Ditch The Switch: A Soft Latching Circuit Roundup

For some of us, there are few sounds more satisfying than the deep resonant “thunk” of a high quality toggle switch slamming into position. There isn’t an overabundance of visceral experiences when working with electronics, so we like to savor them when we get the chance. But of course there’s no accounting for taste, and we suppose there are even situations where a heavy physical switch might not be the best solution. So what do you do?

Enter the latching power circuit, often referred to as a “soft” switch. [Chris Chimienti] has recently put together a fascinating video which walks the viewer through five different circuits which can be used to add one of these so-called soft power switches to your project. Each circuit is explained, diagramed, annotated, and eventually even demonstrated on a physical breadboard. The only thing you’ve got to do is pick which one you like the most.

There’s actually a number of very good reasons to abandon the classic toggle switch for one of these circuits. But the biggest one, somewhat counterintuitively, is cost. Even “cheap” toggle switches are likely to be one of the most expensive components in your bill of materials, especially at low volume. By comparison, the couple of transistors and a handful of passive components it will take to build out one of these latching circuits will only cost you a couple of cents.

Even if you aren’t in the market for a new way to turn off your projects, this roundup of circuits is a fantastic reminder of how powerful discrete components can be. In an age where most projects seem assembled from pre-fabbed modules, it’s occasionally refreshing to get back to basics.

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Finally, An Open Source Multimeter

For his Hackaday Prize entry, [Martin] is building an Open Source Multimeter that can measure voltage, current, and power. It’s an amazing build, and you too can build one yourself.

The features for this multimeter consist of voltage mode with a range of +/-6V and +/-60V. There’s a current mode, basically the same as voltage, with a range of +/-60 mA and +/-500mA. Unlike our bright yellow Fluke, there’s also a power mode that measures voltage and current at the same time, with all four combinations of ranges available. There’s a continuity test that sounds a buzzer when the resistance is below 50 Ω, and a component test mode that measures resistors, caps, and diodes. There’s a fully isolated USB interface capable of receiving commands and transmitting data, a real-time clock, and in the future there might be frequency measurement.

This build is based on the STM32F103 microcontroller, uses an old Nokia phone screen, and unlike so many other multimeters, this thing is small. It’s very small. More than small enough to fit in your pocket and forget about it, unlike nearly every other multimeter available. There’s one thing about multimeters, and it’s that the best multimeter is the one that you have in your hands when you need it, and this one certainly fits the bill.

The entire project is being written up on hackaday.io, there’s a GitHub repo for all the hardware and software, and there’s also a video demo covering all the features (available below). This is a stand-out project, and something we desperately want to get our hands on.

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Homemade Magic Makes The Metcal Go

First soldering irons are often of the Radioshack or Maplin firestarter variety. They’re basically wall power shorted across a nichrome heater or similar with some inline resistance to make it harder to burn down the house. You plug them in, the current flows, and they get hot. Done.

If you stick with the hobby for a while, these eventually get replaced with something like the venerable HAKKO FX-888D or that one Weller everyone likes with the analog knob. These are much improved; having temperature control leads to a more consistently heated tip and much improved soldering experience.

Entering the electronics workplace one comes across the next level of quality soldering iron: high end HAKKOs, Metcals, JBCs, and the like. Using one of these irons is practically a religious experience; they heat in a flash and solder melts while you blink. They even turn off when you put the handpiece down! But they’re expensive to buy (hint: think used). What’s a hobbyist to do?

[SergeyMax] seems to have had this problem. He bit the bullet, figured out how the Metcal works, and made his own base. This is no mean feat as a Metcal might look like a regular iron but it’s significantly more complex than ye olde firestarter. The Metcal magic is based on a oscillating magnetic fields (notice the handpiece is connected via BNC?) interacting with a tip bearing a special coating. In the presence of the changing field the tip heats up until it hits its Curie temperature, at which point it stops interacting with the magnetic field and thus stops heating.

When the user solders, the tip cools by sinking its heat into the part and drops below the Curie temperature again, which starts the heating again. It’s like temperature control with the sensor placed absolutely as close to the part as possible and a nearly instant response time, without even a control loop! [SergeyMax] has a much more thorough description of how these irons work, which we definitely recommend reading.

So what’s the hack? Based on old schematics and some clever reverse engineering from photos [SergeyMax] built a new base station! The published schematic is as rich with capacitors and inductors as one could hope. He didn’t post source or fab files but we suspect the schematic and photos of the bare board combined with some tinkering are enough for the enterprising hacker to replicate.

The post contains a very thorough description of the reverse engineering process and related concerns in designing a cost efficient version of the RF circuitry. Hopefully this isn’t the last Metcal replacement build we see! Video “walkthrough” after the break.

Edit: I may have missed it, but eagle eyed commentor [Florian Maunier] noticed that [SergeyMax] posted the sources to this hack on GitHub!

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SOICbite: A Program/Debug Connector For An SOIC Test Clip

The problem is well-known: programming and debug headers consume valuable board space and the connectors cost money. Especially troublesome are the ubiquitous 100-mil pin headers, not because they’re expensive, but because they’re huge, especially along the z-axis. If you’re building miniature devices, these things can take up a ridiculous amount of space. With some clever thinking, [Simon Merrett] has found a way to re-use something many of us already have — an SOIC-8 test clip — to connect to a special footprint on the PCB without requiring another connector. He calls the system SOICbite.

The SOIC clip attaches to a footprint consisting of eight pads, four on each side of the PCB, plus five non-plated-through holes, which serve to anchor the clip in place. The idea of mating a PCB footprint directly with a removable connector isn’t entirely new — Tag Connect has been doing this for a while, but the connectors are expensive and single-sourced. On the other hand, SOIC test clips of varying quality are available from a number of vendors, including dirt-cheap deals on your favorite websites. The one disadvantage we can see is that the SOICbite footprint must be at the edge of the PCB to properly mate with the clip. The savings in space and cost may well make up for this, however.

[Simon] has made his KiCAD footprint available in a GitHub repo, and has offered to host footprints for any other CAD package there as well. So, fire up your preferred tool and draw one up for him to get these things widely adopted, because we think this is a great idea.

For the commercial alternative, check out our coverage of Tag Connect back in 2014.


Captivating ESP32 Camera Hack

You can never have enough DIY devices at home, so when you look at an ESP32 module that comes with the camera, you automatically start getting ideas. [Daniel Padilla] wanted a way to deploy DIY camera modules without the hassle of configuring them so he made one that looks like an access point and starts streaming as soon as you connect to it.[GitHub]

The code he provides allows the ESP32 to appear as an Open Access Point which you can connect to from a PC or smartphone. The awesome sauce here is that the ESP32 resolves all DNS requests to a redirect in a similar manner to what happens when someone connects to an open Wi-Fi access point in a mall, Instead of a captive portal page that asks the user to authenticate or accept terms and conditions, [Daniel Padilla]’s code instead redirects to the streaming page et voila! Instant camera stream, and it is that simple.

We love this project because it is an elegant way to solve a problem, and it also teaches newbies about captive portals and their implementation. We covered a cheap ESP32 Webcam in the past and this project also comes with code for you to get started. We would love to see what you come up with next.

This Clapperboard Prints Movie Posters

The clapperboard is a device used in video to synchronize audio and video. Its role in movies is well known and its use goes back in one form or another to the 1920s. [Gocivici] is a big movie fan and created a clapperboard that is able to print out posters of recently announced movies when the clapper is clapped.

The poster is not a big, full color job, but rather a black and white one, roughly the size of a movie ticket. [Gocivici] keeps his movie tickets in a journal and wanted to be able to keep small posters in there along with them. A thermal printer is used to print the poster along with the title, the release date, and some information about the movie. In addition to the printer, the hardware involved is a Raspberry Pi, a switch, and an LED. The clapperboard itself is 3d printed and then painted. A bit of metal is used to keep the clappers apart and give a bit of resistance when pressing them together. A nice touch is a metal front, so you can use magnets to keep your posters on the board.

[Gocivici] has detailed build instructions up along with a video (available after the break) showing the printer in action. The 3d models are available as well as the code used to create the posters after grabbing data from TMDb. If you need your clapperboard to be as accurate as possible, take a look at this atomic clock clapperboard.

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Repurposed Plastic Protects PCBs

An errant wire snipping across the wrong electrical pins spells the release of your magic smoke. Even if you are lucky, stray parts are the root of boundless malfunctions from disruptive to deadly. [TheRainHarvester] shares his trick for covering an Arduino Nano with some scrap plastic most of us have sitting in the recycling bin. The video is also after the break. He calls this potting, but we would argue it is a custom-made cover.

The hack is to cut a bit of plastic from food container lids, often HDPE or plastic #2. Trim a piece of it a tad larger than your unprotected board, and find a way to hold it in place so you can blast it with a heat gun. When we try this at one of our Hackaday remote labs and apply a dab of hot glue between the board and some green plastic it works well. The video suggests a metal jig which would be logical when making more than one. YouTube commenter and tip submitter [Keith o] suggests a vacuum former for a tighter fit, and we wouldn’t mind seeing custom window cutouts for access to critical board segments such as DIP switches or trimmers.

We understand why shorted wires are a problem, especially when you daisy-chain three power supplies as happened in one of [TheRainHarvester]’s previous videos.

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