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151  Using Arduino / General Electronics / 109.3035 ohm resistor on: March 02, 2013, 03:29:03 pm
I was shopping for some precision resistors on Mouser's website and got distracted by the really high precision resistors. I found that there is a standard value in a Vishay part of 109.3035 ohms, 0.005% tolerance. That value seems to ring a bell some how but I can't dredge up any reasons for it. Does anyone know what the application of that value would be? It would be nice to have one but they are about $100 so I'm going to pass.  smiley More common values in that precision are around $15-$20 in case anyone needs one.
152  Using Arduino / General Electronics / Re: Differential amplifier noise on: March 02, 2013, 02:35:01 pm
Your breadboard has lots of antennas, which is not helping with your noise problem. Twist the two wires going into in+ and in- together and see if that affects your noise level. If those wires are closely coupled, the effect is that the coupled noise is equal on both wires and will cancel at the +/- inputs of the differential amplifier. If that helps then maybe you can figure out a way to mount your amplifier on your load cell and shorten up/twist the wiring to good effect.

Not sure what to suggest about the one separate gauge. I can't tell how it is wired or whether or not the cable is shielded.
153  Using Arduino / Sensors / Re: Analog pressure gauge / potentiometer coupling? on: March 02, 2013, 01:53:46 pm
What you need is a pressure potentiometer. Just kidding.

There is a company, Wika, that makes a gauge called the Intelligauge that is a mechanical gauge with an auxiliary 4-20mA output. The problem is, I don't know what the cost in a 10,000 psig model. I noticed there was a 160 psig model for sale on ebay for $100. Maybe it would be in your curiosity budget to buy one and disassemble it to find out how it works. Maybe you could contact the seller and see if a higher range is available for the same price.
154  Community / Bar Sport / Re: Youtube bar jokes series: on: March 02, 2013, 12:01:37 pm
You have to be a U.S. ham radio operator is get it, but one of my favorite bar jokes goes like:

A Ham operator walks into a bar with a parrot on his shoulder. The bartender asks "where did you get that" The parrot answers "Dayton".

Lefty (WA6TKD)

I get it. You really can get anything at HF.
155  Using Arduino / Sensors / Re: Problem with amplifying a pressure sensor for measuring blood pressure on: March 02, 2013, 11:17:12 am
Quote
Is that more clear?

I always got it. Calibration/tweaking can be done in software or externally via hardware adjustments. Depends on the application and users requirements. If it's for one's own project then software is cheaper as one has the freedom of recompiling the sketch at any time when external things are changed. If however the project is designed to support changes in sensors or other external changes by users of the project then external calibration might be more desired. At the refinery I worked at we used thousands of sensors wired into central control systems, and we always required that any sensors used had the means to calibrate them externally so that we did not have to have 'custom' loop calibrations for every sensor wired to the central systems.

Different strokes for different folks, there is no single "best method", just what works best for you.

Lefty

When I first read this thread, I wondered if I had the means to calibrate a 0 - 37 kPa transducer. It turns out my smaller Chandler deadweight tester will actually go down that low. I wouldn't be able to match the exact full scale, but 1 psi increments would probably give a good enough calibration.
156  Using Arduino / Sensors / Re: Problem with amplifying a pressure sensor for measuring blood pressure on: March 02, 2013, 10:57:46 am
Don't trimpots usually have much higher tolerances and produce noisier signals?

Trim pots of the 10 and 20 turn variety are often used in precision high quality instrumentation circuits for calibration purposes. One trick is to not let the pot control a larger range then it needs to be, so if you need a +/- 5 percent range of adjustments you might use say a 100 ohm 20 turn trim pot that has it's end terminals wired to 1% 10K fixed resistors. That way any drift in the pot due to tempco would have a much smaller effect in the overall circuit compared to just using a 20K ohm trim pot.

Lefty

Quite true. Also as microcomputers and SOC's get cheaper and cheaper, nowadays it is becoming quite common to use a fixed resistor and do the calibration in software during final test of the instrument. When it's time for a periodic instrument calibration, if anything changes, the calibration values are simply updated in EEPROM or whatever.
157  Using Arduino / Sensors / Re: Problem with amplifying a pressure sensor for measuring blood pressure on: March 02, 2013, 10:48:01 am
I
Quote
t seems to me you just need to select a precision resistor that gives you the gain you need for the maximum signal level that doesn't exceed your ADC input range.


So you can get non-standard value resistors?



Quote
It seems to me you just need to select a precision resistor that gives you the gain you need for the maximum signal level that doesn't exceed your ADC input range. Use the instrumentation amp's gain equation to get the actual gain that results from the value of the gain resistor you actually use.

Let me state it more simply. Decide what maximum gain you need, based on not exceeding your ADC's input spec. Solve the data sheet equation for the resistor value that will give you that gain. Select the standard value of the precision (1%, 0.1%, 0.05%, etc) resistor that is closest to the calculated value without causing the amplifier to exceed the maximum gain. Then, if you need the actual amplifier gain in your calculations, solve the data sheet equation for gain based on the resistor value you actually use. Is that more clear?



158  Using Arduino / Sensors / Re: Frequency input issue for tachometer on: March 01, 2013, 07:43:14 pm
My final goal is a sequential shiftlight not a tachometer, and it's working good smiley-grin

Ah, okay, I didn't realize that from your initial posts. I see now that a faster update makes sense in that context. You'll be able to update pretty quickly, especially if you use integer math.
159  Using Arduino / General Electronics / Re: resonator vs crystal on: March 01, 2013, 07:37:19 pm
Quote
This all too common sloppy practice of calling a crystal resonator just a crystal and a ceramic resonator just a resonator has lead many beginners off to a rather poor understanding of such components and how they actually work.

Nothing personal, just a pet peeve.  

Lefty

How long has that sloppy practice been going on?

Not sure but, too long in my opinion.  smiley-wink
But I will go along with your pet peeves if you go along with mine.

Lefty

I'm cool with that.  smiley I just wondered because I never heard crystals called anything else until recently. I thought "crystal resonator" was coined to distinguish crystals from "ceramic resonators" which are more recent than crystals as a frequency generating/controlling device.

Well it would different if people short cut it to crystal Vs ceramic but instead call it crystal Vs resonator when both are resonators just rubs me the wrong way. It's not unlike when a new comer might (and often do) asks if he should "use a transistor or use a mosfet to control a relay" and one then has to explain what the T in mosfet stands for.  smiley-wink

Lefty

Ah, I get it. I agree.
160  Using Arduino / General Electronics / Re: resonator vs crystal on: March 01, 2013, 07:22:16 pm
Quote
This all too common sloppy practice of calling a crystal resonator just a crystal and a ceramic resonator just a resonator has lead many beginners off to a rather poor understanding of such components and how they actually work.

Nothing personal, just a pet peeve.  

Lefty

How long has that sloppy practice been going on?

Not sure but, too long in my opinion.  smiley-wink
But I will go along with your pet peeves if you go along with mine.

Lefty

I'm cool with that.  smiley I just wondered because I never heard crystals called anything else until recently. I thought "crystal resonator" was coined to distinguish crystals from "ceramic resonators" which are more recent than crystals as a frequency generating/controlling device.
161  Using Arduino / General Electronics / Re: resonator vs crystal on: March 01, 2013, 07:11:00 pm
Quote
This all too common sloppy practice of calling a crystal resonator just a crystal and a ceramic resonator just a resonator has lead many beginners off to a rather poor understanding of such components and how they actually work.

Nothing personal, just a pet peeve. 

Lefty

How long has that sloppy practice been going on?
162  Using Arduino / Sensors / Re: Problem with amplifying a pressure sensor for measuring blood pressure on: March 01, 2013, 06:11:14 pm
Is it really necessary for you to be able to tweek the gain of your instrumentation amplifier? It seems to me you just need to select a precision resistor that gives you the gain you need for the maximum signal level that doesn't exceed your ADC input range. Use the instrumentation amp's gain equation to get the actual gain that results from the value of the gain resistor you actually use.

I imagine you are balancing the transducer at zero pressure in software. I don't think you've talked about how you intend to calibrate it. Maybe the data sheet span voltage is good enough for your application, or maybe the transducer comes with a calibration sheet.
163  Using Arduino / General Electronics / Re: How to protect analog output (current protection) ? on: February 28, 2013, 05:59:06 pm

Hi all,

Thanks to my Arduinos I can generate some real analog values. It can be made through the onboard DAC of the DUE, or with an external DAC like MCP4912.

I am looking for a component to put after the DAC, that does the following job :
- under a certain amount of current (for ex 10mA), the output voltage is exactly the DAC output
- when there is an overload, more than 10mA, the output voltage can deviate from the DAC output to limit current to 10mA
- as soon as load is back to normal, output voltage is back equal to the DAC output
To sum up, from the DAC point of view, there is no overload. this one always outputs its voltage in safe circumstances.

So, what is the best and easy way to do this ? are there integrated circuit for this ? Should I use an operationnal amplifier ?

THanks for your answers.





Google "foldback current limiting". That sounds like what you are looking for.
164  Using Arduino / Sensors / Re: Frequency input issue for tachometer on: February 26, 2013, 05:22:25 pm
Everything is good now !

Just with the RC network before the darlington, no more bad ready, the frequency measure is steady smiley-grin

I need to improve the speed now, my tachometer has a refresh rate of 1sample/second, it's really too slow  smiley-sweat

Thank you for your help smiley
I'm glad it works.

One of the difficult decisions with a digital tachometer is what to use for an update speed. I prefer an analog tachometer for that reason. You could  consider using a digital bar graph or if possible a sort of "pie chart" to simulate the good things about an analog readout. Keep the digital for "cruising".
165  Topics / Device Hacking / Re: how can i run arduino IDE on android? on: February 25, 2013, 04:01:03 pm
The Arduino IDE runs on my Raspberry Pi, albeit slowly. I have a full gcc AVR toolchain installed as well as a copy of avrdude. So, we know it's possible to install the Arduino IDE on ARM processors. You might have more luck with a starting point if you explore how it was ported to the RPi.
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