Hackers often find uses for pressure sensitive materials, detecting footfalls during walking or keypresses in a synthesizer being two examples. [Marco Reps] decided he’d make a hi-res, body-sized pressure sensitive mat mainly for computer-guided physiotherapy, though he wouldn’t rule out using it for gaming (twister anyone?). That meant making the equivalent of a body-sized matrix circuit of around 7000 sensors, as well as a circuit board with a multitude of shift registers. The result has a surprisingly good resolution, capable of making clearly distinguishable the heel, arch and front part of a foot.
His choice of pressure sensitive material was Velostat, a polymeric foam available as large sheets. The foam is impregnated with carbon black to make it electrically conductive, but being a foam, its resistance changes when pressure is applied. The first layer of the mat is made up of one centimeter wide strips of copper tape laid out lengthwise and spaced one centimeter apart. That’s followed by the Velostat and then another layer of copper tape oriented horizontally this time. The pressure sensors are the sandwiches formed by where the tapes overlap. In the first video below he shows how he measured and graphed the Velostat’s dynamic range to help decide to use one centimeter squares. He also puts together a smaller prototype, with good results.
For the body-sized mat, we count around 50 by 140 overlapping areas for a total of around 7000 one square centimeter sensors. And of course to measure each sensor in that large matrix, as you can imagine, he made up a custom circuit board with shift registers. The board works by applying positive voltage to the columns one-by-one, while each time going through all the rows and reading their voltages. Making the board was in itself was an adventure, taking a chance on a Chinese manufacturer asking only $2. But watch the second video below where he evaluates the result, including trying unsuccessfully to delaminate a board. Sadly he forgot to include places on the board for diodes, one for each column, and fixing that is another adventure he walks us through. Patience was definitely a prerequisite here, not only for making the mat, and fixing the diode problem, but also for connecting up 96-pin ribbon cables. We applaud his efforts, and his results. Check out the second video below for the making of the large mat and the circuit board.
ALS robbed one of [C. Niggel]’s relative’s of the use of their upper body. This effectively imprisoned them in their house; ALS is bad stuff. Unfortunately too, the loss of upper body mobility meant that they couldn’t even use the computer to interact with people and the outside world. However, one day [C. Niggel] noted that the relative’s new electric wheelchair was foot controlled. Could this be adapted to a computer mouse?
He looked up commercial solutions and found them not only prohibitively expensive, but also fraught with proprietary drivers and all sorts of bad design nonsense. With all of the tools out there today there was no reason this couldn’t be quickly prototyped and sent to the relative in need.
He used a combination of conductive thread, neoprene, and velostat to build the pads themselves. The pads were balanced with some adjusting resistors in series. The signals are sent to an Adafruit Feather board which interprets them and converts it to a PS/2 standard.
The first version of the mouse used separate pads glued to a MDF board with contact cement. However this, along with some other initial design flaws, resulted in premature failure of the mouse. [C. Niggel] quickly returned to the lab and produced a new version with more robust construction and mailed it off. So far so good!
The 2015 Midwest RepRap Festival, a.k.a. the MRRF (pronounced murf) was just announced a few hours ago. It will be held in beautiful Goshen, Indiana. Yes, that’s in the middle of nowhere and you’ll learn to dodge Amish buggies when driving around Goshen, but surprisingly there were 1000 people when we attended last year. We’ll be there again.
A few activists in St. Petersburg flushed GPS trackers down the toilet. These trackers were equipped with radios that would send out their position, and surprise, surprise, they ended up in the ocean.
Speaking of crowdfunding campaigns, The Mooltipass, the designed-on-Hackaday offline password keeper, only has a little less than two weeks until its crowdfunding campaign ends. [Mathieu] and the rest of the team are about two-thirds there, with a little more than half of the campaign already over.
When [Michelle] was making a sign language translation glove, she needed a bunch of flex sensors. These flex sensors cost about $10 a pop, meaning her budget for the project was eaten up by these bendy potentiometers. Since then, [Michelle] figured out a great way to make extremely inexpensive bend sensors using anti-static bags and masking tape, allowing her to start her project once again.
The build works by sandwiching Velostat plastic bags – the same electrically conductive bags all your components arrive in – between layers of masking tape. A jumper wires is attached to a strip of Velostat attached to a piece of masking tape. Between two of these anti-static/masking tape assemblies, another piece of Velostat is placed. After laminating all these pieces together, [Michelle] had a primitive yet very functional flexible potentiometer.
After attaching one of these flex sensors to an analog input of her dev board of choice, she had a wonderful and inexpensive flexible sensor. You can check out this sensor in action after the break.