Measuring temperatures
#1
Posted 01 June 2011 - 10:51 AM
I thought of two ways;
1. using a Negative Temperature Coefficient Resistor (I got one of 1kΩ at 25°c)
2. using a LM335 Precision Temperature Sensor
Offcause I can try to get it working, but before I reinvent the wheel, anyone has done something simular and/or can point me into the right direction?
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#2
Posted 01 June 2011 - 11:34 AM
#3
Posted 01 June 2011 - 11:47 AM
I don't need high precision, I just want to have a very cheap solution.For high precision temperature measurement (i.e. -/+ 0.1'C), you should rely on PT100 probes. Essentially a platinum league having a very well-known behavior with temperature.
Just to experimentEverything depends on what is your target.
The NTC is the simples way to measure the temperature, but it is far from being precise.
We are using (still now maybe?) the LM335 sensor, and it is pretty good.
In both cases you should provide a very stable and precise voltage reference.
Do you have a schematic and perhaps a code sample? I guess I need to use the VRef-pin on the Netduino?
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#4
Posted 01 June 2011 - 12:45 PM
http://ww1.microchip...edoc/20092a.pdf
SPI interface stable and accurate and you can choose Celsius
or Fahrnheit.
Chic
I don't need high precision, I just want to have a very cheap solution.
Just to experiment
Do you have a schematic and perhaps a code sample? I guess I need to use the VRef-pin on the Netduino?
#5
Posted 01 June 2011 - 12:56 PM
Might I suggest you take a look at the TC77
http://ww1.microchip...edoc/20092a.pdf
SPI interface stable and accurate and you can choose Celsius
or Fahrnheit.
Hi chizz,
First post? Welcome to the Netduino community
Thanks for your suggestion! It looks like a very nice chip, going to check it out besides this experiment.
But I should state the goal of my experiment; to do some more advanced stuff with an analog input, instead of just using potmeters. That's why I came up with the two suggestions I mentioned in the opening post.
My .NETMF projects: .NETMF Toolbox / Gadgeteer Light / Some PCB designs
#6
Posted 01 June 2011 - 01:00 PM
Basically you should use the NTC as a voltage-divisor. You have to connect another resistor R1 between the Vref and the analog input, then the NTC from this input to the ground.
The R1 value depends on the NTC characteristic: the manufacturer should give it.
After that you should measure the voltage on the input using the ADC.
Since the voltage on the input will have this behavior:
Vadc = Vref * Rntc / (R1+Rntc)and the Rntc varies linearly(*) along with the temperature, you may write a C# function to obtain the desired value from the ADC read.
(*) within a range, usually around a point, typically 25'C
Hope it helps
Cheers
PS: we are using this probe very often. It is cheap, but with a limited precision.
#7
Posted 01 June 2011 - 01:31 PM
I'm looking for a simple way to measure temperatures with the Netduino. It doesn't need to be 100% accurate, as long as I have stable readings.
There is the TMP35 / TMP36 / TMP37 sensors. These are analog sensors and give readings of 10mv per C for the 35 and 36 and 20mV per C for the TMP37. All three have different rabges - I have a couple of TMP36's but had the same problems as others in the Community - variable readings. I'm waiting to see what comes out of the analog investigations as at the time I only had the N+ - I may hook it up to the mini (and maybe the scope) and see if there is a difference. At least with the scope I can see if I get stable voltage - that will certainly point to the N+.
There is also a class available for the TMP102 breakout board in the foums here. This one uses the I2C bus.
Hope this helps,
Mark
EDIT: Just remembered - I hooked the TMP102 to the N+ and an LCD a while ago using the code from the forums.
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#8
Posted 01 June 2011 - 03:19 PM
Just starting with netduino, though been doing embedded programming for ever
I am looking for some starter projects to get the feel of the netduino, I'll
mibbe just start on a TC77 I/F myself.
best regards
chizz
Hi chizz,
First post? Welcome to the Netduino community
Thanks for your suggestion! It looks like a very nice chip, going to check it out besides this experiment.
But I should state the goal of my experiment; to do some more advanced stuff with an analog input, instead of just using potmeters. That's why I came up with the two suggestions I mentioned in the opening post.
#9
Posted 01 June 2011 - 06:09 PM
Mario, any change you would be so kind to provide me a schematic for that sensor please?We are using (still now maybe?) the LM335 sensor, and it is pretty good.
My .NETMF projects: .NETMF Toolbox / Gadgeteer Light / Some PCB designs
#10
Posted 01 June 2011 - 10:27 PM
#11
Posted 02 June 2011 - 04:19 AM
Anyway, our schematics are pretty complex due to the precision requested.
According to the LM335 specs, the simplest circuit you may try is as follows.
(NOTE: refer to the last circuit shown at page 8)
Connect the LM335 from ground to the analog input of the Netduino;
Place a 10K pot in parallel (as the circuit shows);
Connect a 12K resistor from +15V to the analog input.
At this point you should adjust the pot so that the input will be +2.982V at 25'C (refer page 6).
That's all.
The only problem is that you cannot overflow the Vref limit, that is +3.3V.
Anyway just try this basic circuit, then we will make some improvement.
Cheers
#12
Posted 02 June 2011 - 11:23 AM
So I actually have to put a 15V power supply in the cirquit of my netduino? Not impossible, but I don't want to damage a netduino Is it actually safe to do it that way?The only problem is that you cannot overflow the Vref limit, that is +3.3V.
Anyway just try this basic circuit, then we will make some improvement.
Cheers
My .NETMF projects: .NETMF Toolbox / Gadgeteer Light / Some PCB designs
#13
Posted 02 June 2011 - 11:40 AM
#14
Posted 02 June 2011 - 11:57 AM
My .NETMF projects: .NETMF Toolbox / Gadgeteer Light / Some PCB designs
#15
Posted 03 June 2011 - 02:57 PM
My .NETMF projects: .NETMF Toolbox / Gadgeteer Light / Some PCB designs
#16
Posted 03 June 2011 - 06:21 PM
Hmm I found this schematic on two Arduino-based sites:
https://sirleech.wor...o-and-lm335-11/
This is exactly the circuit I meant: it is the basic circuit suggested by the LM335 specs.
Cheers
PS: the specs show what are the legs also.
#17
Posted 12 June 2011 - 02:55 PM
My .NETMF projects: .NETMF Toolbox / Gadgeteer Light / Some PCB designs
#18
Posted 13 June 2011 - 07:05 AM
With 6.25mv/C, and a 424mv offset to 0C... you just need to take the ADC port value in milivolts (3300/ADC value) and subtract 424, then divide by 6.25f to get your reading in celcius.
// gain = 6.25 mV/Deg C int mv = sensor.Read() - 424; // 424mv = 0 return mv / 6.25f;
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