"Precision" Digital scales

[Christopher]

Hi all

For my school project I have decided to make a digital money box, for pennies etc, which weighs each coin as it is put into the hole and then finds out what value the coin is based on the mass of the coin. This value is then added to the value in the eeprom and displayed on 7 segments (Everyone loves 7 segments!).

The problem I am having is with the scales, I'm going to buy a couple of cheap Chinese 0.01g resolution (Crappy I know, but this is just a small project) digital scales, and I need to get the value out of it. My options are:

• hack the loadcells, 12 bit adc etc (don't want to do this, the board already does it and converts the voltage to grams for me)
• find debug points on the board and get the value from that? hack the micro etc
• read the value from the seven segment displays (easiest way, also the dirtiest)

My question to you now is, if anyone reading this has as digital scale, could they please post a teardown or a picture of the insides (PCB)? This would really really help me, and I'd be really really grateful!

Or any other thoughts about the extraction of the mass would be very useful.

[Rerouter]

considering you told us nothing about which currency you wish to measure, i will assume australian, if so you only need 1g accuracy to diffentiate every coin, still the second digit wouldnt hurt,

i would definatly reccomend just breaking into the 7 segment display, you have 7 wires that will drive the segments, and 1 transistor per 7-seg to pull down its earth, tie as many of the segment pull downs as you want to interrupt lines, and have your micro read the values of the 7 segments each time an interupt is triggered, and store them into a array for each segment, that means only 8 pins for 1 digit, 9 for 2, and so on,

also to me this seems a little tidier than the other 2 solutions :/

edit: if you want the dot point that will cost you an extra pin,

[Christopher]

Well all I want is to extract the value from the scales, the rest of my project is simple after that. I'm going to be measuring UK coins, which are 3.56g for a penny (I'll accept a tolerance of about +-0.2g, depending on the weights of the actual coins I have are).

Yeah, this is probably what I will do Rerouter, all depending on how the LCD is hooked up to the IC. If it is one wire per segment, I'll use either a 16 bit shift register (Which I already have) or a picaxe per display (I don't like Picaxe, but I have these available to me) and just connect em up to my main micro by I2C. I think I'm going to borrow my friend's scales later to see what the insides look like, if he'll let me.

The main thing I want is to keep the original hardware intact, with a little port on the side to connect everything up so I can use em afterward.

Cheers

[deephaven]

If you go for reading the segments beware of any multiplexing that might be going on - i.e they may be scanned rather than having a constant logic state per segment.

[Christopher]

If you go for reading the segments beware of any multiplexing that might be going on - i.e they may be scanned rather than having a constant logic state per segment.

Ah brilliant! I guess I'll probe it with a Scope to see if it's the case if I go for that. Surely just checking all of the lines for a logic 0 before updating the value in memory would be the best way?

I really want to find some way of getting the value of it through a couple of pins on the board, though.

I'll be using 74HC165s for this, in parallel. I'll be outputting over 7 segments with 74HC595's in parallel also, taking up 6 pins total. Would you say this is viable (I'll be using an arduino or most likely a msp430)?

[deephaven]

If you go for reading the segments beware of any multiplexing that might be going on - i.e they may be scanned rather than having a constant logic state per segment.

Ah brilliant! I guess I'll probe it with a Scope to see if it's the case if I go for that. Surely just checking all of the lines for a logic 0 before updating the value in memory would be the best way?

I really want to find some way of getting the value of it through a couple of pins on the board, though.

I'll be using 74HC165s for this, in parallel. I'll be outputting over 7 segments with 74HC595's in parallel also, taking up 6 pins total. Would you say this is viable (I'll be using an arduino or most likely a msp430)?

If they are multiplexed you might be able to be clever and use the common of each digit to clock the segment states into the the 74HC595's. You will need to check voltage levels to make sure they're within the specs of the 595's.

[Spikee]

You can use the AD620 to interface the loadcell.

Just watch this video:

[olsenn]

You can buy cheap gram scales used if you don't mind the smell of marijuana

[Christopher]

UPDATE!

Looked through the digital scales, massive chip-on-board blob. Basically useless, I think I'll interface directly with a loadcell, thanks for the video link, quite informative.

Debugging by the 7 seg was no use, the LCD was connected directly to a 16 pin header on the board (Through a solderless technology, not sure of the name, but a conductive strip). I was thinking it _could_ use the HD44780 interface if I'm lucky but I can't really be bothered with that, I'll look  at the signals through a scope if I get a chance.

So yeah, cheers all, time to get a cheap scale and interface with the loadcell. Would the loadcell in a 0.01g resolution be better than that in a 0.1g scale? I guess so, but it could just be the marketing techniques they use (Limited to 0.1g in software). Brill! I'll get back to you once I have something working.

[T4P]

It's one of those brownish flexible cables right?
It's called a flex cable

[Christopher]

No it connects directly to the board, theres open pads on the board and a spongey conductive "thing" which connects the LCD to the board.

I've seen this before on other things I've taken apart, not sure what it's called though.

I think interfacing with the loadcell itsself is the best option! Cheers.

[baljemmett]

No it connects directly to the board, theres open pads on the board and a spongey conductive "thing" which connects the LCD to the board.

I've seen this before on other things I've taken apart, not sure what it's called though.

An elastomeric connector ('zebra strip')?

[Christopher]

Yep that's it! Nice one man

[Jacob]

Most precision scales provide the subsequent with a weight of units: Grams, Ounces., Troy Ounces., and Pennyweight. In addition, some perfection scale also provide the with a weight of models of Carats...

[poptones]

I think you're cheating. Don't take that personally, but I used to work on scales as a field engineer. Scales are pretty simple tech, make one for extra points. All you need is a spring, some brass shims to act as a hinge, and a coil of wire tuned by a slug of iron. The slug is attached to the brass shim and the weight is laid on top.

http://www.elab.ph/forum/index.php?topic=1824.0

Use a shim of brass to act as a spring and hinge and attach the slog of iron to the end of the brass. Calibrate it using your coins, done. I've made scales built like this weigh gallon buckets of paint down to 10g accuracy and that's as they travel down an assembly line.