Go Down

Topic: measuring weight using hacked scale + INA128 + firmata --- help! (Read 3429 times) previous topic - next topic

Hi,

I hope it is ok to post this basic 'newbie' question.  I'm a food scientist / psychologist and am exploring the possibility of using the arduino uno + sensors in food science & psychology research.  At the mo, I'm trying to measure weight via the Arduino.  My end objective is to post a blog for fellow 'no electronic skills' scientists so others can build such devices.  I'm also linking up the arduino to this research software I am writing (www.xperiment.mobi).

After googling, although I could not find a clear guide how to build this,  I settled on this guide http://forums.netduino.com/index.php?/topic/1284-connecting-a-beam-load-sensor-to-netduino/.  Specifically this post:
Quote

This is the IC that I have INA128P. Look at the schematic on the first page.
1. Connect S- to pin 2 of the INA128, connect S+ to pin 3 of the INA128.
2. If you have a bread board connect the netduino +5v and ground pin to the power rails on the bread board.
3. Connect E- and E+ to the breadboards power rails.
4. Connect pin 7 on the INA128 to the +5v rail.
5. Connect pin 4 and pin 5 of the INA128 to the ground rail.
6. Either use a resistor for the gain you want or do what I did and use a pot as a variable resistor. Use a multi meter to determine which two pins of the pot will show a resistance change when you turn the pot and connect those to pins 1 and 8 of the INA128.
7. Connect pin 6 of the INA128 to the analog pin of the netduino.


I bought these:
-cheap electronic weighing scales from tescos http://www.tesco.com/groceries/Product/Details/?id=262683786
-INA128 amplifier.  Here's info on it: http://www.ti.com/lit/ds/sbos051b/sbos051b.pdf

I dismantled my electric scale and soldered the 4-wires from the strain gauge to thicker wire (to let me more easily interface to the bread board).  Luckily, the wires were appropriately labelled on the existing circuit board:


As shown in the image (slightly photoshopped to make things more clear, such as only one wire interfaces with each of the pins of the amplifier), I then linked up the breadboard as advised in that post (I've used a 100ohm resister, but also tried using a variable resister).  Running an Arduino loaded with Firmata Standard, and running the Firmata test program (http://firmata.org/wiki/Main_Page), I get nothing - the analog pin just fluctuates around '400', and to the same degree as the unused analog pins do.  Pressing down on the sensor does nothing :( .  To check that I had not blown the strain gauge I resoldered up the weighing scale and it still works (rather incredibly).

I would very much appreciate your help on this, and your patience with my very limited knowledge of electronics.  Hopefully I am doing something silly. 
With many thanks,
Andy.

Dr_Ugi

#1
Apr 17, 2012, 05:35 pm Last Edit: Apr 17, 2012, 05:37 pm by Dr_Ugi Reason: 1
I would check your wiring:

This is the IC that I have INA128P. Look at the schematic on the first page.
1. Connect S- to pin 2 of the INA128, connect S+ to pin 3 of the INA128. - OK
2. If you have a bread board connect the netduino +5v and ground pin to the power rails on the bread board.- OK
3. Connect E- and E+ to the breadboards power rails.- OK
4. Connect pin 7 on the INA128 to the +5v rail.- Looks like you have connected pin 6 to +5 to me
5. Connect pin 4 and pin 5 of the INA128 to the ground rail. - Looks to me like you have used 4 & 8
6. Either use a resistor for the gain you want or do what I did and use a pot as a variable resistor. Use a multi meter to determine which two pins of the pot will show a resistance change when you turn the pot and connect those to pins 1 and 8 of the INA128. - Looks like 1 & 5
7. Connect pin 6 of the INA128 to the analog pin of the netduino. - Looks like 7


I think you are perhaps misunderstanding the pin numbering.  In your photo, the pins number anti-clockwise from bottom left to top right:

 8 7 6 5
|-------|
D         |
|______|
1 2 3 4

Dear Dr_Ugi,
thanks so much for your reply.  You were right!  It was indeed a silly mistake on my behalf - I had misunderstood how the pins were numbered.
I very much appreciate your help on this.
With kind regards,
Andy.


I hope it's ok I ask another question. 

I am thinking about recycling the 4pin plugs and the curly wire from an old telephone and using these to interface the Arduino to the Kitchen scales.  A gut feeling though is that I should minimize the wire distance between the load sensor and the Arduino as this would introduce unwanted resistence which will impact on accuracy.  Fingers crossed I am wrong!  I'm a big fan of re-purposing things :)

Many thanks in advance,
Andy.

PeterH

You didn't ask a question.

If you wanted to know whether that was a sensible idea - I don't see any problems with it from the electrical point of view and I don't think there is any need to be concerned about wire lengths affecting resistance. If you are in an electrically noisy environment you might find you pick up some electrical noisy but if you're only reading a slow-changing analog signal I would not expect that to be a concern.
I only provide help via the forum - please do not contact me for private consultancy.


Go Up