The third sensor in my collection is the force sensor. The Vernier Dual-Range Force Sensor costs around $109. My version was < $20 in parts in addition to the Arduino. Yes, it’s not as nicely calibrated but you should be able to get it close.
What is a force sensor really? A weight scale held sideways. Or the other way around, a scale measures the gravitational force an object exerts and translates it into a weight unit. So I use a scale as the sensor, basically a luggage scale from Amazon with a rating of 40kg. It’s available for around $3.50 (shipping from China) or a bit more from an Amazon warehouse near you. That scale is rated for 40kg which would be equivalent to almost 400N, much more than the 50N of the Vernier sensor.
When you try to push on the hook assembly of that scale, it actually starts to show negative numbers, so it would actually work for both directions. But not really inside the case, the metal parts are held in place by little plastic tabs. They are not indented to carry any force! But it’s easy to remove the strain gauge from the housing and create a new frame for it. An Erector Set or something similar would probably work very well. I did not go that far [yet];-)
The other needed part is an instrumentation amplifier because the strain gauge sensor delivers only very small voltage variations. I decided on the Analog Devices AD620 which I found on Amazon as well. With shipping, it was about $10.
I found a nice video @ the NerdKits site: weighscale that explains the principles very well. My circuit follows the idea pretty closely except I’m using a real Arduino instead of the `bread boarded MCU in the video’.
One word of advise: Be careful when you want to play with something like this. The wires on my strain gauge were not well attached. So I lost one and had to remove the silicone cover, solder a new wire in place and recreate a cover. Try to prevent it, it’s no fun.
I created a Fritzing part for the AD620 because I could not find one:-( It’s available here: AD620.fzpz (You will have to remove the extra .zip suffix, WordPress did not want to let me upload the file without it:-()
My little sketch is here: ForceSensor.tar It does a zero-point calibration first and then measures the force in the main loop. As always, that can be easily adapted to some data collection/plotting setup on the host computer. The factor to translate the values to Newtons really depends on your sensor and the gain of the AD620. So you will need some known weights and calculate it.