Circuit Boards You Can Stretch: Liquid Metal Nanomaterials Make A Strange Flex

If you think polyimide-based flexible PCBs are cool, wait until you get a load of what polymerized liquid metal networks can do.

Seems like [CNLohr] has some pretty cool friends, and he recently spent some time with a couple of them who are working with poly LMNs and finding out what they’re good for. Poly LMNs use a liquid metal composed of indium and gallium that can be sprayed onto a substrate through a laser-cut stencil. This results in traces that show the opposite of expected behavior; where most conductors increase in resistance when stretched, pol LMNs stay just as conductive no matter how much they’re stretched.

The video below shows [CNLohr]’s experiments with the stuff. He brought a couple of traditional PCB-based MCU circuits, which interface easily with the poly LMN traces on a thick tape substrate. Once activated by stretching, which forms the networks between the liquid metal globules, the traces act much like copper traces. Attaching SMD components is as simple as sticking them to the tape — no soldering required. The circuits remain impressively stretchy without any apparent effect on their electrical properties — a characteristic that should prove interesting for wearables circuits, biological sensors, and a host of real-world applications.

While poly LMNs aren’t exactly ready for the market yet, they don’t seem terribly difficult to make, requiring little in the way of exotic materials or specialized lab equipment. We’d love to see someone like [Ben Krasnow] pick this up and run with it — it seems right up his alley.

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Seek And Ye Shall Command

If we count all the screens in our lives, it takes a hot minute. Some of them are touchscreens, some need a mouse or keyboard, but we are accustomed to all the input devices. Not everyone can use the various methods, like cerebral palsy patients who rely on eye-tracking hardware. Traditionally, that only works on the connected computer, so switching from a chair-mounted screen to a tablet on the desk is not an option. To give folks the ability to control different computers effortlessly [Zack Freedman] is developing a head-mounted eye-tracker that is not tied to one computer. In a way, this is like a KVM switch, but way more futuristic. [Tony Stark] would be proud.

An infrared detector on the headset identifies compatible screens in line of sight and synchs up with its associated HID dongle. A headset-mounted color camera tracks the head position in relation to the screen while an IR camera scans the eye to calculate where the user is focusing. All the technology here is proven, but this new recipe could be a game-changer to anyone who has trouble with the traditional keyboard, mouse, and touchscreen. Maybe QR codes could assist the screen identification and orientation like how a Wii remote and sensor bar work together.

Choosing The Optimal Sampling Rate For Your DIY Heart Rate Monitor

With wearables still trying to solidify themselves in the consumer health space, there are a number of factors to consider to improve the reliability of such devices in monitoring biometrics. One of the most critical such parameters is the sampling rate. By careful selection of this figure, developers can minimize errors in the measurement, preserve power, and reduce costs spent on data storage. For this reason, [Brinnae Bent] and [Dr. Jessilyn Dunn] wanted to determine the optimal sampling rate for wrist-worn optical heart rate monitors. We’ve shared their earlier paper on analyzing the accuracy of consumer health devices, so they’ve done a lot of work in this space.

The results of their paper probably don’t surprise anyone. The lower the sampling rate, the lower the accuracy of the measurement, and the higher the sampling rate the more accurate the measurement when compared to the gold standard electrocardiogram. They also found that metrics such as root mean square of successive differences (RMSSD), used for calculating heart rate variability, requires sampling rates greater than 64 Hz, the nominal sampling rate of the wearable they were investigating and of other similar devices. That might suggest why your wearable is a bit iffy when monitoring your sleeping habits. They even released the source code for their heart rate variability analysis, so there’s a nice afternoon read if you were looking for one.

What really stood out to us about their work is how they thoroughly backed up their claims with data. Something crowdfunding campaigns could really learn from.

Reliability Check: Consumer And Research-Grade Wrist-Worn Heart Rate Monitors

Wearables are ubiquitous in today’s society. Such devices have evolved in their capabilities from step counters to devices that measure calories burnt, sleep, and heart rate. It’s pretty common to meet people using a wearable or two to track their fitness goals. However, a big question remains unanswered. How accurate are these wearable devices? Researchers from the Big Ideas Lab evaluated a group of wearables to assess their accuracy in measuring heart rate.

Unlike other studies with similar intentions, the Big Ideas Lab specifically wanted to address whether skin color had an effect on the accuracy of the heart rate measurements, and an FDA-cleared Bittium Faros 180 electrocardiogram was used as the benchmark. Overall, the researchers found that there was no difference in accuracy across skin tones, meaning that the same wearable will measure heart rate on a darker skin-toned individual the same as it would on a lighter skin-toned. Phew!

However, that may be the only good news for those wanting to use their wearable to accurately monitor their heart rate. The researchers found the overall accuracy of the devices relative to ECG was a bit variable with average errors of 7.2 beats per minute (BPM) in the consumer-grade wearables and 13.9 BPM in the research-grade wearables at rest. During activity, errors in the consumer-grade wearables climbed to an average of 10.2 BPM and 15.9 in the research-grade wearables. It’s interesting to see that the research-grade devices actually performed worse than the consumer devices.

And there’s a silver lining if you’re an Apple user. The Apple Watch performed consistently better than all other devices with mean errors between 4-5 BPM during rest and during activity, unless you’re breathing deeply, which threw the Apple for a loop.

So, it seems as if wrist-worn heart rate monitors still have some work to do where accuracy is concerned. Although skin tone isn’t a worry, they all become less accurate when the subject is moving around.

If you’d like to try your own hand with fitness trackers, have a look at this completely open project, or go for the gold standard with a wearable DIY ECG.

Got Me Feeling Blue

Sleep schedules are an early casualty in the fight to be productive. Getting good sleep is an uphill battle, so anything that can help us is a welcome ally. We all know about the phone and computer settings that turn down the infamous blue hues at sunset, but what about when you want more blue light? Maybe you want to convince your body to stay awake to pre-acclimate for a trip across time zones. Perhaps you work or live in a place that doesn’t have windows. Menopause introduces sleep trouble, and that is a perilously steep hill.

[glowascii] takes the approach of keeping-it-simple when they arrange six blue LEDs under a flesh-tone patch, which isn’t fooling anyone and powers the lights with a USB power pack. Fremen jokes aside, light therapy is pricey compared to parts some of you have sitting in a drawer. Heck, we’d wager that a few of you started calculating the necessary resistor sizes before you read this sentence. Even if you don’t need something like this, maybe you can dedicate an afternoon to someone who does.

DIY therapy has a special place in our (currently organic) hearts, such as in this rehabilition glove or a robot arm.
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LED Shirt Does It With Tulle

Given that we are living in what most of humanity would now call “the future”, we really ought to start acting like it. We’re doing okay on the electric cars, but sartorially we’ve got some ground to make up. Helping with this effort is [Amy Goodchild], who put together a fancy LED shirt for all occasions.

The basis of the shirt is an ESP8266 running the FastLED library, hooked up to strings of WS2812B LEDs. It’s a great combination for doing quick and simple colorful animations without a lot of fuss. The LED strips are then fastened to the shirt by sewing them on, with heatshrink added to the strips to give the thread something to attach to. Tulle fabric is used as a diffuser, hiding the strips when they’re off and providing a more pleasant glowing effect. Everything is controlled from a small box, fitted with an arcade button and 7-segment display.

It’s a fun piece that’s readily achievable for the novice maker, and a great way to learn about LEDs and sewing. We’ve seen other similar builds before, such as this glowing LED skirt. Video after the break.

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Clean Air And A Gentle Breeze In Your Hoodie

Hoodies are great, and rightfully a hacker’s favorite attire: they shield you from the people around you, keep your focus on the screen in front of you, and are a decent enough backup solution when you forgot your balaclava. More than that, they are also comfortable, unless of course it is summer time. But don’t worry, [elkroketto] has built a solution to provide the regular hoodie wearer with a constant breeze around his face, although his Clean Air Bubble is primarily tackling an even bigger problem: air pollution.

Wanting to block out any environmental factors from the air he breathes, [elkroketto] got himself a thrift store hoodie to cut holes in the back, and attach two radial fans that suck in the air through air filtering cloths. A 3D printed air channel is then connected to each fan, and attached on the inside of the hood, blowing the filtered air straight into his face. Salvaging a broken drill’s battery pack as power supply and adding a 3D printed clip-in case for the step-up converter, the fans should provide him a good 5 hours of fresh air. Of course, one could also add a solar charging rig if that’s not enough.

Keep in mind though, while a wearable air filter might sound particularly useful in current times, [elkroketto] specifically points out that this is not for medical use and won’t filter out any airborne diseases.