A weighty matter

Haven’t had much time for electronics recently, and I can’t say I wanted to be stuck indoors much when the weather was too good to miss. So, I’ve made no progress on lots of fronts.

Tomorrow starts with a trip to the local tip. That prompted julie to ask me to take her defunct eBay digital scales to get rid of them. Hmm… I nearly bought a load cell the other week to have a play with, I thought I may as well offer to fix them since if it all went titsup then at least I’d have a load cell 😉


Control Panel

Control Panel

That’s pretty much all it was doing. You couldn’t actually weigh anything any more.

Rear of control panel - 4 obvious screws

Rear of control panel – 4 obvious screws

Hmmm… 4 screws removed but it still doesn’t want to know. I wonder…


Let’s have a look inside…

Main board, front

Main board, front

Bit of a mess really...

Bit of a mess really…

You can just see some kind of IC poking out at the back there. I was assuming it was a PIC or something:

Not a PIC

Not a PIC

That turns out to be a 2K-bit I2C EEPROM. On closer inspection, there appears to be a “blob” under the display. That pretty much rules out doing too much with this then since it’ll be too hard (if not impossible) to figure out what the hell is going on in there.

On the plus side, the wires that come from the base are very well described:

Yellow     VIN
Green      GND
Pink       3.8V
Black      GND
White      S-
Blue       S+

Time for a look in the base unit with the load cell to see what circuitry is in there…

Battery Compartment

Battery Compartment

That's All Folks!

That’s All Folks!

There is nothing in the base (apart from the load cell, obviously) save this clunky little single-sided PCB that is used only to connect the wires.

From this and with what’s connected to the load cell, we can see that the 3.8V line (pink at the control panel) is used to “excite” the load cell and that the +/- signals from it are returned on the blue and white wires.

So, what to do. There are no leaky or bulging caps, loads of solder joints that could be on the way out and nothing obviously fixable. Just dismantling it made it work again! I suppose I could go over all the joints but I can’t really be arsed. I have a cunning plan…

We don't need no steenkin' blob

We don’t need no steenkin’ blob

I “sampled” an INA125 Instrumentation Amplifier from TI a few months back. Can’t remember why now :). I’ve had that Arduino Pro Mini 5V lying around for about 18 months (and I’ve got another unused one too), I have loads of 16×2 LCD displays so I think I may as well replace that crummy circuit board with some home made stuff (which may still be crummy but it will be my crummy).

On balance, I’ll probably use a 4-digit 7-segment LED display with a MAX7219 (yup, got loads of them cheap from China via eBay). I will keep the half of the circuit board with the buttons on it though – just connect them to a similar transistor based on/off circuit as used on the current board. The range of the scales is 0-16Kg so I guess < 1000g we just show grammes (0-999), everything from 1.000Kg to 9.999Kg and then 10.00Kg to 15.99Kg. This will need calibrating ideally using 2 known weights with a decent range between them. I’ll cross that bridge when I come to it.

The advantage of using the Pro Mini rather than just putting together (say) an ATmega328p and associated discrete components is that it has a reasonable voltage regulator on it that will take up to 12V (on the RAW pin) and deliver 5V to the mega328p that’s on there. Since I’ve got a nominal 6V from the batteries or something around 7.5V from the supplied PSU (I measured 11.7 on that thing and have been using a decent adjustable PSU from RS – at 7.5V say). I know it’s not hard to add a linear regulator, neither is it any hardship to deploy the Pro Mini (finally).

I’m not sure why they used 3.8V for the load cell input. I think I’ll use the 5V output from the arduino. The INA125 can supply accurate reference voltages at 5V and 2.5V but needs an input voltage in excess of +1.25V over the reference. I can’t get that with 4 x “C” cells unfortunately. Will just have to play with the gain resistor to get a sensible range from the load cell – sounds like a job for a multi-meter.

So, something to play with as the nights draw in. And if I stuff it up, it don’t matter because she wanted it thrown away anyway. What’s not to like 🙂


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