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. This is why i choose to sew the contacts as diagonal stitches of conductive thread - to minimize the overlap of conductive surface. But only 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.
I am also selling these handmade Thread Pressure Sensors via Etsy. Although it is much cheaper to make your own, purchasing one will help me support my prototyping and development costs >>
As in all my Instructables the materials used for the sensor are basically cheap and off-the-shelf. There are other places that sell conductive fabrics and Velostat, but LessEMF is a convenient option for both, especially for shipping within North America. But they also ship to Europe within about 10 days.
Velostat is the brand name for the plastic bags in which sensitive electronic components come packaged in. Also called anti-static, ex-static, carbon infused plastic (So you can also cut up one of these black plastic bags if you have one at hand. But caution! Not all of them work, so test them first!)
To make the sensor fully fabric one can use EeonTex conductive textile (www.eeonyx.com) instead of the plastic Velostat. Eeonyx normally only manufacture and sells its coated fabrics in minimum amounts of 100yds, but 7x10 inch (17.8x25.4 cm) samples are available free of charge and larger samples of 1 to 5 yards for a minimum fee per yard.
Step 1: Materials and Tool
- 1.5 mm neoprene from http://www.sedochemicals.de
- Conductive thread from www.sparkfun.com
also see http://cnmat.berkeley.edu/resource/conductive_thread
- Stretch conductive fabric from www.lessemf.com
also see http://cnmat.berkeley.edu/resource/stretch_conductive_fabric
- Fusible interfacing from local fabric store or
also see http://www.shoppellon.com
- Velostat by 3M from http://www.lessemf.com
also see http://cnmat.berkeley.edu/resource/velostat_resistive_plastic
- Regular thread
- Machine poppers/snaps
- Pen and paper
- Fabric scissors
- Sewing needle
- Popper/snap machine (handheld or hammer and simple version)
Step 2: Create Your Stencil
Step 3: Prepare Your Materials
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.
Step 4: Sew Your Stitches
Do the same on the other side of neoprene.
Step 5: Sew Things Together
Step 6: Poppers
Step 7: Visualizing
If you happen to be working a lot with poppers and circuits you might like to modify a set of crocodile clips to have poppers on one end. Otherwise you can just clip on to the poppers.
To visualize with a multimeter, create the following setup (see pictures)
Set multimeter to measure resistance (in Ohm).
Multimeter plus to one side of fabric pressure sensor (doesn't matter which side) and multimeter minus to other side of fabric pressure sensor. Apply pressure and watch the resistance value change. You might have to adjust the range if you don't see anything. If you have a constant connection then either you forgot to put the Velostat in between or somewhere your two pieces of conductive thread are touching. Ops.
To visualize with an LED or vibration motor, create the following setup:
Connect the plus of two AA batteries or a 5V source to one side of the pressure sensor (doesn't matter which side) and connect the other side of the pressure sensor to the plus of an LED or either side of the vibration motor (switching plus minus only affects the direction of the vibration motor, whereas an LED only works in one direction). Connect the minus of the LED or the other other side of the vibration motor to the minus of the power supply.
Apply pressure to the fabric pressure sensor and control the brightness of the LED or the strength of the vibration.
In the video I have hooked up the pressure sensor to an arduino (www.arduino.cc) and am visualizing the change in resistance with a simple application written processing (www.processing.org).
For Arduino microcontroller code and Processing visualization code please look here >> http://www.kobakant.at/DIY/?cat=347