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.

Step 2: Sizing

Use the knife/razor to score your PCB into two plates that mirror each other. I went with approximately one-square-inch squares, but you could do any two shapes so long as there is copper in between.

Cut your foam into the same shape as the plate.

Solder one wire to each plate. You'll want to make sure the solder is going to hold the wire in place, so clean the copper beforehand if necessary and use plenty of solder.

Step 3: Connecting the Pieces

Glue the three pieces together. Only glue along the outline of the FSR, otherwise it will not conduct well. For mine, I just glued the top and bottom of both plates to the foam.

Step 4: Test It Out

Grab a multimeter and measure the resistance across your FSR. Your values will vary, but I got about 200 kiloohms at rest and 9 kiloohms when almost completely depressed. If your plates have a larger surface area, or the foam in between is thinner, these values will be smaller.

Step 5: Notes

Variations

• Use it to Dim an LED (video + code)
• Use it to Make some noise (video)
• Try different kinds of foam (test resistance across the foam first to make sure it's conductive)
• Cut unusual shapes
• Test different foam configurations (e.g.: multi-layered foam)
• Test different plate materials (e.g.: aluminum foil on cardboard/plastic/wood)
• Make humongous FSR arrays

Links

SensorWiki FSR page explains FSR theory and use, with examples
Protolab explanation of FSR use in the context of other sensors

Thanks to Dane Kouttron and Zach Barth for introducing this technique to me, and leaving a few FSRs around the eclub.

Comments

[martinmilchevski2]

Hello, where i can find the conductive foam. Thanks

[IaciB]

I can find the arduino code

[fra1985]

Hi, I am very new on this, I need to build one of this concept with an FSR long a stripe of about 10cm and attached a display led where I can see the weight, is it possible ? and if yes, how can i do it ? thanks for your help is an important project for me :)

[aramsden2012]

would this concept work if the weight on the pressure plate was that of the weight of a pill or a door key? Am creating a school project and need to create an extremely sensitive pressure plate!

[SarahM178]

I'm looking to be able to apply potentially hundreds of pounds to a force sensor and detect the weight applied. Would this work for this project?

[zggtf211]

I wouldn't count on it. Your better off just getting a few load sensors from alibaba or ebay for a few bucks. These aren't very accurite.

[carlosb]

Hello, thank you for the tutorial, I want to know if there are any way to connect to arduino in order to get the variables changing when applying force, I need to do a surface that will be able to detect when people hands over it and then create a reaction in a liquid (non newtonian) using speakers / changing frequencies depending force applied.

can it be possible?

thanks!

[tcanedy]

if you wanted to measure force in the hundreds of pounds could this work?

[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.

[wblakesx]

The wear factor is a bother wirh fpoams i've seen. How tp make my own...the next bit

[ashcocks]

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

[ac-dc]

oh the irony

[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!

[leevonk]

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