/ / Introduction
Just a reminder
Microcontrollers cannot produce varrying voltages. They produce a high (5V) and a low (0V). But you can "fake" analog output by using pulsewidth modulation. Just like animation creates the illusion of movement, pulsewidth modulation, manipulates the intervals bewtween on (the pulsewidth) and off in such a way that it fools the eye and appears to be between on and off.
/ /
Wire up a flex sensor (a flex sensor uses the same circuit as a photocell). Make the lights fade and blink.
Wire up a pressure sensor (an fsr uses the same circuit as a photocell). Make the lights fade and blink.
The more pressure applied, the lower the resistance.
/ /Make your own flex sensor
Using:
- Neoprene 2 mm thick HS -from seattle fabrics
- Velostat by 3M from http://www.lessemf.com/plastic.html
- Conductive thread from http://www.fine-silver-productsnet.com
- Stretch conductive fabric from http://www.lessemf.com/fabric.html
- Fusible interfacing
- Regular sewing thread
you will make your own fabric bend sensor. The sensor actually reacts (decreases in resistance) to pressure, not specifically to bend. But because it is sandwiched between two layers of neoprene, pressure is exerted while bending. Allowing one to measure bend (angle) via pressure.
TOOLS:
- Pen and paper
- Fabric scissors
- Iron
- Sewing needle
- Possibly pliers for pulling needle through neoprene
What to do
- Cut out the stencils for the neoprene, Velostat and stretch conductive fabric tabs. Trace it twice onto your piece of neoprene and cut these out. Now mark the inner stitches and placement of conductive fabric patch onto these cutouts. Caution! The tracing should be identical and NOT mirrored.
stencil
- Fuse the interfacing to one side of your stretch conductive fabric
- Place the two pieces of conductive fabric on top of the neoprene cutouts and fuse these together
- Thread a needle with about 50 cm of conductive thread and stitch from the side into the first marked stitch hole and then make the four stitches and at the end connect to the stretch conductive fabric tab with at least five stitches. Cut the thread and repeat on the second piece of neoprene.
The reason the stitching on both sides must be identical is so that when they lie on top of each other (facing each other) the stitches crisscross and overlap in one point. This has the advantage that the stitches will be sure to cross (make contact at these points) and second that the point of contact is as small as possible. If the conductive surfaces are too big, the sensitivity of the sensor is no longer good.
- Start sewing around the edges of the two neoprene pieces. Make sure to displace them by the 1.5 cm marked on the stencil. And don't forget to insert the two pieces of Velostat before closing the sensor all the way.
The Velostat will go in between your two conductive stitches. And this is what creates the pressure sensitive change in resistance. The Velostat lets more electricity through, the harder you press the two conductive layers together, with the Velostat in between.
You can add more or less pieces of Velostat to control the sensitivity of the sensor. - Hook it up to a multimeter and set it to measure resistance (Ohm). Bend or press the sensor and the range should lie between 2K and 200 ohm.
- To visualize the change in resistance in the bend sensor you just made you can also hook it up to your computer via a microcontroller (Arduino) and use a little bit of code (Processing) to visualize it.
Code is found liz_arum_classes:examples:processing:physical_computing:2009:_080809_Read6AnalogIN:_080809_Read6AnalogIN.pde
When you have the Processing application open you only have to pay attention to the movement of the bar that is connected to your sensor, since the Arduino has 6 analog inputs in total and this application is programmed to visualize all 6 of them... in case you want to plug in more than one sensor.
http://www.instructables.com/id/Neoprene-Bend-Sensor-IMPROVED/
/ /Assignment
Create a piece to present to the class that uses an fsr. Embed it in some material. Think about action and reaction.
What do people apply pressure to? What can happen when they do?
Do you want to help, surprise or inspire someone?
- A toothpaste or paint tube
- A sponge
- A Stress ball
Experiment!!
/ /Make your own fsr
Stitching conductive thread into neoprene creates a pressure sensitive pad. The conductive surface is minimized by stitching only a few stitches on either side with conductive thread. This creates a good fingertip pressure sensor.
The resistance range of these pressure sensors depends a lot on the initial pressure. Ideally you have above 2M ohm resistance between both contacts when the sensor is lying flat. But this can vary, depending on how the sensor is sewn and how big the overlap of the adjacent conductive surfaces are. Sewing the contacts as diagonal stitches of conductive thread minimizes the overlap of conductive surface. The slightest touch of the finger will generally bring the resistance down to a few Kilo ohm and, when fully pressured, it goes down to about 200 ohm. The sensor still detects a difference, right down to about as hard as you can press with your fingers. The range is non-linear and gets smaller as the resistance decreases.
Materials:
- Neoprene 1.5mm thick
- Velostat by 3M from http://www.lessemf.com/plastic.html
- Conductive thread from http://www.fine-silver-productsnet.com
- Stretch conductive fabric from http://www.lessemf.com/fabric.html
- Fusible interfacing
- Regular sewing thread
- snaps/poppers
you will make your own fabric bend sensor. The sensor actually reacts (decreases in resistance) to pressure, not specifically to bend. But because it is sandwiched between two layers of neoprene, pressure is exerted while bending. Allowing one to measure bend (angle) via pressure.
TOOLS:
- Pen and paper
- Fabric scissors
- Iron
- Sewing needle
What to do
- Decide on a shape for your pressure sensor. Consider that you will need to create two separate tabs for the two layers of conductive fabric and that these should not touch each other. Sketch the shape for your sensor onto some paper or cardboard, including both tabs. You will also want to plan where to make your conductive thread stitches in the center or the sensitive area of your pressure sensor. One stitch is the minimum and the more stitches the more sensitive your sensor will be, in the sense that you will hit the least resistance with much less pressure. So best to do as few stitches as necessary to evenly cover the area you want to cover.
- Trace your stencil onto the neoprene twice and cut both out.
-
Cut two small pieces of conductive fabric the size of your tabs or slightly smaller and iron these on to the neoprene with the fusible.
-
Mark with a fabric pen or a permanent marker where you will be stitching with the conductive thread. Make sure the markings on each side are identical so that when you lay both sides on top of each other the identical stitches are sure to cross each other in an X like manor and not match up. This way each two stitches will be sure to cross each other and make direct contact in only one point.
- Take the conductive thread and stitch into the neoprene from the back so that the knot stays on the outside of the sensor. Now stitch your stitches but there is no need to go all the way through the neoprene so that they are visible and vulnerable on the outside. You can dive into the neoprene and at the same time this isolates the conductive thread. When you have finished you stitches you will want to bring the thread to the patch of conductive fabric that is fused to the tab. If you plan ahead you can aim to end close by. With about 5 to 7 stitches attach the conductive thread to this patch and then cut it.
Do the same on the other side of neoprene.
-
Trace the stencil once onto the Velostat, but cut out the shape from the Velostat 2-3 mm smaller than the stencil and don't include the tabs.
- Layer your piece of Velostat between the two pieces of neoprene with the conductive stitches facing inwards.
- Thread a needle with regular thread and sew around the edges. Do not sew too tight or you will have high initial pressure. If you want to increase the resistance, lower the sensitivity then add one or two or more layers of Velostat in between.
