cheap load cells [found]

ManAtWork

I'm looking for an affordable way to measure forces in the 0..200N (40lbsf) range. Building them by myself out of strain gauges would be complicated and industrial load cells are way too expensive. My idea was that there might exist some luggage scale that has a serial port for read out, comparable to those calipers with serial interface.

I'd like to measure lift and drag in a "poor man's wind tunnel" (test object mounted on top of a roof rack on a car ). To measure the force vector I need at least three of them or even 6 to also measure torque. And I'd like to connect them to a data logger (P2 board) for analysis. I alrady thought about pointing a GoPro at the display of a luggage scale but I don't want to buy 3 or 6 cameras. And it would be much more convenient to have the data in numerical format.

I don't care much about absolute precision. 10% error would be acceptable as I only need a qualitative comparison of different configurations.

JRoark

Amazon has some unbelieveably cheap load cells. Here is just one as an example:

https://www.amazon.com/ALAMSCN-Digital-Weighing-Portable-Electronic/dp/B08KRV8VYP

ManAtWork

Thanks @JRoark , I haven't thought that this is so easy. Should also be no problem to write a driver for the P2 and the HX711 module.

I've ordered 3 of the 10kg load cells. This should be perfect to support 10..20kg weight and handle the lift + drag forces.

JonnyMac

I did a really simple HX711 driver for a work experiment last year. Since it's so simple, I used bit-bang SPI instead of smart pins.

pmrobert

Excellent catch @JRoark! One of my next projects is a small engine dyno. Load cells are essential for this. Still working on how to place variable load on the engine.

Circuitsoft

Many of those load cells are made for bathroom scales. Might be easier to just harvest such a scale and grab an hx711 breakout board to read it with. Although, given the precision you're looking for (or lack thereof) you can probably just use an input similar to your thermocouple trick.

frank freedman

Check sparkfun. They have an HX711 on a small board as well as load combinator board. They also have a Microe click board. They list the load cell (looks like bathroom scale cell) for $4.50. At that price, may be cheaper raiding the local goodwill for a scale to can out the cells. A (much) pricier option might be the Tekscan product lines. https://tekscan.com/products-solutions/force-sensors/a301

ManAtWork

I don't wanted to pay $500 for an industrial load cell but I don't care if a cheap one costs $5 or $10. My time required to disassemble the scale and figuring out how to connect the (then unknown) components costs more than the $5 saved.

@JonnyMac said:
I did a really simple HX711 driver for a work experiment last year. Since it's so simple, I used bit-bang SPI instead of smart pins.

That's perfectly OK. As this is no product development but rather a one-time experiment efficiency and code size doesn't matter at all as long as it works.

ManAtWork

The load cells and the HX711 boards have arrived. I've soldered the load cell wires to the board and connected a few dupont wires to my P2 breakout board. With only ten lines of code it worked within less than 15 minutes.

CON
  _xtlfreq = 25_000_000
  _clkfreq = 200_000_000        ' clock frequency

  pin_c = 30
  pin_d = 31

OBJ
  hx : "jm_hx711"

PUB main ()
  hx.start (pin_c, pin_d)
  repeat 
    repeat until (hx.is_ready ())
    debug (uhex (hx.read ()))

I still need to calibrate it but as a rough guess, if I place a screwdriver (17g) on the cell the ADC value changes from $6E8xx to $6F7xx. The last two digits are noisy. One count of the 3rd last digit corresponds aproximately to 1g. So I can resolve ~1g with a range of 10kg or a ratio of 1:10k. That's quite good for such a cheap piece.

Thanks @JRoark and @JonnyMac, you made my day!