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46  Using Arduino / General Electronics / 3.3v and 5v logic level converter question on: October 05, 2012, 02:24:32 pm
Due to the ambiguity of the search terms I have been unable to find an exact answer to this scenario.

I am using various devices, some which require 5v and others 3.3v. Some use i2c, but they're not sharing the same i2c pin. Here's a concrete scenario:

device 1: 3.3v, using i2c, pins A5 for clk, A4 for data.
device 2: 3.3v, uses pin D7 for clk D8 for data.
device 3: 5v, uses pin D2 for data tx/rx.
device 4: 5v lcd, uses software serial pin 5.

I have not hooked up device 4 yet. I have a logic level converter hooked up for device 3 and it works great. I don't want to fry anything by attempting to simply hook up device 3 directly to pin 2. Is this only a problem when the pin being used is shared between differing voltage devices, or do you need to use a logic level converter for everything when you have devices using differing voltage. My intuition says you only need it when the pin is shared, but I don't want to be wrong and let out the magic smoke. smiley-neutral
47  Using Arduino / General Electronics / Re: 6v to DC input jack seems to work fine? on: October 05, 2012, 02:14:43 pm
Thanks for the info. Yea, even when under load the voltages from the 7.5 and 9v wallwarts are around 11-13v. Not good. The current ratings on them vary from 450ma to 1A. I would have thought the 450ma one would have dropped to the spec voltage under load, but it didn't. The 6v one is the only one that appears to work, but I guess I'll find out for sure if there are any problems once I test it under a higher load (lcd screen, ethernet shield).
48  Using Arduino / General Electronics / 6v to DC input jack seems to work fine? on: September 28, 2012, 12:54:17 pm
I've done a lot of reading up on DC input jack voltage, and how ideally this should be 7.4v or so minimum. I have a bunch of extra unused wall warts around, 6v, 7.5v, and 9v. Took my crappy analog multimeter out to test some of their voltages with the arduino powered up. I first compared it to known reference voltages from 9v and 1.5v batteries to get an idea of the measurement error of the meter. With that in mind I found to my dismay all my 9v adapters were putting out 13v or so, and even the 7.4v was putting out 11v. As stated, these readings were with the arduino powered up. Noticed the vreg got a bit warm with all these adapters. With these results I figured the 6v one would be my best bet, but of course murphy dictated this one actually put out precisely the advertised 6v even under no load. At any rate, the arduino seems to work fine with the 6v adapter, and the vreg is running nice and cool. I suspect I will probably run into problems with this, though? I measured the 5v rail on the arduino and it's within spec no matter which adapter I use. Unfortunately I'd probably need a real multimeter to get the kind of accuracy needed to figure that out, however.
49  Using Arduino / General Electronics / Re: Accurate resistence measurement without reference resistor? on: September 20, 2012, 03:06:04 pm
Docedison, I looked at the attached datasheet for the AD592, and it says it has an accuracy of +/- 0.5C and not 0.3c? So far it seems like the SHT75 is my best bet since it also measures humidity (whose accuracy is dependent on the temperature).

From an electronics standpoint, I was curious if I'm better off running cat5 to the sensors outside (which of course would be subject to the complications and limitations of the I2C standard) or if having a second arduino which would transmit serially over the cat5 is a better solution. My concern with a second uno outside is that the temperature changes to the uno board could affect the accuracy too. Also, two unos seems like overkill when it could all easily be done on one, but obviously I need that one inside so it can display information on an LCD screen. I would need to run a cable out regardless since the sensor measurement assembly needs to be fan aspirated to get accurate temperature readings.

Sorry for the wandering nature of this thread, if it's preferred perhaps a topic in the projects forum would be better suited to this discussion.
50  Using Arduino / General Electronics / Re: Accurate resistence measurement without reference resistor? on: September 19, 2012, 10:36:14 am
Doc, retro, and others, thanks for the helpful information. The 87 I was planning on simply getting a used one on ebay when the time was right. You can usually get them for quite a discount and there are always ones up for auction. As for the reason, I'd ideally like my temperature sensor to be accurate to the tenth of a degree Fahrenheit. This is for a weather station project. From all my research on the matter, it seems the best I'll be able to do is ~0.2C (0.36F) accuracy , and that's with an integrated sensor package such as the SHT75. I did want to add a couple thermistors to complement the 75 (for backup and accuracy reasons), but it appears in order for that approach to be feasible I'd have to purchase a .05% tolerance 10k resistor for the reference, and either use a 12-16bit ADC alongside a 0.1% thermistor, or else use some type of multisampling solution to improve the arduino's native ADC resolution. Not sure if it's worth all that hassle, especially since they'd also each have to be calibrated too, and I'd have to use ones with known steinhart-hart coefficents or else an extensive thermistor table. The one I am testing with right now has no part number or anything so it's truly a mystery.
51  Using Arduino / General Electronics / Re: Accurate resistence measurement without reference resistor? on: September 18, 2012, 08:23:39 pm
I think the capacitor current idea would probably only shift the problem to "how accurate are the reference capacitor values" instead of how accurate is the reference resistance value. I have suspected the arduino's ADC wouldn't be up to the task of taking full advantage of a 0.1c accurate or better thermistor anyway. For whatever it's worth I was using the 3.3v and analog reference vs the 5V. Seems the only real solution is a fluke 87 or better to pin down the reference resistance value, and then a quality 12 or 16bit ADC to poll the thermistor. A third option, of course, is an integrated chip thermometer solution, I've had my eye on the SHT75 for that reason. No need to worry about calibrating those either since they're supposedly already calibrated. It just would have been nice to have some accurate thermistors employed in addition to that to get a more accurate average temperature reading.
52  Using Arduino / General Electronics / Re: Accurate resistence measurement without reference resistor? on: September 18, 2012, 04:49:55 pm
Yea, I assume it's 1%. I just wanted an accurate reference value. I figured this wasn't possible due to the errors of the components involved (ADC, resistors, cheap multimeter). I do have another thermometer to test against but not sure it does much good if I can't get an accurate reading for the 10k 1% reference resistor.
53  Using Arduino / General Electronics / Accurate resistence measurement without reference resistor? on: September 18, 2012, 02:50:47 pm
Is it possible to get an accurate (say 0.05%) measurement of a resistor using the arduino, but without having to rely on a reference resistor? The best I have to work with right now is a 10k 1% resistor. I'm trying to use a 10k thermistor and would like to get an exact value for the 10k reference resistor. Unfortunately, I don't have a multimeter and I think that only something like a fluke 87 could give me an accurate reading anyway (from what I've heard the cheaper ones don't have as accurate an ohmmeter). I'd like to get an 87 but that's currently not in my budget at the moment.

I suspect this isn't possible, but figured I'd ask anyway.
54  Using Arduino / Sensors / Re: Finding sensors best suited to weather station project on: September 11, 2012, 02:55:19 am
Good points retrolefty. I am trying to "do it right" from the get go with this project, but I guess there comes a point where it becomes hard to impossible to get the kind of accuracy I want. Still, I want to do whatever I can to ensure the accuracy is as good as possible with this type of setup. In case anyone else is interested in this type of project I'll try and keep updating here as I progress.

I have ordered the uno r3, should be here by the end of the week. That should at least give me something to start testing and messing around with. In the meantime, I've been doing some more research, and have not been able to find a decent (at least 0.1c) integrated humidity/temp sensor package yet. Therefore, I've been seeing if a separate thermistor/humidity sensor approach would work, and it seems that humidity sensors need to take the current temperature into account when performing the RH% calculation. If using separate sensors, this could lead to some calibration nightmares that might not be there with integrated packages. In addition, I have not found a very easy, straightforward way to read capacitive humidity sensors with the arduino, still searching around but so far it seems pretty complicated and something which would require its own circuit. Basically the capacitive sensors seem to require an AC waveform of usually 1khz or higher in the reading process. I'm still quite green to this stuff so it's proving a bit much so far. I did however find this somewhat helpful thread:

http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1264143673

Another issue I've come across is calibration for the thermistor. The 0c ice bath part is probably do-able, but good calibration will also need data gathered at exactly 25c and a third point (like 100c) as well. I don't see any easy way to calibrate those other points without expensive calibration equipment which I don't have. 100c is outside the range of most thermistors as well, so sticking one in boiling water will ruin it in that case. Still trying to figure out more info on this part.
55  Using Arduino / Sensors / Re: Finding sensors best suited to weather station project on: September 08, 2012, 08:00:27 pm
Ok, searched digikey and couldn't find any sensor modules with better than 0.1c accuracy. I did find a nice +/- .05c thermistor which would come out to 0.09f accuracy provided it was read by an ADC with enough resolution. I still wonder, however, if component noise and the like might preclude that level of accuracy even if I did have a 16bit ADC? I figure there's a reason there aren't any sensor boards with better than 0.1c even on digikey  smiley-small

I tried searching on mouser and to my dismay found I wasn't able to filter by accuracy at all. Plus, I've heard mouser tends to charge a lot for shipping so it's best to order from them when you have a lot of components to get. newark also didn't bring up any promising results.

I may end up going with the .05c thermistor and a higher resolution adc, provided it's worth the extra effort/cost and is feasible.
56  Using Arduino / Sensors / Re: Finding sensors best suited to weather station project on: September 08, 2012, 05:06:51 pm
Thanks Chagrin. I will try searching digikey/etc and see if I come up with anything there for integrated temp sensors and such. All the integrated temp sensors that hook up to the arduino's digital pins I've found so far have only had at best 0.2c accuracy, and I'm looking for something that has at worst 0.1f accuracy.

In case this doesn't work out, does anyone know offhand of a newbie friendly 12 or 16bit ADC shield/addon that works well with the arduino? My searches so far have yielded solutions which aren't too newbie friendly, though maybe I could still figure it out in the worst case.
57  Using Arduino / Sensors / Re: Finding sensors best suited to weather station project on: September 07, 2012, 09:31:45 pm
Hate to keep replying to myself here, but I've found that probably the best approach to get the accuracy I want (+/- 0.1F or better) is to get a 12 or 16bit ADC and hook up the thermistor to that, and then the ADC to the arduino. Unfortunately this process seems a bit much for someone who has no microcontroller experience, unless I'm missing an easy plug n play ADC for arduino shield out there.
58  Using Arduino / Sensors / Re: Finding sensors best suited to weather station project on: September 07, 2012, 12:12:01 am
I figure someone might ask why 0.2C isn't good enough. In a weather situation, especially with things like freezing rain, you want something accurate to at least a tenth of a degree fahrenheit. 0.1c == .18 fahrenheit, so even that is outside the desired range. Ideally, I'd like something that could take advantage of a 0.1% 10k thermistor, but haven't found such a solution yet for the arduino. It may be I'd have to go with a different microcontroller entirely, which would be a shame since the arduino seems like such a newbie friendly way to get into microcontrollers.

Thanks for the info copiertalk. To be honest I'm wary of going with OS stuff, given my experiences with them along with what I've heard from friends who've had even their higher end weather stations. Their stuff tends to have QC issues, are unreliable, and prone to failure. I had my eye on the sparkfun anemometer package but I just want to start out with the basics (temp sensor, pressure sensor, fan aspiration, relative humidity sensor, and possibly a wireless solution for outside siting) to begin with, and then possibly add things like anemometer and rain gauge down the road sometime.
59  Using Arduino / Sensors / Re: Finding sensors best suited to weather station project on: September 06, 2012, 08:24:13 pm
OK, I found the datasheet on the dht22, I wasn't thinking and hadn't tried searching for it. It appears it's +/- 0.2C. I've also been unable to find humidity sensors with better than +/- 2% RH accuracy (perhaps the tech just doesn't exist). After doing some more research, it appears a 0.1% accurate thermistor could approach 0.1C accuracy on the arduino, the big limitation being the arduino's ADC. I haven't been able to find dedicated arduino thermistor reading boards.
60  Using Arduino / Sensors / Finding sensors best suited to weather station project on: September 06, 2012, 01:48:00 pm
Ok, so I am very new to arduino (don't even have one yet) but have extensive programming and C experience (was a CS major). I've always wanted to have a weather station but have never liked the limitations and non-customizable nature of the commercial offerings. Once I heard about arduino I did a ton of reading up on it as it was an interesting subject for me, and it seems the perfect project for me to take on if I take the plunge into the arduino world.

Obviously I want to take the best approach when putting something like this together, and that has led me to researching what would be the best sensor solution for this type of project. There are a ton of sensors out there, but the two major categories appear to be the thermistor approach, and the digital sensor solution. I want as accurate sensors as possible while keeping the price reasonable. According to this site (scroll to the very bottom), the best accuracy you can hope for from an arduino using the analog approach would be half a degree centigrade. I'd like something with at worst +/- 0.1C accuracy, along with a fast response time. Would an infrared temperature sensor be a feasible approach for this type of application? I also found this integrated humidity and temp sensor, but could not find any datasheet on its accuracy for temperature and humidity. My concern with humidity sensors also is their tendency to go bad after a while. I remember the oregon scientific weather station humidity sensors were notorious for this problem. Therefore, if possible I'd like to get a humidity sensor that has a decent lifespan instead of only a year like the OS ones, and is accurate enough for a good dewpoint calculation. Anyone have ideas on decent sensors that would be well suited to the arduino?

I'm not going to pursue other sensors besides temp, humidity, and barometric pressure one for now. The rain gauge and wind sensors I can always add at a later time.
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