In the last edition of Don’t Fear the Filter, we built up two examples of the simplest and most-used active filter of all time: the two-pole Sallen-Key lowpass. This time, we’re going to put two of these basic filter blocks in a row, and end up with a much sharper lowpass filter as well as a bandpass filter. For the bandpass, we’ll need to build up a quick highpass filter as well. Bonus!
I claimed last time that the Sallen-Key lowpass would cover something like 80% of your filtering needs. (And 72.4% of all statistics are totally made up!) These two will probably get you through another 10% or so. Honestly, I’ve never built a standalone active highpass, for reasons we’ll see below, but the active bandpass filter that we’re building it for is a great tool to have in your belt, especially for anything audio.
Back the late 2000s, when netbooks were the latest craze, some models would come with an inbuilt 3G modem for Internet access. At the time, proper mobile Internet was a hip cool thing too — miles ahead of the false prophet known as WAP. These modems would often slot into a Mini PCI-e slot in the netbook motherboard. [delokaver] figured out how to use these 3G cards over USB instead.
It’s actually a fairly straightforward hack. The Mini PCI-e standard has a couple of pins dedicated to USB data lines, which the modem in question uses for communicating with the host computer. Unfortunately it’s not quite as simple as just soldering on a four-wire USB cable. The modem relies on the 3.3V power from the Mini PCI-e slot instead of the 5V from USB. No problem, just get a low-dropout 3.3V regulator and run that off the USB port. Then, it’s a simple enough matter of figuring out which pins are used to talk to the SIM card, and soldering them up to a SIM adapter, or directly to the card itself if you’re so inclined. The guide covers a single model of 3G modem but it’s likely the vast majority of these use a very similar setup, so don’t be afraid to have a go yourself.
Ever since the Raspberry Pi was released to an eager public just over five years ago there is one project that seems to have been tackled more frequently than any other using the small computer from Cambridge: that of making a laptop with Pi for brains. Perhaps you feel you have had your fill of Pi laptops both good and bad, but it’s still a project that can bring up some surprises.
Does [Eben] carry a silver marker with him at all times, laptops for the signing of?[Archie Roques] is a young maker from Norwich, UK, and at the Raspberry Pi birthday party in early March he had rather an unusual laptop. He’d done the usual thing of mating the official Pi screen, a bluetooth keyboard/touchpad, Pi, and battery, but as always it’s the detail that matters. His case is a carefully designed sandwich of laser-cut plastic that somehow manages the impossible task of containing all the laptop internals while not being too bulky.
For power he at first used a 4 AH LiPo cell from a dead tablet with a Pimoroni LiPo power board, but since he hit problems with the Pimoroni board supplying both screen and Pi he’s switched to an off-the-shelf power bank. Unusually this laptop also has built-in audio, using another Pimoroni product, their speaker pHAT.
Where this laptop has a flaw though is in the display hinges. He has plans for a beautifully made 3D printed hinge, but for now he’s using a piece of tape, which though functional does not add to the aesthetic. When we saw it in Cambridge the keyboard was fitting more snugly than it does in the photos on his write-up, so perhaps he’s fixed some of its issues. Despite the in-progress hinge it’s a very usable little Pi laptop, and though (Hint, [Archie]!) he hasn’t yet published the design files for it, we’re sure when he does we’ll see other people building the same machine. They won’t be quite as exclusive as [Archie]’s model though, while he was in Cambridge he managed to get it signed by [Eben Upton], founder of the Raspberry Pi Foundation and judge for the 2017 Hackaday Prize.
[Jack Eisenmann] is no stranger to building impressive DIY CPU’s on vast stretches of breadboard. This time [Jack] has done away with the seventeen breadboards he used in his last 8-bit computer and instead has gone a step further and designed a set of generously utilised PCB’s for the CPU. The result is the DUO Enterprise.
The CPU design is based around an 8-bit data bus and a 24-bit address bus. As usual, a minimal yet carefully chosen instruction set allows [Jack] to do all the heavy lifting in software as part of the compiler and operating system he is working on. There is no sign of a display yet, instead the computer communicates via a dumb terminal. We love the aluminum foil for shielding! Check out the video, below, to see what we mean.
His final goal with DUO Enterprise is to allow anyone to utilise its computing power by submitting programs and calculations. Heads up [Jack], our neural net needs training soon.
[Pete Juliano, N6QW] built a 20 M QRP CW transmitter using just a handful of parts. That in itself will not raise any eyebrows, until you find that he built it using one of the very first RF transistors manufactured all the way back in 1955. That’s from before the time most of us were born and not many years after the invention of the transistor in late 1947.
QRP in HAM-speak technically stands for a request to “reduce power” or an offer of “should I reduce power” when appended with a question mark. A QRP transmitter is designed to transmit at really low powers. The accepted upper power limit for QRP transmitters is 5 W, at least for modes like CW using FM or AM modulation. [Pete]’s interest was piqued when he read about a 10 mW 10 M QRP transmitter design in a vintage Radio magazine from the late ’50’s and decided to replicate it. We aren’t sure, but it appears he had a Philco SB-100 RF transistor lying around in his parts bin. The SB-100 was one of the first surface-barrier transistors and could output 10 mW at frequencies up to 30MHz.
[Pete]’s rig was originally putting out 0.4 mW with a 3 V supply, and oscillating at 14.060 MHz in the 20 M band. The design appears to be a simple Colpitts oscillator with just a few parts assembled in dead-bug style on a piece of copper clad laminate. After adding an output transformer, he managed to increase the power output to about 25 mW. Check out [Pete N6QW] sending out a CQ shout out from his QRP transmitter in the video after the break.
[Afonso]’s 77-year-old grandmother lives in a pretty remote location, with only AM/FM radio reception and an occasionally failing landline connecting her to the rest of the world. The nearest 3G cell tower is seven kilometers away and unreachable with a cell phone. But [Afonso] was determined to get her up and running with video chats to distant relatives. The solution to hook granny into the global hive mind? Build a custom antenna to reach the tower and bridge it over to local WiFi using a Raspberry Pi.
The first step in the plan was to make sure that the 3G long-shot worked, so [Afonso] prototyped a fancy antenna, linked above, and hacked on a connector to fit it to a Huawei CRC-9 radio modem. This got him a working data connection, and it sends a decent 4-6 Mbps, enough to warrant investing in some better gear later. Proof of concept, right?
On the bridging front, he literally burned through a WR703N router before slapping a Raspberry Pi into a waterproof box with all of the various radios. The rest was a matter of configuration files, getting iptables to forward the 3G radio’s PPP payloads over to the WiFi, and so on. Of course, he wants to remotely administer the box for her, so he left a permanent SSH backdoor open for administration. Others of you running remote Raspberry Pis should check this out.
We think it’s awesome when hackers take connectivity into their own hands. We’ve seen many similar feats with WiFi, and indeed [Afonso] had previously gone down that route with a phased array of 24 dBi dishes. In the end, the relatively simple 3G Pi-and-Yagi combo won out.
Part two of the project, teaching his grandmother to use an Android phone, is already underway. [Afonso] reports that after running for two weeks, she already has an Instagram account. We call that a success!
As if Windows Update wasn’t bad enough, one has to deal with a plethora of attention-hungry programs and utilities all begging for a continual stream of patches from the Internet. It’s exhausting, but unfortunately also par for the course. Many of these updates are to close security vulnerabilities that could otherwise expose your computer to undesirables. The Internet of Things will only expand the amount of hardware and software you need to keep updated and protected on a daily basis. Now, it’s your dishwasher that’s under attack.
The Register reports that Jens Regel discovered the bug in a Miele dishwasher with a webserver. It’s a basic directory traversal attack that can net the intruder the shadow password file. Armed with this, it’s simple to take over the embedded Linux system and wreak havoc on your local network.
It’s not particularly surprising – we’ve talked about IoT security and its pitfalls before. The problem is, a dishwasher is not a computer. Unlike Microsoft, or Google, or even the people behind VLC, Miele don’t have infrastructure in place to push out an update to dishwashers worldwide. This means that as it stands, your only real solutions are to either disconnect the dishwasher from your network, or lock it behind a highly restrictive firewall. Both are likely to impede functionality. Of course, as always, many will ask why a dishwasher needs to be connected to the Internet at all. Why indeed.
![Does [Eben] carry a silver marker with him, laptops for the signing of?](https://hackaday.com/wp-content/uploads/2017/03/archie-with-laptop.jpg)
