• I would like to connect a load sensor and two Hall-effect sensors to my Espruino Board 1.4b. Does it matter where I connect the wires to on the board for a sensor that doesn't have a module?

    Would it matter for the load sensor what area I connect it to on the board? The sensor has a ground wire and might need a specific voltage. Also with the load sensor, it will need to be converted to a digital signal (so I'm guessing an ADC spot) . I think voltages matter in an ADC conversion and the load sensor has the capacity of sensing up to 50kg. I'm also not familiar with the voltage conversion equation enough to really know what I need to do with the load sensor so that it senses up to the 50kg max.
    If it also wouldn't be to much trouble, does anyone know a good setup that would work with a load sensor like the kind I have?

    For the Hall-effect sensor, my main concern is using a resistor with the sensor. How would that work with the Espruino Board? And the resistor is needed.

    Link to Load sensor page:

    Link to Hall-effect sensor page:

    Thanks in advance!

  • No datasheet, no mfg part number, and the only information is a an excerpt from a spreadsheet and a video (i don't watch videos), and the load cells require external components not listed or described on the product page. Typical sparkfun - awful vendor, just awful, and their prices are high too. This week I had a ($3 shipped - no fancy shipping here) package from CHINA beat their shipment, despite them having a 1 day head start and shipping domestically (I needed something I couldn't find elsewhere, otherwise I avoid sparkfun and adafruit like the plague).

    The impression that I get from other resources is that you need external components to amplify the output of the load cell to get useful data. But since I didn't watch the video, I can't say anything regarding that specific part.

    The hall switches are easy:
    One side goes to +5v (VBat) other side goes to ground, middle pin goes to any 5-volt tolerant input (any pin not marked 3.3v here http://www.espruino.com/ReferenceESPRUIN­OBOARD ). You need to do pinMode(pin,'input_pullup'); on the pin, since it's open collector. Hall switches that work with 3.3v supply exist (and these could use any pin), but those ones you got, datasheet says 3.5v minimum (again, typical sparkfun)

  • Yes, as @DrAzzy says it's pretty poor of Sparkfun not to give any real data on it - my guess is they don't because it's a pain to use. The problem is that the resistance in the sensor changes by a tiny amount - probably too little for Espruino's 12 bit ADC to be much use.

    Honestly I'd suggest you try and get a refund, and buy:


    And the sensor that they also sell - note that it has 4 wires so is a complete 'Wheatstone bridge', where the sparkfun only has 3.

    Then you just wire up that module (which contains a 24 bit ADC) and connect it to I2C pins, then the rest is pretty easy...

  • To be clear, gordon's link is just an ADC - I think the sensor you want to go with it is one of these : http://www.ebay.co.uk/itm/271744729693

    (you can probably find same items with better prices/shipping rates to your country from other vendors on ebay)

  • Thanks!

    Just to give a bit of background: These sensors usually work by attaching a resistor to a piece of metal that stretches. As it moves, it stretches the resistor and changes the resistance, but only by a tiny amount.

    It makes it quite hard to sense with a microcontroller without any external components - and once you've added amplifiers and things externally it's probably easier to just buy a module that is meant for the job.

    To make matters worse, the resistance not only changes with the weight, it changes with temperature as well (and probably other things like moisture and atmospheric pressure). It wouldn't normally be a big deal, but because you're measuring such small differences it really adds up.

    So normally there's a wheatstone bridge arrangement of resistors on the sensor itself such that everything balances out nicely and resistance can be read accurately. Unfortunately yours doesn't seem to have that - so while with some effort you could wire it up through an op-amp or feed it directly into an accurate ADC, you might find that it wasn't actually that accurate anyway.

  • So from what I'm understanding (and looked up) the load sensor that I have is only half a Wheatstone bridge and cannot be used alone?

    I'm also wondering about using a household digital scale and then hacking it to get an input. Would that work?

  • Yes - the sensor can be used (with a suitable ADC), but because it's only half a wheatstone bridge it's not going to have such good accuracy (one day the reading could be different to the next).

    For using a digital scale, I think it'll depend which model you have. However a scale like this one would be perfect - you can get a simple square wave, the duty cycle of which is the weight.

    Putting that into Espruino is trivial - just connect up to any IO, and the software looks like:

    setWatch(function(e) {
      var weight = (e.time-e.lastTime)*someScaleFactor;
    }, YOUR_PIN, { repeat: true, edge: 'falling' });
  • See this posting and the last comment by me http://forum.espruino.com/conversations/­258182/#comment11952575

    Also this ADC from ebay http://goo.gl/zC8B2n should allow you to read the very small signals from the strain gauge



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Connecting a load sensor and a few Hall-effect sensors

Posted by Avatar for Progma @Progma