As you may/maynot know, my project uses an accurate weigh scale
See the CarbCounter Plus Project for more info

The Problem is that DigiyKey does not stock or list any suitable loadcells or strain guages (Actually no strain guages at all!)

I have emailed DigiKey and the response is that they do not have anything suitable.


The scale requirements are an accuracy of 1 gram (+/- 0.5gram) over a range 1gram to 5Kg.
I have tested this using a load cell that had 4 strain guages arranged in a bridge and it works fine.

Does anyone have any other ideas on how I can acheive this resolution and range with parts that Digikey has listed.

Currently seperating the weigh scale out of the project so that it does not conflict with the competition rules.


Mike

I am also looking into pressure sensors e.g MSP6818-ND from Digikey

Mike

Go old school:

Hook up a solenoid to the food tray such that the solenoid, when powered, lifts the tray. Use a lever to decrease the power required (though it'll increase the solenoid throw - you only need a mm or so of movement on the tray end)

Now put some food on the tray. Activate the solenoid. If it lifts the tray you're good to go on.

Use a bare electrode or a very low force pushbutton switch under the tray - when the tray is lifted by the solenoid the switch or contact should disconnect.

Weighing procedure:
1. Start at 0 current in the solenoid, make sure the contact/switch shows the tray is down.
2. Ramp up the current in the solenoid until the switch/contact disconnects.
3. Measure the current in the coil of the solenoid - that is related to the weight of the food.

You have to measure the current due to variations in the power supply, etc, although for a gross measure you could simply use PWM out and use the value you set the PWM to. You could implement a PID here if desired to find the weight with more accuracy, and to prevent noticeable vibrations as you move the tray up and down weighing the food. If implemented well mechanically, and using springs to assist the solenoid, it could still require very low power, but it's not ideal. You could also implement it using a magnet on the tray with a coil below it - force the magnet up (or down).

In theory the current required to lift the load from the contact is linearly related to the force required to lift it, which knowing gravity indicates weight. However the solenoid is not a linear actuator and you also need to account for the mechanical linkages. You'll probably want to manually calibrate your homebrew, active load cell with several points until you obtain the required accuracy and range.

There are several other ways to measure weight/mass, but this one seems the most interesting to me.

Good luck!

-Adam

Hi I found a 500gm scale to 0.1gm resolution. It consists of a small loadcell about 15mm high with a separate section for the electronics. I had to get the expensive one at US$21 the US$12 ones in same range were sold out. There are no connections to the outside world and I have not looked to see what may be available inside the box. It certainly looks like a loadcell is available in the size range you are looking for. Size of the whole unit is about 20mm tall and 100mm square.

Best regards,
Clive

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Hi Clive,

Hopefully by the end of this project you will have all the resources, design and code you need to build your own unit, or someone will start manufacturing them, and I hope it helps you more accurately calculate the insulin dosage, as well as helping Log carb intake, BG readings etc etc

Question are you still MDI or are you pumping?
Either way I look forward to your input for the rest of the project, especially when it gets to the additional hardware/software "Features" that you would like to see added.

Thanks,

Mike