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76  Using Arduino / Sensors / Re: change two identic I2C address from SHT21 on: April 12, 2013, 10:50:45 am
The electrical characteristics of SHT2x are defined in Table 1....
Exposure to absolute maximum rating conditions for extended periods may affect the sensor reliability (e.g. hot carrier degradation, oxide breakdown).

Digital I/O Pins (SDA, SCL) to VSS: min-0.3  max VDD + 0.3

So if you have power connected to an [Arduino] IO pin sitting at 0.3V, and SCL/SDA pulled up to 5V (which is also the absolute Max) then the input pins are 4.7V over spec.
Similarly, pulling SCL/SDA up to 3.3V puts them 3V over spec.
Okay, I get it. Vdd becomes 0.3V when you power the part down.

Here is the response I got from Sensirion in June 2009 when asking them the same question about the SHT75. Since I was using a software solution to control the non-standard two wire interface anyway, this is actually the way I dealt with it. I was successful controlling the power to the parts as well but ended up just providing separate data lines. I agree with your assessment that it would be exceeding the max ratings to switch the power pin.

Quote
Thank you for contacting us.
 
Yes, this approach works.
Also the other way (sharing a CLK line and reading at separate DATA lines) has been done successfully. To my knowledge, this is easier to implement.
 
Thank you
Michael

When you used the term "crap design" I had a knee-jerk reaction. My apologies.
77  Using Arduino / Sensors / Re: change two identic I2C address from SHT21 on: April 12, 2013, 09:08:33 am
We have two different solutions being suggested - I am suggesting multiplexing the control signals and nothing else.
You are suggesting multiplexing by powering devices off.
I am suggesting having active control signals with no power applied overstresses the part.
You do not seem bothered by that.

I am not trying to prove anything. I only suggest that good electrical design practices be followed.
Okay, I see what you are saying now but I don't get how the max values are exceeded from your example. Explain it again using 3V3 for Vdd.
78  Using Arduino / Sensors / Re: change two identic I2C address from SHT21 on: April 11, 2013, 11:16:34 pm
How do you figure adding more hardware to control the power (and stressing the parts in the meantime) is faster than writing 2 bits to select 1 of 4 channels in a multiplexer chip designed for I2C multiplexing?

We have no idea if it's necessary to add more hardware to control the power. That's why I suggested that OP post their schematic. If OP is running a 3V3 Arduino already, then all they have to do is use a spare port to power the SHT21. If OP is running a 5V Arduino, the most they will need to do is switch the 3V3 to their part with a high side MOSFET.

You are the one that suggested OP "add more hardware" in the form of a multiplexer board. If that is the solution because OP is not capable of anything else, then so be it.

I'm only suggesting that your hardware multiplexer solution is certainly not the only solution and maybe OP can solve their problem without having to spend more money.

I have explained that when I made my first suggestion that I was thinking about my own solution to the same problem which involved a 3V3 processor. It was a mistake to not consider that OP might have a 5V processor. You have made many mistakes when offering help on this forum. Give me break! What are you trying to prove?
79  Using Arduino / Sensors / Re: change two identic I2C address from SHT21 on: April 11, 2013, 03:18:42 pm
thanks for giving my thoughts an other direction.

Well a multiplexer is hardware solution but what is with the speed? Will it be fast enought?

cutting off the sensor from the power is to hard but with some transistors (mos-fet) i could time the dataline. I guess that would be faster than the multiplexer.


Quote
component(SJ2 = SJ3). Do you know what it is and for what?
 http://www.dsscircuits.com/images/schematic/PCA9544A%20Breakout%20PCB.pdf
Does anyone have some idea?



I think you need to post the schematic diagram for your project. It's hard to give advice without that.
80  Using Arduino / Sensors / Re: change two identic I2C address from SHT21 on: April 11, 2013, 12:08:40 pm
That's a crap way to design things, in clear violation of datasheet recommendations:

The electrical characteristics of SHT2x are defined in Table 1....
Exposure to absolute maximum rating conditions for extended periods may affect the sensor reliability (e.g. hot carrier degradation, oxide breakdown).

Digital I/O Pins (SDA, SCL) to VSS: min-0.3  max VDD + 0.3

So if you have power connected to an IO pin sitting at 0.3V, and SCL/SDA pulled up to 5V (which is also the absolute Max) then the input pins are 4.7V over spec.

You're absolutely right. I was thinking about my own application which uses a 3v3 Arduino. If the OP is using a 5V Arduino then they have already had to deal with running the chip off a different voltage and making sure the SCL/SDA lines are correctly interfaced so they can use that knowledge to do it correctly the way I suggested.

My suggestion was to point out that using an I2C multiplexer was not the only solution to the problem. Chill.
81  Using Arduino / Sensors / Re: change two identic I2C address from SHT21 on: April 11, 2013, 10:42:07 am
Hallo everyone,

i am  very new in this field. I just got the Leonardo Board and also a few Sensirion Sensors (SHT21).

What I wanna do:
I wanna connect a few sensors (up to 3 or 4) Sensirion to the Leonardo board. The SHT21 Sensors works via I2C. :-)

In the datasheet of the Sensor (http://www.sensirion.com/fileadmin/user_upload/customers/sensirion/Dokumente/Humidity/Sensirion_Humidity_SHT21_Datasheet_V3.pdf) is written that all Sensors are set with the same I2C address. :-(

I am using some already written files (https://github.com/misenso/SHT2x-Arduino-Library) from Christopher Ladden and modified by Paul Badger. Thanks a lot

With one SHT21 Sensor it is working fine. Now I wanna add a second Sensor.

Problem: I have to change the I2C address of the Sensor. How does it work?
than updating the code ....

I would be very pleased if anyone could give me a hand.

Greetings
Coolf


I had to look up the SHT21 as my experience is only with the SHT75. It looks like the SHT21 is very similar to the SHT75 with the biggest change being in the two wire communication. At any rate, I used two SHT75's sharing SCL and SDA by powering the chip itself from spare Arduino pins. It only consumes a few hundred microAmps and you can just turn them on and off by toggling the pins when you want to read the values. Remember to give the chip time to come up to spec, around 15ms it looks like, before you try to read it.
82  Using Arduino / Sensors / Re: change two identic I2C address from SHT21 on: April 11, 2013, 10:15:24 am
Hoi CrossRoads,

thank you for the quick responds.

Oh well an other component to order.  Is there any way to use the I2C Sensor at an analog or/and digital channel?


If you just need another I2C port and don't want to buy any more hardware, you could consider using a software I2C solution. Google "software i2c arduino".
83  Using Arduino / General Electronics / Re: Wheatstone bridge? on: April 09, 2013, 05:48:34 pm
One of the nice things about using a simple resistive divider network with a thermistor is that the accuracy of the excitation voltage doesn't matter because it drops out of the equation and you are solving simply for the resistance of the thermistor as a function of the resistor you place in series with it. Therefore, the accuracy is just dependent on the precision of the series resistor, the thermistor, and the ADC. A little easier to characterize over ambient temperature.

RT = RS / (1024/ADC - 1) where RT is the resistance of the thermistor and RS is the resistance of the series resistor.

If you are using the ADC from the Arduino, it is probably the least accurate link in the chain, so you can ignore the rest.
84  Using Arduino / General Electronics / Re: Thermocouple calibration on: April 09, 2013, 09:22:55 am
The data sheet says.

Note that the MAX31855 assumes a linear relationship between temperature and voltage. Because all thermocouples exhibit some level of nonlinearity, apply appropriate correction to the device’s output data.

No kidding, for the cost of the chip why did they not do this ?
There are published tables for all the thermocouple types.
I can't use them with this chip as its already digitised the output.

I can probably knock up the same linear circuit cheaper with amps but with much more hassle.

I've been having look at other bits, borrowed aquarium thermometers , Weather monitors,etc.

Amazing, 0 and 100 they all agree but I suppose that is not too surprising.
Elsewhere though , wow some big disagreements.


Now that I read the data sheet, I see it's not very accurate. +- 2degrees C isn't that great.

I don't know what your budget is but check the price of lab grade thermometers. You can use hot cooking oil to make a bath to check the calibration of your sensor and create a lookup table to linearize it.
85  Using Arduino / General Electronics / Re: Thermocouple calibration on: April 08, 2013, 07:06:25 pm

Quote
Quote from: Boardburner2
140 reference , do not have one, only things that boil at that temp appear to be toxic and unobtanium.
Quote
Then how do you know it's inaccurate at this temperature?

He doesn't know, but rather he knows that his various meters, arduino, IR don't agree at that around that temperature value.

A typical quandary, man with one watch always knows the time, man with two (or more) watches never quite sure. That is way primary calibration standards are so important and usually expensive if guaranteed accuracy is required or desired.

Lefty


Yep, it's a dilemma.
86  Using Arduino / General Electronics / Re: Thermocouple calibration on: April 08, 2013, 06:13:43 pm
You might try loading your water with some common table salt. There are many references about the water solute boiling points with common ingredients available as always on Wiki.. A really great resource.

Bob

Unfortunately, that will only raise the boiling point about 3 degrees Celsius before the solution is saturated and will dissolve no more salt.
87  Using Arduino / General Electronics / Re: Thermocouple calibration on: April 08, 2013, 05:26:13 pm
Quote from: Boardburner2
High temp uses custom amp , ADC type circuit with a lookup table, it's this I need to calibrate.

So are you okay with calibrating this now? You haven't posted a schematic diagram of this circuit.
88  Using Arduino / Sensors / Re: Solar Tracker - Trouble Selecting Light Sensor on: April 08, 2013, 04:48:15 pm
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I bought few LDRs to test things out,but the problem is, even when its exposed to low intensity evening light, the resistance hits zero.

Even in bright light an LDR's resistance is not zero. Show us your circuit and a part number/data sheet for the device.

Even I thought the same, but my multimeter proved me wrong. Even the evening Sun (4 PM) made the resistance 0.

Am I missing something?
Probably. What scale is your multimeter set to? What does the data sheet say the dark resistance is supposed to be? Is it shorted out?
89  Using Arduino / General Electronics / Re: Thermocouple calibration on: April 08, 2013, 04:43:22 pm
Quote from: Boardburner2
Shield is based on max 31855 uses standard ,un extended themocouple with plug into proper socket on the board.
If that's the case, I'd use it as my calibration standard.

Quote from: Boardburner2
High temp uses custom amp , ADC type circuit with a lookup table, it's this I need to calibrate.
Good luck.

Quote from: Boardburner2
140 reference , do not have one, only things that boil at that temp appear to be toxic and unobtanium.
Then how do you know it's inaccurate at this temperature?
90  Using Arduino / General Electronics / Re: Thermocouple calibration on: April 08, 2013, 03:09:11 pm
What are you using for your 140 degree calibration source? What sort of circuit is your shield?
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