This is an Instructable to show you how to make a pressure-sensitive pad - which can be used to create digital toys or games. It can be used as a large scale force sensitive resistor, and although playful, it could be used for more serious projects to explore smaller user-interfaces of all kind which require a light touch from the hand, to the force of a body sitting down, to a stop from your feet! It could create anything from a burglar alarm to a dancing game! The tech: Velostat and Metal Foil are combined to make a thin pad which changes resistance upon pressure. What you do with it is up to you!
This pressure pad solution actually started with a desire for one young boy, Josh, aged 8, to play with his friends in the playground. Josh is blind, due to a condition called Norrie Disease. His journey was captured in the BBC documentary, The Big Life Fix, where myself and another designer, Ruby Steel, were tasked with making the playground not only more accessible to Josh, but if possible, to make games which where sight was not the only defining interaction.
After some pretty unconventional ideas - ranging from IR Retroreflective Fiducials, to BLE Beacons - we ultimately settled on a simpler solution of creating a 'Digital Playground' - by this we meant that we wanted to create an entire playground that was a bit like the old Dance, Dance, Revolution game pad - where if you stepped on a pad, it would play a sound... if you stepped on a special sequence of pads, then alternative play would be unlocked. I think there is something cool about taking an idea like this and *blowing it up* in scale! (Yet it would also function as a small game too.)
Primarily, the technology was functioning as a fun expedience for all, and in additional, it would also allow us to assign specific sounds to the beginning and end of a 'road', all of which connected to central navigational 'Hubs'. We called these 'yellow brick roads', so his friends would appreciate their navigational intent, and help Josh if nearby while he was learning. In fact he was so quick to learn, he needed less help than we imagined! Full project here. (LINK)
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Step 1: To Make the Pressure Pads - You Will Need:
Foil: Copper Foil (often called EMI Foil online)* - LINK
Velostat: Conductive Pressure Film, also available at Adafruit, etc. - LINK
Laminate Pouches - LINK
Laminator: I suggest one that is A3, but it can be as big as the pads you wish to create. However, I would suggest getting one that does not 'bend' the sheets too much - ideally 'straight through', as shown in later steps. LINK
Solder, Wires, Wire Strippers, Blow Torch & Heat Shrink - useful for sealing wires to whatever controller you are using: Arduino UNO is fine, though I have suggested using a Bare Conductive TouchBoard to play the music, and itself is based on Arduino architecture.
*NOTE: It should be said, that the foil need not be self-adhesive, as this property is not vital. Nor does it need to be copper, but aluminium was simply too flimsy at the available thicknesses. So feel free to experiment!
Step 2: Cut Velostat Template
As mentioned, you can make this any size, as long as it is bigger than the copper.
I went for 24x24cm square.
I also experimented with the thickness of Velostat needed for this application - I actually went 3-ply (three sheet stacked), but you may find one is fine.
The template was simply as I knew I was going to be making over 35 of these!!
Step 3: Cut Conductive [Copper] Foil Template
I went for 20x20cm square - however - note I added a 'D' tab to one side! This was to make for easy soldering.
I realised that these tabs would be placed face-to-face, so they would not overlap. This small, seemingly insignificant detail was design to keep the solder from pressing into the other tab over time. I imagined that if I jumped on an area with solder and wires, it might 'cut through' the Velostat - and hence 'short circuit' the pad, making it always read 'on'.
Copper - face down (white backing paper facing you).3x sheets of VelostatCopper - face up.Note Tabs are not over-lapping, but are on the same side.
Step 4: Soldering to the Tabs
Safe to say, having a good quality soldering iron with a 'chunky' tip will make this easier.
Using some blu-tack to hold the connection wire in place, flow the solder onto the wires and to the copper. Allow some of the strands to fan-out. Apply tape to cover them up, and to offer some strain-relief for the wires, while handling.
Note the final pre-assembly, with alternate position of 'tabs'....ready for lamination.
It's not essential to assign a polarity to the pad, but it can help for more complex installations. (Ground).
Step 5: Laminate
This stack is about 24x24cm, so fits in an A3 laminate pocket.
I left the wires trailing out the bottom of the pocket - at the opposite side to where the pocket is pre-sealed. This is so it is 'pulled' into the machine, and less likely to jam.
Safe to say, that this is not the original intention for laminators, so do take care not to break it by using wires that are too thick. I used the same sort of 1mm dia wires you find in jumper leads, and kept them side-by-side.
Once I sealed one side, I passed it back upside-down, to ensure a good seal.
Step 6: Trim & Prepare Wires
I cut the excess laminate away, leaving a 20mm edge around the Velostat.
I was careful to then cut closely up to the wires, but not cutting through them!
Holding the wires (at the pad side) and then pulling away the excess laminate worked fine to free the wires.
I was them able to strip these - ready for soldering to the larger system...
Step 7: Wiring Up
I was using a heavy-gauge wire on this project, but a thinner one can be used of course.
As shown, I prepared some heat shrink - to be ready to cover over the wires, once joined.
I wrapped the smaller strands around the larger ones, and then soldered.
Finally, heat-shrinking the wires (blue), and then the entire assembly (red)...
(You can of course use a lighter gauge wire, as this was to be installed in a playground, but the thicker the better, as this has lower resistance).
Step 8: Strain Relief
These pads needed to be buried under an industrial playground, and installed by contractors, so it was sensible to assume they might need some strain-relief to ensure they did not break. For this I improvised some fabric tape, and secured this as shown.
It also served to keep out any slight ingress getting in around the wires.
(If unsure about this, silicone sealant could be applied in the gap).
Step 9: Done! (Now What Will You Make With It?)
This is the final pressure pad, ready to install in Josh's playground. More on that project here: LINK.
Of course, you can do smaller projects, or with more or less pads - the trick is to connect to the right processor for the interaction you need.
Many thanks also to Daljinder "DJ" Sanghera who worked through the wee small hours to help me make the pads in time for the BBC film crew to begin filming the builders install them!
Step 10: Arduino/TouchBoard Code & Pressure Pads
The code is basically a combination of three Arduino fundamentals:
1. THE PAD: Is essentially a variation on ANALOGUE INPUT tutorial: https://www.arduino.cc/en/tutorial/AnalogInput
2. THE TRIGGER: Is essentially incorporating the POTENTIOMETER tutorial: https://www.arduino.cc/en/tutorial/potentiomete , such that the two can work together. Lastly, the TouchBoard is essentially a more integrated version of the mp3 player...
3. AUDIO PLAYER tutorial: https://www.arduino.cc/en/tutorial/potentiomete , which will play once the desired event has occured by stepping on the pad.
Below is how we did this, but of course you can improvise as you wish.
For the A Single Pad, I suggest using some variation of the code (attached here - as an .ino file)
Let me explain how to do it, and what is going on...
- The Pressure Pad is essentially a variable resistor, so it will change resistance when you step on it. We want it to play a sound when we get a sure signal of someone stepping on it.
- This pad may have a value which remains fixed (say 112Ohms), but most likely, it will change, either upon installation (we put a 1kg tile on top of it and glued it down (perhaps it goes to 82Ohms)....you might do something different).
- This is why we include a 500Ohm (LINK) 'trim pot' to allow us to adjust when we want the pad to be considered pressed and when we want to ignore it.
- Consider it a bit like a 'see saw' - we want it to be in a state of definitely on *or* off - not teetering on the edge of one or the other.
- The second 'trim pot' (1kOhm (LINK)) is to allow us to adjust when the pad should play a sound.
- Going back to our 'see saw' - let's say we have a definite 'down' press - how 'hard' (how much resistance change) do we want to see before we play a sound? This allows us to adjust that, and say we want a +/- of say 50Ohms, then we can change this here.
- There is a 'pull down' resistor of 200Ohms also. (LINK)
- One could of course do this in the code, but when working on an installation like this, it's more practical to have an analogue adjustment (with a screwdriver), than to re-upload the Arduino every time.
- The circuit diagram is drawn to look close to that of the Arduino Shield (so forgive GND being on top), and hope this helps.
- The Arduino Prototyping Shield (LINK) is so as to make for easy connection to the music player: which in this case is a Bare Conductive TouchBoard (LINK), and although useful for this, need not be used, if an mp3 player can be connected to play more easily (and cheaply). If you do want to use it, however, solder header pins to the TouchBoard to allow it to connect to the shield.
- TouchBoards work just like Arduino Unos with the same interface to upload the code.
So this is a great single pad, and others have done some cool variations - like EmilyG here (LINK).
However, if you want to take it to the next level and essentially make a 'game' out of multiple pads, with secret moves/sequences to press them in to 'unlock' all sorts of different hidden sounds, then check this next Instructable out (LINK) - taking it from small scale to large scale! Many thanks to Sam Roots for this!
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Step 11: Digital Playground
Participated in the