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1  Using Arduino / Sensors / Re: Capacative Pressure Sensor on: October 12, 2012, 03:25:57 am
my goal for is to detect if I'm still lying in bed sleeping when I should be up.

I thought that was managed by something called "will power".

Yeah not for me, I use two and sometimes three alarm clocks, I sleep through them all on occasion.
This is actually a serious problem for me.
2  Using Arduino / Sensors / Capacative Pressure Sensor on: October 11, 2012, 11:34:31 am
Hi everyone, I'm trying to put together a "pressure sensing pad", my goal for is to detect if I'm still lying in bed sleeping when I should be up.

My pad would consist of two metal plates(foil) with foam in between them, when a weight is applied the foam is compressed and the two plates moves closer to one another. This would change the capacitance right?

What I need is for my Arduino to measure the change in capacitance. Is this possible?
3  Using Arduino / Sensors / Re: OPamp on: October 11, 2012, 11:25:04 am
If you have three wires coming out, I think you have two resistor elements in the strain gage, one is on the piece of steel that strains, and the other is in another place where it is an offset for temperature correction.

They make up half of a wheatstone bridge.

You think?

Your assumption is sadly not correct.

The strain gauge is a voltage divider. It works alot like a potentiometer only the strain gauge only changes it's resistance value by a couple of ohms at full load. There is no resistor in there for temperature correction. However by using two Half-bridge gauges, one as a pressure sensor and the other as a reference for the amplifier you'd get your temperature correction. 
4  Using Arduino / Sensors / Re: Load cell on: September 30, 2012, 11:07:29 am
After further experimentation I get less and less certain that I managed to read a weight change, Seems more like I've built a great EMI antenna and amplifier, Putting a weight on the sensor does change the value, but it seems I can accomplish the same change by resting my finger on the sensor.
5  Using Arduino / Sensors / Re: Load cell on: September 30, 2012, 08:52:07 am
While looking at the two op amp instrumentation amplifier circuit(2.3):

I noticed that the equation: Vo=(sig+ - sig-)(1 + R1/R2 + 2R2/Rg) doesn't make sense since they state that R1=R2 and R3=R4
If R1=R2 then Vo=(sig+ - sig-)(1 + 2R2/Rg) would be true, but then there wouldn't be anything that defined the relation between (R1, R3) & (R2, R4)
6  Using Arduino / Sensors / Re: Load cell on: September 30, 2012, 06:52:53 am
YES! I got it to work. Using two LM358Ps set up as a three Op Amp Instrumentation Amplifier. The problem I had with that circuit before must have been the low resistor values I used, This time having done the math, and using high resistor values (100K, 10K, 150K, 6.7K) I got it to work really nicely. Next thing to do is to try the two Op Amp Instrumentation Amplifier, Why you ask? Well because then i only need to use one LM358.

I'm very happy right now. And thanks to all of you guys for helping me out.
7  Using Arduino / Sensors / Re: Load cell on: September 18, 2012, 11:25:34 am
I think you need WAYmore gain and a proper instrumentation amp (very very good CMRR).  The change in resistance of a strain-gauge is measured in tiny fractions of a percent, so gains of 1000 or more aren't unreasonable.

This might be useful:  there a gain-resistor of 10 ohms is used, meaning a gain of 6000.  Sounds more likely than 50!

I agree, still I was hoping to see some small change at least with a gain of 470, but it might be the amp as well, I'm thinking of trying one of the INA amps preferably with an easy gain adjust   
8  Using Arduino / Sensors / Re: Load cell on: September 18, 2012, 07:49:25 am
Uhh, my head hurts, no luck with the instrumental amplifier setup. I no longer have any idea what to do next.
9  Using Arduino / Sensors / Re: Load cell on: September 18, 2012, 06:04:11 am
Using a better multimeter I get a 3mv change with 11,5Kg keep in mind that I'm still using the LM358 to subtract the values only with zero gain i.e no resistors,

I might try using 3 OPamps in a Instrumental amplifier setup and see where that gets me. 
10  Using Arduino / Sensors / Re: Load cell on: September 18, 2012, 05:50:47 am
Yes I checked the voltage swing with my multimeter

4.1-3.4mV swing at 0-20kg

But it seems like I cant replicate that, most likely thanks to my shitty multimeter.

I also just tried a huge gain value of 470 which didn't work.

I'm going to dig out my brothers multimeter and try to get a decent reading with that.
11  Using Arduino / Sensors / Re: Load cell on: September 18, 2012, 04:57:19 am
After some meditating yesterday I thought I had figured it out,

If Rf=Rg and R1=R2
   Vout=Rf/R1 x (V1-V2)

So I changed the resistor values,

Vout = 5V
V1-V2=0.100V (This i wasn't actually sure about but i figured that it'd be something like that)

Which meant that:
5=Rf/R1 x 0.100

Based on that I chose the new resistor values Rf=4.7K R1=100
4700/100=47 (which is close to 50 so I thought it'd be enough)

Sadly this still isn't enough gain to get any reading on the voltage change, which I find strange but I'm going to try to double it and see if that works.

12  Using Arduino / Sensors / Load cell on: September 17, 2012, 10:24:51 am
Hi. I'm having some trouble hooking up my load cell to my Arduino.

I'm using two wheatstone half-bridge load cells. One is used as a "dummy/reference" for the other.

I'm amplifying the voltage difference using a LM358 set up as a Differential Amplifier.

I wasn't certain about the values on Rf and Rg so I followed

Here is some Data:
The load cells has a total resistance of 1965 and 1966 Ohms
I'm exciting the cells with 5V
With 150Ohm resistors on Rf and Rg the baseline Voltage on Vout = 870mV
R1 and R2 are both 1KOhm

Applying a load does not change the Value in either case.

What am I doing wrong is the Resistor values completely off or what?
13  Using Arduino / Sensors / Re: OPamp on: September 07, 2012, 11:23:56 am
I don't really need any higher resolution than 1 bit, I intend to use the load cell for weight increase detection not measuring, I'd like for my Arduino to detect if the base value of the load cell increases by say 10Kg not 5Kg not 1Kg, In fact I'd be very happy if it didn't even notice a 5Kg increase.

So a high resolution or great accuracy is not something I'm in need of (Low noise is though).

The less expensive the better (not that I'm cheap I'm just thinking large scale).

I bet the ADC you talk about fat16lib, is great it might even make my life easier, And if you feel that It's what's best suited for my application than I'll try it.
14  Using Arduino / Sensors / Re: OPamp on: September 07, 2012, 09:13:45 am
can u show us a picture of ur assumptions about the internals of that "load cell"?
Something like this,

 ------ Readout

So my "assumption" is that it's a voltage divider circuit, You apply a voltage across the "resistors(R)" And read the voltage at the "readout",

My load cell isn't a full-bridge Wheatstone Bridge it's a half-bridge
15  Using Arduino / Sensors / Re: OPamp on: September 07, 2012, 07:37:09 am
I'm really starting to get that drowning sensation, you know water over your head  smiley-razz

The ISL28130CEZ isn't "available" where I'm at, I can of course get a hold of it but I'd like to get things moving.
The LM358AN has been suggested by some, It's seems to do the things I need, Sadly I have little experience with OPamps beyond the Cmoy, so I'm not really sure.

I intend to
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