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 of a coffee pot and updating our intranet with the info. The coffee pot will sit on the scale and at regular intervals the data will be submitted. The maximum weight of a full coffee pot will probably be less than 5 lbs (2.2 kg). Since this project is just for fun, I would prefer that the parts would be inexpensive.

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. What parts do I need in order to measure these small changes in weight to a relatively lightweight object? 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.

Update: Here is the parts list that I've compiled so far:

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    You should look at a netduino as it supports ethernet connections. – Dean Aug 15 '11 at 18:47
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    I think the netduino is for programming in .NET. Also, I am not really interested in the ethernet part yet. I am only trying to figure out what types of sensors and things I will need. – Andrew Aug 15 '11 at 19:10
  • Doesn't the carafe usually sit on what is basically a hot plate in the bottom of the machine? If so, you might need to weigh the whole machine. OTOH, if you have an insulated carafe this might not be an issue. Maybe you can rig up a temperature status or control while you are at it... – Chris Stratton Aug 16 '11 at 5:58
  • It's a standalone (insulated) airpot, that does not sit on a warming plate, like one of these: google.com/products/… – Andrew Aug 16 '11 at 14:42
up vote 9 down vote accepted

If you want ease of implementation, value for money, and good accuracy - then a cheap commercial digital "kitchen scales" is worth considering. These are commonly available in 2 kg and 5 kg versions and no doubt other ranges as well. By using a simple lever arrangement you can increase or decrease sensitivity mechanically.

You can buy load cells and instrumentation amplifiers and roll your own, but the ones in some kitchen scales are superb and the cost is liable to be far less than you can buy the parts for new.

Note that a load cell beam CAN be damaged by overloading. The gauges usually are designed to "bottom" shortly past full scale. If you used say a 5kg cell and used it to only say 2kg or 3kg and added your own stops so it never reached full scale then you would completely avoid overload damage.

The best (and in some cases also the cheapest) kitchen scales use a "real" load cell with 4 strain gauges in a bridge plus hopefully a 5th temperature compensating strain gauge (places on the load cell where stress/strain is caused only by temperature changes. The photo below shows a typical load cell used in low cost scales.

enter image description here

Performance of cheap kitchen scales varies from not-overly-marvellous through to unbelievably superb.I bought some 2 kg kitchen scales here a few years ago which were essentially linear to 0.1% across the range and which were temperature immune for practical purposes.

Mine looked very like these. Similar appearance no guarantee.

Mine looked very like these

Factors which relate are accuracy, linearity, repeatability and temperature independence. The first 3 are close travelling companions but not identical.

Accuracy checking: Obtain several hundred one and two cent coins. These will probably weigh close to 1.000 gram and 2.000 gram each. If not that then some other fixed mass. This is partially so banks can check amounts by weighing. They are amongst the best value for money calibration wights you can but. Make a few larger test weights by using eg coins to calibrate them. Say 100 200 400 800 gram would be easy. These can be made out of almost anything stable. Even eg plastic screwtop jars with water in - as long as they are airtight.

My scales would track linearly for any number of coins aded or removed and whether removed or added one by one or N at a time. Superb. Some scales are not so good.

Some scales are poorly temperature compensated. Mine can have a hairdrier waved over them on high until they are toasty hot (50C plus) with little of no display deviation. Superb.

Interfacing to Arduino:

  • Once you have found ones which are accurate enough for you, you need to interface them to the Arduino.

    Access to existing amplified signal: Most will have an analog voltage(amplified strain gauge signal) which is converted to digital by the display controller. Signal level should be in the volt or few range - easy for an Arduino ADC to measure.

    Access scale controller's digital signals: If you can't get at the analog signal you may be able to access the controller at the digital stage. Long ago people have gone so far as decoding 7 segment multiplexed display signals for processor interface - but if it's that hard then finding ones with analog signal access would beeasier.

    Voltage to frequuency ADC: Some may use a V to F (Voltage to frequent) converter. These are easy to read by either pulse counting in finite time or by seeing how long it takes to make N clock cycles. You could also add a V to F converter to existing scales using th amplified strain gauge voltage as input and reading the output frequency with the Arduino - but a "proper" ADC approach is probably preferable.

    Instrumentation amplifier + ADC: Failing all the above you can use the classic instrumentation amplifier plus ADC solution. Integrated high performance instrumentation amplifiers make this easy enough, but using what's in the scales already is liable to make more sense.

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    What, exactly, is a "Two Cent" coin? I don't think I have ever seen one. – Connor Wolf Aug 16 '11 at 9:51
  • @Fake Name - In NZ (New Zealand) we had 1 2 5 10 20 50 cent coins. The 1 2 5 have now been removed from circulation. I reatined a quantity for use as test weights. In many but not all countries the mass of coins is arranged so each is an integral number of grams. This allows them to be easily counted by weighing a quantity using suitably accurate scales and dividing by a coin's mass. Obviously this works for non integer gram masses if coin weights are consistent but makes use of a calculator necessary. In country of choice substitute low value integer gram mass coins of choice. – Russell McMahon Aug 16 '11 at 10:50
  • Using coins or other items as test masses relies on their mean mass being close to a consistent constant value when a large quantity are weighed. NZ 1 cents = 1 gram, 2 cents = 2gram. I do not know how accurately or tightly they cluster around these values but for practical purposes they are effectively perfect using up to several hundred coins and scales with 1 gram resolution. – Russell McMahon Aug 16 '11 at 10:53
  • I understand the logic behind using coins as test-masses. However, I was trying to point out that their use is high localized. The coins in every country are different, and you can't just discuss using them without some sort of reference to where the coins you are using are from, and (ideally), somewhere that people can look up what the weights of their local coinage is. – Connor Wolf Aug 16 '11 at 12:23
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    @Fake Name - coin masses are highly standardised in integer gram amounts in many countries. I have a reasonably extensive international loose change collection gathered in recent years during travelling. I spend a happy hour or few a while ago weighing various samples of all the coins I had. While hardly an eclectic collection the trend was very very very clear. I don't see the need to point to tables for roubles, pesos, drachma, shekels, ... . Weigh a few of your target coins or ask your bank and you'll rapidly find out. Weight based coin counters are very widely used by banks world wide. – Russell McMahon Aug 16 '11 at 13:15

I'd get a digital scale from a department store and hook up to it's existing electronics. This solves two problems: getting strain gauges, and building a physical mechanism for transferring the load to them properly. Most of them seem to be produce readings out to half a pound. Between that and some quick calibration, you should be in business.

For example, see: http://www.youtube.com/watch?v=fPzUtzFJFus

  • this is a good idea. I hadn't thought of hacking a digital kitchen scale because I didn't think it would be easy to do. Looks like it may be the cheapest/easiest solution! – Andrew Aug 15 '11 at 21:22
  • Kitchen scales would be designed for better resolution down at the weights your using, but double check that they can support the weight in question. An average looking coffee pot I found holds 10 cups; that's 5.2lbs of water, plus the weight of the glass. I don't think any of my digital kitchen scales go up that high. It'd probably be easier to squeeze more resolution out of the bathroom scale than to ensure the kitchen scale can handle heavier weights. – Jay Kominek Aug 15 '11 at 21:35
  • Hmm...I've seen 11 lbs capacity kitchen scales, but I will also be dealing with someone pressing on the coffee pot in order to dispense the coffee. Would it be possible to damage the scale by applying too much pressure/force on the scale? – Andrew Aug 15 '11 at 22:06
  • @andrew My guess is that the scale will have stops, so pressing down past the maximum travel will just transfer additional load through the frame. You can verify this for yourself when you take it apart. :) – Nick Johnson Aug 16 '11 at 0:04

you'll need a load cell that works with your weight ranges, an arduino, and an wifi or ethernet shield

  • thank you. I have updated my question to include your list. But could you be more specific? Where can I find the load cell that you mention? – Andrew Aug 15 '11 at 22:08
  • what weight range are you looking for? – neufuture Aug 16 '11 at 2:56
  • probably somewhere between 5 lbs and 20 lbs – Andrew Aug 16 '11 at 14:51

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