[time-nuts] temperature sensor

Bernd Neubig BNeubig at t-online.de
Wed Jun 25 12:45:45 UTC 2014


Hi,

the time-nut approach for temperature measurement would be to use a temperature sensor crystal - like the good old Hewlett-Packard guys did many years ago.
If you do not look for ultra-linearity of the frequency vs. temp response, there are several possible types of crystal cuts possible. The simplest one is the Y-cut or the slightly rotated Y+5° cut, which has a slope of about 90 to 95 ppm/K @ room temperature.
Smaller sensor crystals are tuning-fork type crystals, which come in the same small cylindrical package as normal watch crystals.
For further reading an application note for such a crystal from AXTAL can be found here:
http://www.axtal.iwebfree.de/cms/iwebs/download.aspx?id=87592

Vy 73
Bernd  DK1AG


-----Ursprüngliche Nachricht-----
Von: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] Im Auftrag von Didier Juges
Gesendet: Mittwoch, 25. Juni 2014 02:12
An: Discussion of precise time and frequency measurement
Betreff: Re: [time-nuts] temperature sensor

After having tried just about every solution under the sun, my opinion is that within the ambient temperature range (up to at least 100°C) and homebrew budgets, nothing beats an NTC thermistor. 

They are inexpensive, have a large output and interface most easily with a microcontroller's ADC in ratiometric mode requiring a single precision resistor. Even cheap ones have a 1% tolerance which is more precision than you will ever need.

My favorite is a 10k at 25°C with a B factor of 3380 at 25/85 that costs $1.25 at Digikey. The math to derive the temperature from the ADC reading is simple (you do need the log function) and a mundane 12 bit ADC gives you temperature with a fraction of a degree resolution and much better than a degree absolute precision if you actually needed it. With a 2.5V reference (like those in my favorite Silabs uC), self heating is negligible, even in open air (a fraction of a degree).

This is implemented in my Thunderbolt Monitor kit software to measure ambient temperature.

If you need better resolution than what you can get by directly measuring the voltage at the junction of the resistor and thermistor (due to limited resolution of the ADC), add one op-amp and three resistors, or use the PGA if your microcontroller has one.

Didier KO4BB


On June 24, 2014 6:43:07 PM CDT, Chris Albertson <albertson.chris at gmail.com> wrote:
>On Tue, Jun 24, 2014 at 3:24 PM, Mark Sims <holrum at hotmail.com> wrote:
>
>> ...
>> However it is disappointing that no one has stepped up to tackle the 
>> temperature problem.
>
>
>I can post what I have.  It's a uP based PWM fan controller.  It is a 
>stand alone device that does not know anything about the FE5680 but it 
>would be easy to add into any GPSDO using the GPSDO's existing uP 
>provided there were enough extra analog pins available.  It uses the 
>Arduino software environment but I built it using a bare 8-pin DIP.
>The problem with it is I do not have a good temperature sensor, The 
>ones I tried are noisy.
>SO
>I'm looking forward to your sensor info.
>
>My first controller used a comparator chip.  Then I figured the uP was 
>the same cost and same 8-pin package but could do things like PID, data 
>logging, led status blinking and "whatever".  This is a start on the 
>code that works just like the comparator and uses a pot for the user to 
>adjust a set point.
>
>If you see a way to make this better post the changes.
>Some things I will do are (1) use better sensor, (2) use PID library,
>(3)
>measure ambient air temp.
>FanController.ino
><https://dl.dropboxusercontent.com/u/28915695/FanController.ino>
>
>BTW is there any design info on the FE5680 controller.  Schematics of 
>source code?
>
>
>
>--
>
>Chris Albertson
>Redondo Beach, California
>_______________________________________________
>time-nuts mailing list -- time-nuts at febo.com To unsubscribe, go to 
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--
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