Hacking a milligram balance (scale) with a Parallax Propeller microcontroller----make money online

For an upcoming project, I'd like to develop a dispensing system that can measure out a desired mass of material. The idea would be to use a microcontroller, dispensing valve, and electronic balance to provide feedback. I already have an American Weigh Scales miniPro-100, and decided to hack it so that my microcontroller can discover how much mass is on the balance. It can then regulate the dispensing valve appropriately.

The balance contains a Cirrus CS5530 24-bit ADC. I tapped the clock and data lines and found bursts of data that occurred at 7.5Hz. The clock is about 150KHz. I used a Parallax Propeller running assembly in one of its cores to capture the data stream and load it into my main program.

hey everyone for my current project I'd

like to use a microcontroller to

dispense a material onto a balance like

this and then have the microcontroller

stop the dispensing when it's weighed

out a certain amount of material so to

do that this this scale doesn't have any

electronic interface so today's task was

to crack this thing open and figure out

a way that I can get a microcontroller

to read the values from the balance and

then later I'll do a video showing how

I'm going to meter out the material and

turn it off turn off the dispensing when

we get up to a specified mass so I

thought I'd just show you how to take

this thing apart it's actually built a

little bit differently than most

consumer electronics that I've taken

apart most of them you'd probably start

by cracking the case seen here that the

plastic shells but this one actually

doesn't have any clips around here so I

put a few dents in mind by trying to

take the plastic shells apart when that

actually wasn't necessary so in this

case the platter comes off just like

that and then there's a couple more

screws holding down this stainless plate

and then there's a couple more screws

here and then the top shell comes off

and a couple screws to hold the circuit

board down and then the circuit board

comes out just like this now of course I

added these wires here these these were

not here in the stock configuration so

let's loop the zoom in on the circuit

board and I'll show you what I found

okay I'll just give you a circuit

overview as near as I can tell these

wires here are where the strain gauge

connects to the input of this thing this

is a weighing device and so it's its

main piece of sensing tech here's a

classic strain gauge which I'll talk

about in a minute and there really isn't

any analog path to speak of in fact the

the sense wires from the Wheatstone

bridge go through a ferrite you know

bead inductor and then they go right

into this guy which is a serious logic

see 5530

24-bit analog to digital converter so if

I had to guess I would say that that

chip accounts for most of the cost of

this

whole project or this whole item here

maybe besides the strain gauge there's a

couple of small voltage regulators one

here and one here and I think they're

both set to just 5 volts the input to

this thing is just a typical unregulated

wall wart type thing so if the input

voltage is you know 9 volts unregulated

they put a couple 5 volt regulators in

there this guy is an LCD driver and it

has a little bit of RAM inside of it

it happens to be an HT 1621 and the

microcontroller which is an ADC 51 part

here drives this whole thing so the flow

is pretty simple I mean basically the ad

pumps the data or the microcontroller

runs the ad and then sends the data into

the display Ram here and that's that's

basically it the other side of the board

doesn't have a whole lot going on these

are the ferrite beads that suppress a

little bit of high frequency noise

coming from the Wheatstone bridge and

then we've got your your classic plain

old LCD here with some buttons and just

a few transistors also this has to be

one of the nastiest looking solder jobs

I've ever seen in a piece of commercial

equipment okay so if the job here is to

sort of tap the signal and get the

actual value of the thing that's being

masked out of this circuit we start to

look for ways that we can get into sort

of tap the lines since the analog signal

path doesn't really exist it basically

goes straight from the Wheatstone bridge

right into the ad converter through

those beads there's really no chance of

us adding another ad converter because

this one is actually set up with the

reference voltage going into the

Wheatstone bridge so be very difficult

to to tap the analog side so I pulled up

the datasheet for this ad converter and

as it turns out the the data line and

the clock line go directly into this

microcontroller and it's the data is

read out in sort of a synchronous way so

I I tapped the clock and data out from

the ad chip line and looked at it on the

scope and this is what it looks like

okay so the bottom trace is the clock

going from the microcontroller to the AG

converter and the top trace is the data

line coming from the AG converter to the

chip and if I press on the scale a

little bit you can see the higher order

bits up here are changing when I when I

pressed on the scale so this agrees with

the datasheet I looked up the the

datasheet for that serious ad part and

it confirms that it's sending data in

for byte chunks and so you can see these

clusters of clock cycles down here so

currently the scope is set to 50

microseconds per division but it's it's

unnormal trigger mode so if i zoom out

you can see that that cluster now it's

50 milliseconds per division you can see

that that burst of data happens

periodically and it turns out it's

running at about 7.5 Hertz so seven and

a half times per second the

microcontroller decides to query the ad

converter and pulls that data in in that

little burst now one difficulty is that

the burst is actually quite fast and so

if we zoom in a bit and then scale over

to take a look at some of those clock

cycles when data is being sent the the

clock time is actually quite fast it

turns out to be six and a half

microseconds from rise to rise on the

clock there so obviously we can't pull

that with a microcontroller because it

would just be ridiculously fast and

another problem is that we couldn't

really easily use an interrupt in a lot

of microcontroller architectures because

the rate is just so high 150 kilohertz

even though this burst only happened

seven and a half times per second during

this burst received period if we try to

use an interrupt the processor would

have to be fast enough to handle the

interrupt before the next clock cycle

came in so this is definitely going to

be a problem and especially if the the

processor has to handle other interrupts

you might get some interference between

different sources of information

so I came up with a different solution I

ended up using a parallax propeller the

propeller is a really interesting chip

it's a 32-bit architecture chip but it's

built in a way that has eight cores and

it's unlike a traditional

microcontroller and that there's no

interrupts and so all the cores run

synchronously and share a common memory

space so that you can talk from core to

core by sending information into the

shared memory and each core also has its

own memory to do certain tasks so I

initially tried to program one core in

the propellers native programming

language called spin and it was not fast

enough to receive the signals and so I

put a weight instruction to wait for a

rising edge on the clock line and by the

time the next instruction fired five or

ten microseconds had passed so there's

no way to to capture the data because

it's coming in quicker than this thing

can runs in instructions so the makers

of this knew that the the real power of

this chip would be you know realized by

programming it in assembly so they also

allow you to do that in fact it's

interesting and that parts of the chip

can be programmed in assembly and parts

can be programmed in spin in fact even

side by side functions can switch back

and forth between assembly and spin

which is pretty cool so I ended up you

know hunkering down and writing the

assembly to capture the data from this

line one of my requirements for this was

that the scale itself not be harmed by

what I you know what I added to it

originally when I was thinking of

getting the data out I thought well I

could intercept the clock line and then

basically turn off the connection

between the microcontroller and the ad

and then turn on the connection from my

microcontroller to the ad so basically

just switching the ad back and forth

from their microcontroller to mine but

that would cause the panel to get weird

and I've noticed this thing goes into

fault modes and then doesn't recover so

that's kind of a pain in the butt

the propeller is a 3.3 volt device and

so I just added a couple current

limiting resistors here since this

circuit is five so even though you know

this thing is pulling 0 to 5 volts these

current limiting resistors are do a fine

enough job of preventing too much

current from flowing into the chip the

chip also runs very quickly it's a 5

megahertz crystal but there's a 16 X

phase locked loop in there so it's

actually an 80 megahertz clock

internally this is really helpful in

doing assembly instructions most

instructions are four clock cycles per

per instruction so you can actually get

a whole lot done when coding it in

assembly okay so here's my program I'm

definitely not an experienced assembly

coder so I was basically happy just to

get this working I'm sure this could be

optimized a bit but there's plenty of

time at 80 megahertz with only four

clocks per instruction there's actually

very easy to to make this work and so

what I did is I have a loop here that

just times out after one millisecond so

what this thing does is when it times

out it resets it's 32 bit register and

then as the bits come in from the ad it

just fills up that 32 bit register and

when it times out it sends the value

that register back to the main program

loop and then resets and waits for the

next one so this takes advantage of the

fact that the data is coming in bursts

and it uses it knows that the start of

the data is coming because it's just

been a long time and it's timed out so

here's the the debug value coming back

and if I push on the scale a little bit

you can see the value going up and the

units are you know uncalibrated I

noticed that if you press the tare

button on the front of the scale it

doesn't actually change this at all so

the unit conversion and tare functions

are all done in the microcontroller

downstream of the ad so since I've come

this far with the propeller I think I'll

probably just do the whole project using

this chip it's been kind of a while

since I've used it and it does have some

pretty cool features that I haven't

played with in a while since each core

runs at 80 megahertz

possible to do video generation with the

chip without too much work and then your

program can just send values to the core

that's doing video generation and it

doesn't really add all that much

overhead to your program here's a shot

of the strain gauge that's inside the

scale these devices are actually really

simple but there can be made very

accurately all it is is a piece of metal

and when I press on this the metal just

deflects a tiny amount and the strain

gauges themselves are thin thin film

resistors that are just glued on to the

surface of this metal structure so by

pressing on this the metal structure

changes shape very very slightly which

is what that's what strain is and these

thin film resistors will change

resistance very very slightly because

they're actually being distorted a

little bit compressed or stretched and

the whole idea with the Wheatstone

bridge is that some of the resistors are

on the strain gauge for temperature

compensation so if the whole temperature

of the device changes a little bit all

the resistors will change in the same

way and so a temperature will

essentially be compensated out okay I

hope that was helpful see you next time

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