Capacitive And Resistive Touch Sensors For Wearables

When you look at switching solutions for electronic wearables, your options are limited. With a clever application of conductive fabric and thread, you can cobble together a simple switch, but the vast array of switch solutions is much more than that. This one is different. The zPatch from [Paul Strohmeier], [Jarrod Knibbe], [Sebastian Boring], and [Kasper Hornæk] at the Human-Centred Computing Section at the University of Copenhagen gives eTextiles capacitive and resistive input. It’s a force sensor, a pressure sensor, and a switch, all made completely out of fabric.

The design of this fabric touch sensor is based around a non-woven resistive fabric made by Eeonyx. This fabric is piezo-resistive when compressed. This material is sandwiched between two layers of silver-plated polyamide fabric, which is then connected to the analog input of a microcontroller. On top of all this is a polyester mesh, with everything held together with iron-on sheets.

Reading this sensor with a microcontroller is extremely similar to a capacitive touch sensor made out of copper and FR4. All the code is available in a repo, and all the materials to reproduce this work can be found in the various links provided by the team. That last point — reproducibility — is huge for an academic work. Not only did the team manage to come up with something interesting, they actually provided enough documentation to reproduce their build.

In the video below, you can see how this sensor can be used to sense a hand hovering, a light touch, a hard press, or anything in between. Only two analog pins are required for each sensor, making the routing and layout of this eTextile should be relatively easy to integrate into clothing. It’s a great build, and we can’t wait to see the community pick up on these really cool sensors.

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Get You An E-Textiles Sensor That Can Do Both

[Admar] is a software developer who was introduced to e-textiles in 2011. The bug firmly took hold, and these days he gives e-textile workshops at Eindhoven University of Technology. Here, students learn to build a single e-textile sensor that detects both presence and pressure. The workshop presentations are available on his site, which is itself a window into his e-textile journey.

Over the years, [Admar] has discovered that any e-textile project requiring more than a few connections is ripe for some kind of textile-friendly multi-point connector. Through trial and error, he designed a robust solution for use with an embroidery machine. The wires are made from conductive thread and soldered to a row of male header pins to make the transition out of fiber space. This transition requires solder, which quickly gets interesting when coupled with a fabric substrate and no solder mask. We wonder if spraying on mask beforehand would help, or if it would just soak in and stain and get in the way.

You can see the connector in practice in [Admar]’s capacitive multi-touch demo video after the break. He has stacked two pieces of fabric, each with a wire bus made of conductive threads, with the traces at right angles. Both sensors are wired to a Cypress PSoC5 to create a sensor matrix, and then to a laptop for visualization purposes. As his fingers approaches the fabric, the bar graphs roar upward to show increased capacitance. Once he makes contact, each finger appears as a yellow dot illustrating pressure.

E-textile projects aren’t limited to traces sewn by hand or embroidery machine. Circuit boards can be knitted, too.

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Building This TARDIS Is Anything But A Snap

As an avid fan of the show Dr Who, [Adam Sifounakis] saw a model for a laser-cut TARDIS that piqued his curiosity that eventually grew into a multi-week project involving multiple setbacks, missteps, revamps and — finally — gratification. Behold, his sound activated TARDIS.

First and foremost, assembling and painting the model was a fun puzzle — despite a few trips to the store — with a little backtracking on the painting due to impatience. Next, the creation of a pulsing soft white LED circuit timed with an audio clip to really sell the image of a mini-TARDIS proved to be a tedious ordeal, paying off in the end with a satisfying glow through the vellum-diffused windows on the model.

How to trigger the lights? [Sifounakis] initially wanted a capacitive sensor to trigger the sound effects, but that way lay dragons — and madness — so he went with snap-activated effect to activate the TARDIS like the Doctor himself. After struggling with building his own microphone setup, he switched to an electret mic with adjustable gain which worked like a charm. Setting up this TARDIS’ Adafruit Pro Trinket brain involved a snag or two, and after that it was smooth sailing!

Until he hit another hitch with the power circuit too, that is. Luckily enough, adding a capacitor to give the circuit a bit more juice on boot solved the issue. All that was left to do was dismantle and rebuild his circuit after all this troubleshooting and substitutions, and — finally — install it in his model.

With much satisfaction and a final rework of the LED pulsing effect, it was done. Check it out!

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Are You In Bed?

If you’re building an omniscient home-automation system, it’s ability to make decisions is only as good as the input you give it. [Petewill]’s self-made panopticon now knows when someone is in bed. That way, the [petewill]’s automatic blinds won’t open when he’s sleeping late on weekends.

[Petewill] didn’t take the easy way out here. (In our mind, that would be a weight sensor under one of the bed’s feet.) Instead, his system more flexible and built on capacitive sensing. He’d tried force sensors and piezos under the mattress, but none of them were as reliable as capacitance. A network of copper tape under the mattress serves as the antenna.

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StarMAT Greets Visitors With The Imperial March

Strong is the Force, with this Padawan. To coincide with the latest installment of the continuing saga from a galaxy far, far away, [Rohit Gupta] built a Star-Wars themed interactive doormat. The doormat detects a footstep using capacitive sensing and plays a random Star Wars audio clip like the opening theme or the Imperial March or a famous phrase from the movie. Check out the video below the break.

The current setup is temporarily breadboarded, but we are sure it will be popular enough with his visitors to make him tidy it up. The hardware consists of an Arduino with an audio shield connected to a pair of speakers. A capacitive wire loop under the mat and a capacitive sensor tuned to the mat size wire take care of the sensing.

When Earth people step on the mat, the sensor triggers the Arduino to play a random audio clip from the SD card. The capacitive sensing is taken care by the TP223 1-key touch pad detector chip (PDF), which he mounted on a home etched board with SMD parts. The whole bundle is powered by a small “power bank” battery pack like the ones used to charge mobile phones.

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Conjuring Capacitive Touch Sensors From Paper And Aluminum Foil

Stumbling around YouTube, we found what has to be the lowest-tech method of producing a touchpad to make a capacitive touch keyboard, and we just had to share it with you. If you’re afraid of spoilers, skip down to the video below the break now.

[James Eckert] got his hands on a Freescale MPR121 capacitive touch sensor. The chip in question speaks I2C and senses up to twelve simultaneous capacitive sense electrodes; break-out boards are available in all of the usual places. It’s a sweet little part.

So [James] had to make a twelve-key capacitive keyboard on the quick. He printed out a key template on paper — something that he does often in his woodwork — and spray-glued aluminum foil on the back side. The video doesn’t say how many hours he spent with the razor blade tracing it all out, but the result is a paper, foil, and packing tape keyboard that seems to work just fine.

A pin-header was affixed to the foil with conductive paint and more tape. If you’ve ever tried soldering directly to aluminum foil, you’d know why. (And if you’ve got any other good tips for connecting electrically to aluminum foil, we’d love to hear them.)

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Pimp My Keyboard: Automatic Lift Kit And More

Cherry-keyboard-with-lifts

Wondering what the heck a lift kit is? You know those low-riding cars that bounce? That’s the idea with this hack. [Justblair] added automatic height adjustment to his Cherry G80, and hid a few other extras while he was at it. Since there’s a fair amount of room inside the case of this model he was able to hide everything and keep just a single cord to run it all.

Certainly what catches your eye is the keyboard’s ability to rise to a typing height automatically. This is accomplished with a few servo motors and some 3D printed replacement feet. There were some hiccups along the way with under-powered servos, but bulking up to some HXT 900 9G models provide more power than is currently necessary. The automatic feature is thanks to a capacitive sensor built with a wire that loops the perimeter of the keyboard.

Of course to monitor the sensor and drive the servos you need some kind of brain. For that [Justblair] went with an ATmega32U4 breakout board. Since he had to patch into USB for power anyway he added a USB hub and routed one of the ports out the left side of the keyboard as a convenient way to connect other peripherals. There was even room to include an RFID reader which he uses to unlock his sessions (similar to the desk install from earlier this year). There’s still a lot of potential left in that hardware. To make future improvements easier the hack includes an IDC socket as an auxiliary port.

[Justblair] did a great job of sharing his work. His post links to a Github repo for the code and a Thingiverse project for the 3D printed legs. And it wouldn’t be complete without the demo video which is found below.

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