How can we build an inexpensive force meter that can graph force vs time for use by physics teachers?

I'm working on an open-source curriculum project to develop a free physics curriculum. We're trying to build a cheaper version of this dual-range force sensor - https://www.vernier.com/product/dual-range-force-sensor/ that uses a strain gauge. It's attached to a dynamics cart and smashed into stuff to see how force varies with time on a graph. It has a really high sample rate. It's a really good product, but too expensive for most teachers to have a classroom set of.

It needs to be off the shelf and relatively easy for teachers to set up. We've thought about using a linear potentiometer on a spring or a strain gauge. Any advice?

I used to play around with a Basic Stamp but I'm completely new to ardunio programming and have no idea on how to build an interface that can calibrate the force meter and display a graph of force vs time. Are there people who can help with such a thing (maybe paid)?

Why do you think $109 is too much money? Bye the time you have is tall done and packaged and documented your cost will be similar.
Paul

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Fair point, Paul. The total kit cost for materials for 6 groups of students needs to be at/around $400. So, the Bluetooth version that doesn't require an interface (https://www.vernier.com/product/go-direct-force-and-acceleration-sensor/) is $99 and that would break the budget.

[ edit after watching the video, this post does not apply ]

By using an arduino as a separate device. You may be able to have say one arduino and 4 different groups of sensors.
That way you could get 4 different things.
Force, fluid, vibration and something else
So much less per experiment.

Good evening @jdonna, I am afraid I don't fully understand the exact process that your dual force sensor works, but it is my understanding that it reads push and pull. I am not sure if what I am about to suggest is something that will help but maybe someone here can tell you if this will work for you.

What I am thinking is utilizing at least 2 accelerometers facing away from each other linked to a suitable Arduino board. (I am currently only working with Nano V3.0's myself and an Uno shield but that's beside the point) If you run the Arduino IDE on your computer then you can use the Serial Plotter to see on a line graph the data coming from the accelerometers, both forward and reverse (a pull and push per say). Not only that but you can also see other lateral movements from your main line and be able to base a lesson on if the lateral movement directly correlates to the effects of push-pull.

Mind you that is entirely a possible theory of an idea. I hope someone with more experience an knowledge can actually tell you if this will work for you.

While you certainly COULD use a pot the "slide" pots have a lot of friction, and low friction "precision" versions are £100. So are similar devices such as LVDT's.
However a ROTARY pot - precision 10 turn - have low friction and you could easily convert to linear motion with a suiable spring and drum arrangement to measure a pull force.

Altenatively you could use a load cell that would adapt to push or pull forces

Unfortunately S or z shaped load cells that work without introducing torque sem to be VASTLY more expensive than the straiight beam type.

Then all you need is a basic arduino - eg a nano £zilch and a pc to program it and show results.
Easy to do and you have a lot of flexibility in calibrating the sensor, and doing data capture and display.

installed in a housing, the beam can be installed and connected so only a linear force is seen.

as we know, sometimes the enclosure costs more than the sensor and electronics.

OK @jdonna

off the shelf - £10

and some simple hardware to fix it to a bench or stand;

and an ampliffier board like this
https://learn.sparkfun.com/tutorials/load-cell-amplifier-hx711-breakout-hookup-guide/all

Weighing Sensor 24-bit A/D Conversion Adapter Load Cell Amplifier Board HX71HFSG | eBay £3

and an arduino and PC

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I really like the idea of a rotary pot. I ordered some. Seems like there's a bit of friction, but not as bad as the linear. Would a rotary encoder work better? Or probably not enough resolution?

resolution - you get what you pay for. more about rotary encoders here.

A 10 turn pot would give excellent resolution for not much money - and the ones I have have little friction.

My preferred option would be a load cell, simply because of the mechanics of converting linear motion to rotary motion. Also the load cell can measure push AND pull - even a force difference.

You get what you pay for. The cost is in photographically/etching to produce the encoding disk. And testing after manufacturing.
Paul

Guess the old saying still applies.
Resolution costs money
How precise can you afford to be?

The only good way is a load cell or call it a strain gauge. Add a HX711 as johnerrington wrote.
The cheapest scales or fish weight devices or hanging scales on Ebay / AliExpress / Amazon have all a load cell. Buy a few and open them, you'll see. It is cheap, reliable and accurate.

Every other way is not even close.
Adafruit has for example conductive rubber band: https://www.adafruit.com/product/519.

If you are not trying to measure a weight, but rather a impact force, that is something different. Then it is something as those plates for boxing that can measure the force.