DIY Force Sensitive Resistor (FSR)

Make a force sensitive resistor (a pressure sensor) with spare parts instead of spending $5 - $20 each.

Step 1: Materials

Tools

• Soldering iron
• Hot glue gun
• Knife/razor
• Wire cutter

Components

• Solder
• Hot glue
• One-sided copper PCB
• Conductive foam
• Wire

The foam

Conductive foam is what microcontrollers generally come packaged in. If you've received little ATmega microcontrollers or PICs, sometimes they'll be surrounded by conductive foam inside a little case or box. Not all conductive foam is created equal: some of it bounces back into shape faster than others. If you use PIC foam to make your FSR, it will respond quickly, but if you use ATmega foam will take a second to release. The fact that this FSR has a visible deformation is the primary difference from other FSRs.

Comments

[eyesee]

Can not be measured accurately

[theyuki]

i am thinking of building a weighing scale out of this. but how much foam and what is the best dimension that you would think i have to make for it to be able to hold and correctly be able to measure a 1kg weight?

[ratchai]

Would you please describe why we need the conductive foam? it is very new for me. The distance change cause capacitance change. but I don't really know about the conductive foam.

[kylemcdonald]

the conductive foam changes resistance as it is compressed. i imagine that it's a network of resistors, and more of them are touching each other when you compress it. this means the average resistance from one side to the other is decreased because the path is more direct.

[.patrick]

how to make force sensor with 3 pins?????

[corpuscallosum]

What kind of glue do you use? I tried gorilla epoxy, but it does not hold so well...

[kylemcdonald]

i use hot glue. for pretty much any electronics, i always use hot glue :)

[dbsoundman]

For those of you who have actually built this, what did you use to protect the copper? Conformal coating spray is pretty expensive, and I plan on trying to make rather long FSRs so coating with solder might be a bit difficult as well. Suggestions?

[martzsam]

Liquid plastic coating. Like plasti mix.

[Emilious]

what's liquid thin?

[ShaiGar]

:( My first thought was "Awesome, now the jedi can't touch it". Damnit.

[MdP1632]

Or maybe, only Jedi can use it...

[Chromacon]

A few years ago I used a fine grade Pressure Sensitive Material to make an 18 channel control board for lights. I had Vellman dimmers that ran on a 0-10 V control voltage, and I made 18 pads similar to these to make voltage dividers to control them. The guy who gave me the PSM as an industrial sample died though, and I have no supply for more as needed. I am really glad to find the suggestions in this project!

[kylemcdonald]

I'm glad to help/provide a place for links to congregate.

Be sure to check out Hannah's work too: http://www.instructables.com/member/Plusea/ She does a ton with resistive material, and has some links to suppliers.

[tudgeanator]

Would you be able to use a potentiometer instead of the fsr's? I can't think of a reason why not-i just wanted to check.I'm definately going to do this because i'm making an electronics order tonight but i can't get the foam in a small lot and don't want to spend £13 ($26) on foam that i know i will only use once.

[kylemcdonald]

Yes, you can use potentiometers in place of FSRs. You'll just want to use one of the outside pins and the center pin. Both FSRs and potentiometers are variable resistors. The advantage of a potentiometer is that you can put "low" (ground) on one side and "high" (say, 5 V) on the other side, then "tap" to get a value in between. The advantage of FSRs is that they are sensors -- you don't have to "interact" with them using a screwdriver :) Of course, if you have big pots with knobs/dials, that's not necessarily an issue. Keep in mind, if you're using these for the Nandhopper, potentiometers are generally 1k-10k ohm max, so you'll need to use bigger capacitors to make up for this.

[Derin]

I have 100k pots.

[kylemcdonald]

See the explanation of cap selection in the post below. You're looking for something like 20 nanofarads for the caps.

[tudgeanator]

If i used 1K LIN.POT what capacitor would i have to use?

[kylemcdonald]

"f = (1 / (r x c)). For example, when c = 470 pf and r = 10 megaohms, f = 212 Hz (measured at 250 Hz)"

Or, c = 1 / (f x r). So if you want a 500 Hz wave and have a 1 kiloohm resistor, you need a 2 microfarad cap.

[tudgeanator]

k,cheers

[Lftndbt]

I could use some of those... Are these similar to what you would find in a keyboard? Obviously keyboards work generally via two contacts and a spacer sheet. Do you think I could use a keyboard circuit to make some of these?

[kylemcdonald]

As far as I know, most keyboards keys are switches. According to HowStuffWorks there used to be "foam element" keyboards, but I don't know if they were more like switches or thresholded FSRs.

I'm guessing a keyboard circuit wouldn't be very useful for making a matrix of these; but I haven't really dissected enough keyboards, so don't let me dissuade you :)

[ac-dc]

A keyboard made like this would be terrible because it would need more circuitry (expense) to register key presses, have a far lower number of cycles before wearing out, and force slower typing since foam doesn't expand as fast as a spring or bubble membrane does, keytravel would be slow in both up and down directions and not tactile.

[Lftndbt]

I didn't ask if you could make a keyboard from these. I was asking, could I hack one apart to obtain the individual pressure switchs from a key board.

[ashcocks]

you ac-dc, are a very sad person - and properly ignore the 'be nice' comment policy.

[TheMadScientist]

being nice doesn't mean you have to be a kissass. he was just stating the fact that this wouldn't work.

[Lftndbt]

I agree with you comment. But for the cause of good information I would have to say, perhaps he should read the question first before commenting.

[ac-dc]

Indeed TheMadScientist, and I wasn't really even addressing exactly what the prior posters had written. kylemcdonald had already written that a keyboard circuit wouldn't be very useful, and Lftndbt was just asking a question - it is nice to provide info about a question asked.

As for the HowStuffWorks article I'll comment on some strange ideas in it, after quoting it.
http://computer.howstuffworks.com/keyboard3.htm

"Metal contact and foam element keyboards are increasingly less common. ...
Both technologies have good tactile response, make satisfyingly audible "clicks," and are inexpensive to produce. The problem is that the contacts tend to wear out or corrode faster than on keyboards that use other technologies. Also, there is no barrier that prevents dust or liquids from coming in direct contact with the circuitry of the key matrix."

Foam element keyboards never were common, and are rare as hen's teeth these days. I'm wondering if the author really mean silicone dome element switches, the type used in things like some landline cordless phones and cheaper TV remote controls where there is a black round bump on the back which is embedded with conductive material. That type weren't all that rare, I think today's all rubber roll-up types of keyboards still use that design but I could be wrong.

Both are not inexpensive to produce. Relatively speaking the common rubber dome type found today is the inexpensive one, while metal contact (mechanical switches of one type or another) are by far the more expensive type of what were once far more common than they are today.

The contacts on metal element do not tend to wear out or corrode faster, on the contrary a decent metal element (mechanical switching) keyboard tends to last over 5X as long as today's rubber dome plus deposited film circuit types do.

You can't spill anything on most of today's rubber dome keyboards either because the liquid simply goes to the edge of the rubber dome mat where it then gets inbetween the two sheets of film, and because it is a depositing layer it is very thin and unlike the old fashioned copper-on-PCBs, small current shorts from liquid can degrade or even burn through the traces. I've lost several rubber dome keyboards like that, they can be opened and cleaned out but once that trace is gone it's very hard to repair. I've tried attaching a wire with rear window defogger repair solution and a conductive silver pen. The defogger flaked off and was too thick, the silver pen worked but later failed again.

The funny thing is that even in the picture that article provided, you can see the deposited traces in the rubber dome picture are very degraded where they are discolored at the bottom. That particular keyboard they opened had probably failed or became very flaky already and that's why it was chosen to be taken apart. Rubber dome keyboards take less effort to press and are the cheapest to make, that is why they are the most popular (except buckling spring types on laptops due to being shorter to allow for a thinner laptop).

[ac-dc]

By "Both are not inexpensive to produce" I meant metal contact and foam element", not the 3rd type I mentioned which was silicone dome element which are inexpensive, and come to think of it they're also used on lots of other products like car key remote controllers.

[ac-dc]

oh the irony

[leevonk]

lol, ac-dc's comment is perfectly logical and realistic, not mean

[kylemcdonald]

I noticed you don't have any instructables posted, ac-dc. It seems like you're knowledgeable about lots of different topics, though -- I'd love to see what sort of projects you enjoy working on.

[ac-dc]

My interests are more in the unimaginative, pure sciences. I'm not really the inventor type personality, more of a system analyst type, plus taking this particular Instructable as an example I don't need a force sensitive resistor for anything even if other people might. I tend to buy professionally engineered products, it is very rare to have a real need for a product that doesn't exist or at least come close to the goal but can be modifed. When I modify things it seems easy enough to just write a paragraph instead of an elaborately presented instructable. For a simplified example, when I made a charging base for an MP3 player, I just stuck stainless screws on the player case wired to the battery contacts inside, screws to match up to the contacts in an old phone recharger cradle and put a LM317 current limiting circuit in the cradle. Pictures and a description of drilling holes, soldering wires, or telling someone to look at LM317 datasheet for a schematic all seem like obvious details to me. A lot of the instructables seem that way to me, like the ideas are good, the end results can sometimes be desirable, but most of the info is obvious enough it wouldn't need to be written or pictorialized. I do find one thing about the Instructables site disturbing though, that no matter what a project is, is seems as though people want to censor fair analysis of the cons and only focus on the pros of something. I feel it is best to fully weigh the cons too, otherwise there's no good way to compare to other alternatives. This is only a generalized comment, has nothing to do with Force Sensitive Resistors.

[kylemcdonald]

Thanks for the explanation -- it gives me a better idea of where you're coming from. I definitely understand why brief projects (like making a charging base) would not warrant an instructable so much as concise descriptive paragraph. (Maybe we need another site: instructagraphs?) As you pointed out, there are a lot of good ideas here with a lot of "obvious" information. What I like about instructables, though, is exactly this abundance of potentially superfluous information. The closest I've come academically to electronics was a physics class that dealt with e&m, so all the "obvious" info about soldering, drilling holes, and datasheets have been really helpful for me to get an idea about how other people approach electronics. As a corollary: I've dealt with some Processing code in two of my other instructables, and since I've studied computer science I tend to omit a lot more information in that realm -- it all seems "obvious" to me. Yet these omissions are the things I've received emails about asking for clarification. If you're ever excited about something you've put together (or even something professionally engineered that you've modified), it might be helpful to share your hacks with other people who don't see it so clearly. I can't speak for other instructables authors, but I welcome both kinds of comments -- especially from more qualified individuals than myself. It seems like you excel at pointing out the "cons" to other people's "pros", but it'd be great to see you post comments that aren't specifically in response to other comments as well.

[ac-dc]

I admit I do seem overly negative sometimes, but to me it only seems logical. Many other people had already mentioned the positive things so it would be redundant for me to do so. I try instead to mention all remaining aspects I can think of which are often the drawbacks. I have fiddled with a lot of things over the years and one thing I learned was to have retrospect, after I finished a project I end up wishing I did some things differently and I try to analyze where in my thought process did I go wrong in not considering then what I now see I need or want instead. Most often it seemed I had overidealized and not formed a good mental picture of myself using the resulting project, imagining all the realistic ways I might find it suboptimal. I also look around the junk in my basement sometimes and see projects that are just clutter today, that I did again to overcome the limitations of the past project result. Sometimes I have to admit I wasted time on projects and would've rather had the time back to do something else. Years go by in the blink of an eye, I think a lot of the younger folks reading these should be enjoying their summer, you only get about 85 of them.

[tosp]

AC-DC: Try this ... Look at each of your "junked" projects and try to remember what you learned from it. Travel along this thinking to discover how it is that you now know what you know about why that project "didn't work out". Try some meds. But whatever you do to fix you own thinking, PLEASE DON'T DESTROY THE INCENTIVE FOR YOUNG MINDS (of any age) TO RECOGNIZE THAT FAILURE IS A REQUIREMENT FOR LEARNING!

[kylemcdonald]

I think it's fairly obvious this wouldn't be the best sensor for typing -- for all the reasons you mention. I think he meant "could I interface to a bunch of these by reusing circuitry from a keyboard?"

[Lftndbt]

You are correct.... Certain types of keyboards have circuitry which allows you to obtain hundreds of these easily.

[balisticjoe]

How can I get the foam without buying a micro controller?