Free Love – Inflatable Input Device


I’ve been very interested in inflatable structures lately. While experimenting with computer fans and trash bags, I came across this simple physical input mechanism that can sense when an inflatable membrane is squeezed.

It turns out that the MIT Media Lab’s Ben Dalton also stumbled across this idea a number of years ago. My project works on similar lines but with the addition of internet connectivity.


The sensor in this inflatable input device uses a hall effect sensor built into an old computer fan to track the speed of the fan. As the membrane the fan is inflating is squeezed air flows backwards through the fan, slowing it down. For each revolution of the fan, the hall effect sensor makes two 5VDC pulses on the yellow tachometric pin. By tracking the speed of the fan with an Arduino and a ruby script, the program is able to detect when the membrane is being hugged and send a tweet.



  • Foam core
  • Two 3-wire computer fans
  • Plastic trash bags
  • Clothing iron
  • Cardboard
  • Arduino
  • 1 10 k-ohm resistor

The Base

The base contains the fans and acts as a docking station for the inflatable membrane. I created it out of foam core. The fans are hot glued inside the base. The bottom of the base is open to allow air to flow into the membrane.

The Membrane

The inflatable, huggable membrane is made out of cheap white plastic trash bags that I picked up at the grocery store. I cut the edges off with a knife, then cut five pieces of plastic to make a rectangular prism to fit on top of my base with a little extra space at the edges for fusing with the adjacent edges.

Once I cut out my pattern, I began sealing the edges using an iron on the cotton setting. I set one piece of the plastic on the floor and covered it with a small piece of cardboard to protect the other edge from the iron. Then I pulled the extra plastic on the bottom sheet’s edge over the top of the cardboard and placed the other sheet’s edge on top of the bottom sheet’s exposed edge. With the two edges overlapping, I slid the iron across the edge for a few seconds to fuse the two sheets of plastic. If you’re worried about ruining the iron with hot plastic you can place a thin towel in between the iron and the plastic.

This can get a bit tricky when fusing the final edges to seal up the top of the membrane. As you can see above, my membrane was quite wrinkly on the top edge. If I were to do it again, I would be more careful about measuring my pattern before fusing.


Above is the simple schematic needed to hook up the fan’s hall effect sensor to the Arduino. A pull-up resistor is necessary to get a proper reading.

The fan that you are using must have a (usually yellow) tachometric pin in order to work. I’m using two fans to inflate the membrane, but only reading the fan speed off of one. The two fans are hooked up in parallel to a 12VDC source. One fan’s tachometric pin is hooked up to the Arduino.

Here is the completed sensing circuit. It uses digital in pin 5 to read the frequency from the hall effect sensor.


The membrane fits snugly in between two pieces of foam core in the base. (Thanks for the help, Chris!)

And the membrane inflates!


To read the frequency values off of the fan’s tach pin, I borrowed some code I found on the internet called FreqCounter. Using FreqCounter, I count the number of times the fan pulses in a 250ms window. Then I write the value on serial where it is read by Ruby. Ruby does a running average of the incoming values. If the average dips below an average value computed at the beginning of the program, a squeeze is detected and it fires off a random tweet. To make things more personal, I have people type their name when the program starts. Afterwards the random tweets are addressed specifically to them.

Arduino Code:
Ruby Script:

Check out the generated tweets at:


I could see this sort of interaction being useful for more than just hug input. It would be great for punching input or wind input as well. I could imagine games or 3D modeling software using this sort of input device.

In the future I also hope to pursue my original (and much more provocative) idea, an inflatable sex doll input device. I went out to the local Adult Mart and bought this hilarious inflatable sex doll last week. I was hoping that I could inflate it in a similar manner to the trash bags but the material it is made out of was too heavy to inflate.

The fans, which were placed in this cut-out section on the back of the doll, were not powerful enough to fully inflate the thick plastic.


~ by Max Hawkins on March 30, 2010.

3 Responses to “Free Love – Inflatable Input Device”

  1. There’s a typo in my schematic. The fan runs on +12VDC rather than +15VDC.

  2. […] voodoo trigger device is a development on my previous huggable input sensor project. When the inflated bag is pushed or hugged air flows back through the fans, slowing them down. An […]

  3. Hello, its a very nice project… do you think it could work with differential pressure ?

    i am looking to something similar to:

    but am still confused how to keep it inflated. the idea is to have differential pressure sensors connected inside and outside sending the voltage to the fans.

    any ideas will be great!

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