside of this build that [elad] notes is that the selfie stick isn’t stiff enough to prevent the image from shaking around a little bit, but all things considered this is an excellent project that shows a neat and useful trick in the photography/instrumentation world that could be useful for a lot of other projects. We’ve only seen Schlieren imaging once before and it used a slightly different method of viewing the changing densities.