2026-04-12 16:00:00
It is an old trope in submarine movies. A sonar operator strains to hear things in the ocean but dares not “ping” for fear of giving away the boat’s location. Radar has a similar problem. If you want to find an airplane, for example, you typically send a signal out and wait for it to bounce off the airplane. The downside is that the airplane now knows exactly where your antenna is and, these days, may be carrying missiles to home in on it. In a recent post, [Jehan] explains how radar, like sonar, can be passive.
Even if you aren’t worried about a radar-homing missile taking out your antenna, passive radar has other advantages. You don’t need an expensive transmitter or antenna, a simple SDR can pull it off. You don’t need a license for the frequencies you want to use, either. You are just listening.
The key is that radar uses two different effects. One is how long it takes for the echo to return. The other is how much the Doppler effect shifts the frequency. Suppose you are using an FM radio station as a passive radar “exciter.” You can pick up the signal directly and also detect the same signal bouncing off the target. You can compare these two and determine the delay added by the reflection and the Doppler shift.
This does have one limitation. In a regular radar installation, you know that a certain signal delay means the target is somewhere on a circle a fixed distance from your antenna. With passive radar, you wind up with an ellipse instead of a circle. You can’t “scan” a passive signal like you do an active one, either.
But all is not lost. Similar to stellar navigation, you just need to get multiple ellipses by using different broadcast stations. With two stations, you’ll probably narrow the position down to two points where the ellipses intersect. Three different fixes are often enough to get a particular point.
Build your own? Of course. Don’t forget that the best transmitter to use might not be on the ground.
Title image from the post sourced from https://github.com/30hours/3lips.
2026-04-12 13:00:00
Although vapor-compression refrigeration is a simple concept, there are still a lot of details in the implementation of such a system that determines exactly how efficient it is. After making a few of such systems, [Hyperspace Pirate] decided to sit down and create a testing system that allows for testing of many of these parameters.
Some of the major components that determine the coefficient of performance (COP) of a heat pump or similar system include the used refrigerant, as well as the capillary tube diameter or expansion valve design. For the testing in the video three refrigerants are used: R600 (N-Butane), R134a (tetrafluoroethene, AKA Freon) and R290 (propane), with R134a being decidedly illegal in places like the EU. The use of R600 instead of R600A is due to the former allowing for a lower pressure system, which is nice for low-power portable systems.
The test rig has the typical fresh-from-the-scrap-heap look that we’re used to and love from [Hyperspace Pirate], but does exactly what it says on the tin, and is easy for any DIY enthusiast to replicate. Which compressor to pick for a specific refrigerant is also covered in the video, along with oil type and more.
For basic systems you’d use a simple capillary tube, whereas an airconditioner or similarly more complex system would use an adjustable valve design. With the rig you can test the efficiency of different tube diameters, with three sizes available in this version. Unfortunately the electronic expansion valve (EEV) that was going to be used didn’t get a chance to shine due to unforeseen events.
With the R134a and butane a COP of 2.0 – 2.5 was achieved when taking power factor into account, which was reasonable considering a compressor was used that targets R134a. Regardless, if you have ever felt like repurposing that old compressor from a fridge or AC unit, this might be a fun afternoon project.
2026-04-12 10:00:00
Kiki bills itself as the “array programming system of unknown origin.” We thought it reminded us of APL which, all by itself, isn’t a bad thing.
The announcement post is decidedly imaginative. However, it is a bit sparse on details. So once you’ve read through it, you’ll want to check out the playground, which is also very artistically styled.
If you explore the top bar, you’ll find the learn button is especially helpful, although the ref and idiom buttons are also useful. Then you’ll find some examples along with a few other interesting tidbits.
One odd thing is that Kiki reads right to left. So “2 :* 3 :+ 1” is (1+3)2 not (23)+1. Of course, you can use parentheses to be specific.
If you are jumping around in the tutorial, note that some cells depend on earlier cells, so randomly pressing a “run” button is likely to produce an error.
Would you use kiki? There are plenty of array languages out there, although perhaps none that have such poetic documentation. Let us know if you have a favorite language for this sort of thing and if you are going to give Kiki a try.
If you want to try old school APL, that’s easier than ever.
2026-04-12 07:00:00
There’s a long history of devices originally used for communication being made into computers, with relay switching circuits, vacuum tubes, and transistors being some well-known examples. In a smaller way, pneumatic tubes likewise deserve a place on the list; [soiboi soft], for example, has used pneumatic systems to build actuators, logic systems, and displays, including this latching seven-segment display.
Each segment in the display is made of a cavity behind a silicone sheet; when a vacuum is applied, the front sheet is pulled into the cavity. A vacuum-controlled switch (much like a transistor, as we’ve covered before) connects to the cavity, so that each segment can be latched open or closed. Each segment has two control lines: one to pressurize or depressurize the cavity, and one to control the switch. The overall display has four seven-segment digits, with seven common data lines and four control lines, one for each digit.
The display is built in five layers: the front display membrane, a frame to clamp this in place, the chamber bodies, the membrane which forms the switches, and the control channels. The membranes were cast in silicone using 3D-printed molds, and the other parts were 3D-printed on a glass build plate to get a sufficiently smooth, leak-free surface. As it was, the display used a truly intimidating number of fasteners to ensure airtight connections between the different layers. [soiboi soft] used the display for a clock, so it sits at the front of a 3D-printed enclosure containing an Arduino, a small vacuum pump, and solenoid valves.
This capacity for latching and switching, combined with pneumatic actuators, raises the interesting possibility of purely air-powered robots. It’s even possible to 3D-print pneumatic channels by using a custom nozzle.
Thanks to [Norbert Mezei] for the tip!
2026-04-12 04:00:00
Although we can already buy commercial transceiver solutions that allow us to use PCIe devices like GPUs outside of a PC, these use an encapsulating protocol like Thunderbolt rather than straight PCIe. The appeal of [Sylvain Munaut]’s project is thus that it dodges all that and tries to use plain PCIe with off-the-shelf QSFP transceivers.
As explained in the intro, this doesn’t come without a host of compatibility issues, least of all PCIe device detection, side-channel clocking and for PCIe Gen 3 its equalization training feature that falls flat if you try to send it over an SFP link. Fortunately [Eli Billauer] had done much of the leg work already back in 2016, making Gen 2 PCIe work over SFP+.
The test setup involves a Raspberry Pi 5 on a PCIe breakout board and a PCIe card connected to the whole QSFP intermediate link with custom SFP module PCBs for muxing between PCIe edge connector or USB 3.0 connectors to use those cheap crypto miner adapter boards. The fiber is just simple single-mode fiber. Using this a Gen 2 x1 link can be created without too much fuss, demonstrating the basic principle.
Moving this up to Gen 3 will be challenging and will be featured in future videos, involving more custom PCBs. With Gen 5 now becoming standard on mainboards, it would be great to see this project work for Gen 3 – 5 at link sizes of x4 and even x16 so that it might be able to run external GPUs at full bandwidth unlike Thunderbolt.
Thanks to [zoobab] for the tip.
2026-04-12 01:00:14
You probably don’t think about it much, but your PC probably has a TPM or Trusted Platform Module. Windows 11 requires one, and most often, it stores keys to validate your boot process. Most people use it for that, and nothing else. However, it is, in reality, a perfectly good hardware token. It can store secret data in a way that is very difficult to hack. Even you can’t export your own secrets from the TPM. [Remy] shows us how to store your SSH keys right on your TPM device.
We’ll quote [Remy] about the advantages:
The private key never leaves the device, you yourself can’t even extract it, neither can malware. It does not live on your filesystem or in an ssh-agent (in memory)…
Unlike a hardware token, the TPM is locked to your machine. In fact, in many cases, it is soldered onto the motherboard, although sometimes it is plugged in. The post notes that because of this, the TPM is not quite as secure as a hardware token that you can pull out of a USB port and lock up. But it is still more secure than just having your keys sitting on a hard drive.
One caveat: some computers wipe your TPM when you update the BIOS. The post mentions how to get around this. You’ll need some tools, of course, and it won’t work with Windows Subsystem for Linux, unsurprisingly. Once you have the tools installed, the process is pretty straightforward.
We’ll add this to our set of ssh tricks from now on.
