'Easier way' of sensing mass.... low accuracy / resolution needed

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Blazemaguire

Senior Member
Hello all,

Been a while since I last posted, but it's a new school term, new students and some new questions (I'm a teacher) that you guys might be able to help with.

I have a 6th form students studying systems and control and he's come up with a really nice project 'idea' of creating a digital kitchen scales for use by blind people. - The idea is they punch in the desired weight of ingredients using a simple up and down braille button system.

The blind person then fills the bowl on the scale with whatever ingredient and the PIC analyses the mass and when it reaches matches the 'entered' weight it makes a sound to alert the blind person that they've measured correctly.

Great idea, but having never measured mass with a PIC before I started doing some research.

I originally thought 'strain gauge'.. but from what I gather it's necessary to make the gauge part of a wheatstone bridge to get a varying (but tiny) voltage, and then use an expensive amplifier chip to put this voltage into a range that can actually be read by the ADCs on a PIC chip.

I watched a few videos and read a few websites (my student should probably be doing that, I agree!) and getting the above working seems a bit 'overkill' for what he wants to achieve (and also, for his abilities at this stage)

For the purposes of him getting his coursework grade (this is the main concern, not the quality of the product) can any of you suggest a simpler way of sensing mass that might be accurate to intervals of say, 50g?

It really only needs to be a 'demonstration prototype' for this project.. the system 'stages' and the control programming are of more importance than how well it works.

I was looking for a pre-made 'mass sensor' designed to interface with a PIC, but nobody seems to make anything like that.

Any thoughts? - If not, I may have to get him to chose another path for his coursework.

Rob
 
hippy

hippy

Ex-Staff (retired)
I imagine digital scales will have the electronics in them which your student would otherwise need to design so he may get lucky and find a cheap set from which he can find a point to tap-off a signal to the PICAXE which can be used to determine the weight.

The hard part is finding a set which is easy to use and modify without having to buy every version there is out there. Perhaps a search on the internet to see if someone else has already used this trick will turn up a particular brand of scale which can be used ?
 
B

Blazemaguire

Senior Member
I've been looking into that Hippy but so far not found any 'ready to go scales' - My rough idea was to use the guts from a cheap kitchen scales as the sensor anyway.. the trouble is scaling up the voltage... I appreciate the electronics inside the scales will do that already and through its own method display it on an LCD.. it's just where to tap in? - Some people have mentioned interrogating the final wires that go to the scales LCD display and reverse engineering the data to translate the mass measured - However, this was on an Arduino forum, not sure a PICAXE has the programming power to do that, or enough input pins.. Either way, I wouldn't know where to start with that, let alone my student!

Hence why I'm looking at other sensor options... I had ideas of a more mechanical method using a linear potentiometer with a spring counterweight (the more mass, the more the spring 'gives' and hence the further the pot moves)... feed the pot into the PIC as a potential divider and then use that... would you get 50g accuracy though? just wandered if others had done something akin?
 
Technical

Technical

Technical Support
Staff member
Many lab / warehouse / postage scales have RS232 serial output, so would be quite easy to interface.
Google gives quite a few hits on 'rs232 scales', try ebay for a cheap second hand one!
 
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MartinM57

Moderator
Linear spring balance connected to a slide potentiometer (somehow :)) connected as a voltage divider across the supply rails to a PICAXE ADC pin?
 
Buzby

Buzby

Senior Member
S

SAborn

Senior Member
I have done a few load cell circuits with picaxe and the ADS1131 chip from Texas Instruments, although it do require working with a SMD chip.

One of the last projects i used a load cell from a cheap kitchen scale (7 kg cell) that allowed weight readings of within 1-2 grams with a picaxe.

2 picaxe pins are required to read data from the ADS1131 (clock and data)

Your profile dont show where you are located to offer advice on suitable products you might find in your area.
 
A

AllyCat

Senior Member
Hi,

Going back to your OP title "'Easier way' of sensing mass.... low accuracy / resolution needed", strictly, none of the above measure mass but weight!. However, there's not much practical difference, especially if you really only need "low accuracy". For that case, the easiest method might be to use an M2 "Touch" input driven from a simple spring-loaded parallel-plate capacitor (or maybe some existing scales). Perhaps no more than a couple of squares of PCB or sheet metal and some (foam) rubber for initial trials.

Cheers, Alan.
 
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