Guidance on creating a simple weight sensor (digital scale) and POST to the web

I'm creating my first arduino project and would appreciate some guidance. I am basically trying to re-create a digital scale that will POST to the internet. I am going to be measuring the coffee levels and updating our intranet with the info. :smiley: The coffee pot will sit on the scale and at regular intervals, will submit the data as an HTTP request.

Requirements:

  • Since this is just for fun, the parts would preferably be cheap
  • Small sensor for measuring small weight changes. Accuracy is not that critical (cheaper is better)
  • Need to be able to send an HTTP request, so ethernet is a requirement

I am a professional web developer, so the programming part will be easy for me. However, I have no experience working with electronics/sensors/micro-controllers. Again, this is my first project. What parts do I need? Where can I buy them? Where can I learn to wire these parts together? Also, any tutorials you can refer me to would be VERY much appreciated.

Thanks!

Hi i'm new too so i won't be able to help hugely. But I'm thinking you'll need a load sensor to measure the weight: http://www.mindkits.co.nz/store/sensors/others/load-sensor-50kg. That's from a NZ store but I'm sure you can find it where ever you are. I just bought myself one but I haven't played around with it yet. Also here's some more info about the load sensor NerdKits - Digital Scale Strain Gauge Weight Sensor
Here's someone who did something similar to yours http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1260242124

Hope that helps! :slight_smile:

Thanks for the links. However, I don't think that the 50kg load sensor will work. It will be too big. I will be measuring a relatively lightweight object. So I need to find something smaller (5 lbs / 2.2 kg). Can anyone post a link to a smaller load sensor that I can purchase?

Based on the little bit of research I've done so far, this is my basic understanding (please correct me if I am wrong): A Load Cell (or load sensor) is a piece of steel containing one or more Strain Gauges arranged in a Wheatstone bridge. So basically a load cell does all the work for me. Is that true? Do I need an amplifier or anything like that?

Based on the little bit of research I've done so far, this is my basic understanding (please correct me if I am wrong): A Load Cell (or load sensor) is a piece of steel containing one or more Strain Gauges arranged in a Wheatstone bridge. So basically a load cell does all the work for me. Is that true? Do I need an amplifier or anything like that?

Yes, most load cell use strain gauges arranged in a Wheatstone bridge circuit. Some, at extra cost, may come with matching electronics that output a higher level voltage or current for ease of measurement. However most are of the low level bridge types (if it has four or more wires, it's low level) that only output smaller millivolt measurements and one must use a precision DC differential amplifier to raise it to voltage measurement levels suitable for reading on a Arduino in most cases. An Arduino can be set up to use it's internal 1.1vdc reference voltage for a resolution of step size of 1 millivolt or so, but the arduino analog input still lacks the differential input mode that reading most Wheatstone bridges require, so one ends up needing to use two analog input pins and calculate the difference in their readings.

There are external ADC converter chips that have differential input modes and programmable internal gain amplifiers and can easily interface with a arduino via I2C or SPI buss.

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Lefty