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1  Using Arduino / Sensors / Re: Barometric pressure - looking for a UK information source on: November 07, 2013, 02:39:14 pm
The solution I have for absolute altitude,  is to use GPS.

I did briefly look at GPS, but was immediately put off by the seemingly poor results I was getting for vertical positioning. Sounds like I need to revisit that.

If I have one on my desk and I pick it up and hold it 1 foot above the desk,  the difference is usually statistically indistinguisable from noise.  But if I hold it 2 feet above the desk,  I can determine the difference.

That's great. I've now got a target to aim at. Time to crack on with some coding and breadboarding I think. Many thanks for the information and insights.

2  Using Arduino / Sensors / Re: Barometric pressure - looking for a UK information source on: November 06, 2013, 02:49:32 pm
You are misunderstanding the specification there.

There is the absolute accuracy in reading the pressure, and there is the resolution of changes in pressure which can be distinguished from simple noise. Two entirely different things.

You are absolutely correct. Whilst I appreciate the difference between those two parameters, when I first started looking at the datasheets for the devices I struggled to understand the terminology they employed and was further confused by the differing ways the two datasheet presented their specifications.

The 3115 is claiming an absolute pressure accuracy of +/- 4 hPa   which is actually not as good as what the BMP085 claims to have.

I was looking for exactly that figure in my copy of the MPL3115A2 datasheet, but was unable to find it. Which is why I extracted the loose phrase "less than 1 foot / 0.3 m" from the product description and deliberately enclosed it in quotes. (I've just had another look and I still can't find it. A specific reference pointer would be very much appreciated. Edit: Found it +/- .4 kPa.)

Do you have any recommendations for other barometric sensors I could use which offer better absolute accuracy?

3  Using Arduino / Sensors / Re: Barometric pressure - looking for a UK information source on: November 04, 2013, 03:06:45 am
^^^^ Thanks for those links. That will be very useful as I had already been considering capturing the raw data on a SD card and then processing it on a PC.

Do you have any experience with barometric/altitude sensors other than the BMP085?

4  Using Arduino / Sensors / Re: Barometric pressure - looking for a UK information source on: November 01, 2013, 10:46:31 am
One of the sites that search turns up retains the EGBJ QNH as historic data which should help. In any case, bearing in mind the exercise also involves climbing the Malverns, I'm going to be waiting for a spell of settled weather.

5  Using Arduino / Sensors / Re: Barometric pressure - looking for a UK information source on: November 01, 2013, 06:36:57 am
Unless you keep changing the nominal "sea level"  air pressure...

That is indeed my intention. The idea is to end up with a portable altimeter.

I don't think you will get better resolution reports than 1mb. Maybe you should use the QNH from Gloucestershire airport (EGBJ) though, because it is closer to you than Birmingham or Bristol. As already stated, your sensor is not that accurate anyway.

I was going to use the Staverton QNH, but for some reason it's not listed on the website I'm using.

I must admit I hadn't initially realised how poor the accuracy of the BMP085 is. (+/- 2.5 hPa according to Bosch.) However, I've also got a MPL3115A2, with a claimed resolution of "less than 1 foot / 0.3 m", which I can set up alongside it for comparison purposes. The latter, incidentally also seems to have better on chip computational support as well.

6  Using Arduino / Sensors / Re: Barometric pressure - looking for a UK information source on: October 31, 2013, 02:54:47 pm
Thanks for the links Rob. I already use the met office site, but the other one is new to me. However what I'm really looking for is a source which can provide pressure information to within a tenth of a millibar, or even better a hundredth.

Just to provide some context, a difference of 1 millibar at a standard atmospheric pressure of 1013.25 millibars equates to a difference in altitude of ~27.71 ft (or 8.45 m) at mean sea level. Which is considerably more than the claimed accuracy of the sensor.

7  Using Arduino / Sensors / Barometric pressure - looking for a UK information source on: October 31, 2013, 11:56:53 am
I connected a BMP085 pressure sensor to my Arduino Uno and with some help from this bildr tutorial it is happily outputting my local air pressure reading. What I would like to do now is to find out how accurate and consistent the reading is.

To do that I would like to check it at a precise height against a known sea level air pressure. The first requirement is relatively easily met as I live close to the Malvern Hills where there is an old Ordnance Survey trig point at the top of the beacon. Obtaining an accurate high resolution pressure reference is proving somewhat more problematic however.

At the moment I am simply extrapolating the published QNH from a couple of airports - Birmingham (BHX) and Bristol (BRS). Unfortunately this is only given to the nearest millibar, which immediately introduces a potential height uncertainty of ~+/- 15 ft amsl. What I am seeking therefore is a source of timely, accurate atmospheric pressure information (UK) with a better resolution than this.

Any suggestions or pointers gratefully received.

8  Using Arduino / General Electronics / Re: Serial uart port - dual connection on: October 30, 2013, 02:58:35 pm
Yes.

Ta.

You could switch to upload, but probably not for debugging really - the code you are
trying to debug will be looking for GPS data and sending graphics on the serial pins,
which you wouldn't be able to interleave with debug statements unless you can switch
over super-fast!

You're right of course. A little bit of fuzzy thinking and explanation. (On my part.) There are other sensors involved, eg barometric pressure and temperature, and what I am looking for is to retain the ability to selectively work on those routines whilst the GPS and VGA are still in situ.

I'm not unhappy to retain the software serial port, however what's nagging at the back of my mind is a memory of reading here, some time ago, that it can consume almost half of the available computational resources on a uno. I'm not sure how true that is - or even how accurate my recollection is - but I'm assuming if I saw it here then it must be correct  smiley-neutral.
9  Using Arduino / General Electronics / Serial uart port - dual connection on: October 29, 2013, 04:17:18 pm
I suspect this may be a common question. If so my apologies in advance, but I can't seem to find any solid information.

I am currently working on a project where an arduino uno takes input from a GPS unit on the receive pin of a serial port, and sends output to a VGA controller attached to the transmit pin. I intend to do the initial development using a soft serial port, but would eventually look to move them to the hardware assisted serial port.

After the change over has been made it would be nice to be able to occasionally debug/update via the normal usb connection. If I do this will I need to isolate the VGA controller and GPS whilst communicating over the usb link? If so, does this have to be a complete physical isolation, e.g. a two pole switch?

10  Using Arduino / General Electronics / Re: Loud speaker under 1 inch in diameter on: October 11, 2013, 05:05:09 pm
I've no need for a speaker at the moment, but the pencil case is a great idea. I'll be checking the local (UK) Staples next time I'm passing to see if they stock anything similar over here. Thanks for that tip.
11  Using Arduino / General Electronics / Re: Moving coil ammeters on: September 24, 2013, 04:37:48 pm
Good find.

The quality of the find is tempered slightly though by the asking price of a fiver apiece.

Thanks to all for the helpful advice which seems to be, don't trust the dial rating and creep up on them from afar.

Just to check my understanding; the meters may or may not require an additional external shunt. In either case they can be safely checked with a battery and resistor connected in series, chosen such that the current through the meter is less than 500mA 1mA 100uA not very much. (Anyone want to raise on a 5 / (100 / 1000000) = 50k resistor?)

How am I doing so far?

Just for my further edification, should an external shunt be needed, am I correct in thinking it would go in parallel across the meter terminals?

12  Using Arduino / General Electronics / Moving coil ammeters on: September 24, 2013, 12:10:19 pm
Whilst passing one of the junk antique shops in a local village this afternoon, I noticed they had a box containing a variety of old analogue panel meters. Amongst them were some small 200/250/500 mA fsd meters, which looked suitable as both cosmetic and functional enhancements to a couple of my arduino projects.

On enquiry I was informed that they had all come from a house clearance and the shop owner believed them to be working. (Though the foundation for that belief wasn't elaborated on.) She did say however that I was welcome to return and test them in situ prior to any purchase.

It now occurs to me - not having had any prior experience of moving coil meters - that I'm a little uncertain on the most straightforward way to check them out. I'm presuming that I can just take along a small 4 x AA battery pack, a couple of low value resistors - say 75R and 100R 0.6W which I happen to have handy - and then connect them up in series with the meter. Or am I being too simplistic?

I'm not looking for precision accuracy. I just want to ascertain they will move around a bit and point vaguely in the right direction. Comments and suggestions welcomed.




13  Using Arduino / General Electronics / Re: Connecting as an open connection on: September 16, 2013, 02:43:10 pm
Thanks for the replies. I understand the potential issue in regard to the logic level, but I'm still a little unclear how to dynamically effect a 'n/c' state.

I guess "n/c" is achieved by setting  pinMode(INPUT);  (without Pullup resistor )

I hadn't thought of trying that. I did however consider using a tri state buffer, like a 74HC125, and have one arduino output pin to control the impedence and another to select high or low. Would that work OK?

          74HC125
            |\
arduino  ___| \___  UM3561 SEL1
   D3       | /
            |/|
              |
              |
           arduino
              D2


14  Using Arduino / General Electronics / Connecting as an open connection on: September 14, 2013, 03:21:10 am
Earlier on in the week I picked up a UM3561 sound generator chip and I've since been happily making like an emergency services driver - albeit an American one - by hard wiring the select lines on a breadboard as per the following table:

           S
  n  o  V  e
  /  /  s  l       Sel1   Sel2   Sound effect
  c  p  s  1       ----   ----   ------------
--+--+--+--+--      n/c    n/c   Police siren
|            |      Vdd    n/c   Fire engine siren
|   UM3561   C      Vss    n/c   Ambulance siren
|            |       -     Vdd   Machine gun sound
--+--+--+--+--
  V  S  O  O       (- Don't care.)
  d  e  s  s
  d  l  c  c
     2  1  2


I'd now like to interface the device to my arduino, but I'm a little uncertain as to how to connect the select lines up and in particular how to effect a 'switched n/c'. I'd be very grateful if anyone can provide me with some guidance as to the preferred/easiest way to do it.

15  Using Arduino / Microcontrollers / Re: Prototyping with Arduino Uno but Production with Atmel 8051 on: September 13, 2013, 02:54:12 pm
...2nd worst instruction set in the world...

OK, I'll ask. Your nomination for the worst?
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