Weight in grams.

[metalsapo]

HI, im wondering if anyone has ever created a project to sense weight, and i want to weight around 20 gr. to 60 gr. for one day chicks, where can i buy the sensor, i´m starting to program pic16f84A so it is a nice project, thanks guys.
 

[robrenz]

Buy a 100g scale and interface with whats already there. IMO its not worth trying to build the strain gage flexure unit.  Not to mention it would end up costing you more even if you worked for a $1 per hour.

[PA0PBZ]

I did make one from scratch, mostly because I could not use or scrap a scale because of the size of what I wanted. Since that will not be a problem for you just go out and buy a cheap digital scale, anywhere from 100g - 1Kg, depending on the resolution you want. Glueing your own strain gages is not realy funny...
The easiest for you would be to tap the strain voltage after the amlifier and feed it into the ADC of the pic, at least easier than trying to decode the display output. Of course you would need a pic with an ADC, I used a pic with USB for easy interfacing.

If you can't find an amplified output in the scale just take the strain gage output and amplify it yourself, one LM and 2 resistors did the trick fine for me on a single 5V supply.

[metalsapo]

OK, the search starts now, and just to make this post advance, the data from every time the scale is used is to make a data recording inside of the PIC to see the ending results to inform ourselves and the client, the problem here is to get accurate weight for the client to see the weighted chicks, by now we have a scale, but not to tear down, jajajajja, we weight like 400 of them and its tedious, i´m an electronic, i must do something.

[SeanB]

Start by buying a cheap kitchen scale, one that does from 5g to 10kg will do, and use the loadcell and pan from it. Otherwise you will need to buy a 10kg loadcell, which will be a lot more expensive. Google a bit for loadcell amplifiers, and either build or buy one to get the output of the loadcell to a range where you can use a 16 bit ADC ( or an 8 bit one if you are really careful with drift and offset compensation, as well as subtracting a big tare voltage from it) to give a raw data value. Scale it to get the sensitivity you need, and then use a look up table to get the mass to display.

Remember that you are dealing with live animals that move, so you will probably have to do a few hundred readings per second, and calculate a moving average to have a stable reading.

[metalsapo]

In Jameco i found this options, i bought both, so here i go!

FORCE SENSING RESISTOR,0.5
CIRCLE,1oz-22LB,FLEXIBLE

FORCE SENSING RESISTOR,1.5"SQ
1oz-22LBS,2 LEADS,0.1" SPACING

[SeanB]

Those are not loadcells. Force sensing yes, repeatable no. You will find they have a basic accuracy of a few percent, and that if you place a constant mass on them the output will be both noisy, and the output will depend on both position and time. They also have a poor tempco. You do need a precision loadcell, one that has a nice foil strain gauge and compensating gauge mounted on a block, with both positioning and overload protection. You can buy the foils as parts, but you will need to glue them down carefully.

[PA0PBZ]

If you really want results: buy a cheap digital kitchen scale! They cost like $10 - $15, and are perfect for what you want. Strain gages(*) are very hard to get, been there, done that... Then either take the load cell out or tap into the scale to get an analog value, these things are very linear so it's easy to get a meaningful output. If you want to make 1000+ devices there are probably better ways to do it, but it looks like you only want 1.

^{(*) gages or gauges? I see both and since I'm not native English I have no idea what the right spelling is...}

[olsenn]

Go to a head shop, where they sell marijuana paraphenalia... they sell scales perfect for what you're doing and they're pretty cheap.

[hlavac]

Good way to measure weight is using a long coil and have an iron core slide into it as stuff hangs on/compresses a spring. Measure inductance of the coil, for example put it as part of an LC oscillator and count cycles. This can have surprisingly good resolution.

[saturation]

http://www.esnarf.com/5288k.htm

For calibration I use coins, in the US they are made with very tight tolerances for size and weight. 

http://www.usmint.gov/about_the_mint/?action=coin_specifications

You can find them on eBay for $4@ as a postal scale.

[PA0PBZ]

You can find them on eBay for $4@ as a postal scale.

Too bad the PIC interface will cost a bit more using this scale 

[metalsapo]

Thank you guys, maybe girls, your fast answer will let me think, the advantage here is that time is not important, also i can make some experiments with this, and the pic is just because im starting to program again, i learned in the university and i am very rusted, so, i will keep you noticed for every move, thanks again.

[metalsapo]

If you really want results: buy a cheap digital kitchen scale! They cost like $10 - $15, and are perfect for what you want. Strain gages(*) are very hard to get, been there, done that... Then either take the load cell out or tap into the scale to get an analog value, these things are very linear so it's easy to get a meaningful output. If you want to make 1000+ devices there are probably better ways to do it, but it looks like you only want 1.

^{(*) gages or gauges? I see both and since I'm not native English I have no idea what the right spelling is...}

I want 2 or 3, jajaja, ok i will buy one, ill write to you when i get lost in someones circuit to destroy and get the digital info!