Low Power Temperature Sensor

[diggs]

Hi all,

Just wondering what is the best option for a low power temperature sensor. I had read a few things where the DS18B20 was not the most power efficient way of doing things (could be wrong). Would be measuring around the 4 deg mark.

As it will be purely battery powered project, looking for the best low power option with reasonable accuracy/stability

Any info or just point me in the right direction

[scalz]

it depends on your application target. but si7021 is very low power sensors. and you can find breakout on aliexpress (maybe these are clone). a little more expensive than ds18b20...

[etrombly]

You could try this https://www.adafruit.com/products/1782 pretty accurate and a lot lower power. Also look into other options to cut power, like removing any LEDs and sleeping between measurements.

[fleinze]

The problem with the DS18B20 is that it is rated for a minimum voltage of 3.0V. It works below that, but the error might be bigger.
There is a good article about that here:
http://openenergymonitor.org/emon/node/2331
On the other hand my sensor with a DS18B20 works for 4 months now with a coin-cell battery.
The good thing of the DS18B20 is that it is widely available and really cheap.
The Sensbender micro has a temperature/humidity chip (Si7021) that is rated 1.9 to 3.6 V.
This chip is also available as a breakout board on ebay. I did not try it yet but I think this would be the best solution for battery powered temperature-sensors.

[ericvdb]

My 2xAA Nimh or 1 x 18650 Li-Ion battery sensors use a DC-DC boost converter to crank up the voltage to 5V, then with a LDO MCP1702 i step down to 3.3V to power the arduino pro mini, the nRF24L01+ and the DS18B20. First test results show me that over 2 weeks my battery voltage went down with 4%, that should give me theoretically +/- 8 months without charging the batteries.

http://www.aliexpress.com/item/8pcs-Very-small-DC-DC-0-9-5V-to-DC-5V-Step-UP-Boost-Voltage-Converter/32268508188.html
http://www.aliexpress.com/item/MCP1702-3302E-TO-IC-REG-LDO-3-3V-25A-TO-92-3-MCP1702-3302E-1702-MCP1702/32225711061.html

[AWI]

@ericvdb another theory:
4% in 2 weeks => 20% in 6 weeks = battery empty

not to mention the self discharge of NimH cells:

[m26872]

If you're on a small budget there's always the good old fashion thermistor or even cheaper is to use the internal sensor. At least the later requires calibration, on the other hand most people doesn't check their precalibrated DS18B20s or Si7021 which in surprisingly many cases have accuracy out of spec. I've also learned from my experiments that the internal sensor seems to be much better than expected. But, even if it seems useful for lowpower (minimal self heating) applications, I don't know if it's reliable over time.

[diggs]

@m26872 . Yes, I have a couple of K Type thermocouples on the way to do a bit of testing with as well as I will have a requirement for some reasonably extreme temps in some applications, -196 to +150

[tbowmo]

@m26872

Have you seen that the ds1820 / si7021 is out of spec from the factory? (temperature wise).

Humidity on the Si7021 can be affected by a lot of factors, such as high humidity for a prolonged period.. But temperature should be within the specification.

Could also be fake devices..

[m26872]

To clearify; NO I have not seen si7072 out of spec. (I only own 2.) But when I compare a lot of DS18B20s with each other I've found some diverge >1.5C in very harmonic conditions. I don't want to publish my setup or results until I've got I class A Pt100 as reference, but research (google) told me others have seen the same. Probably with fake or clone ones.

[diggs]

l was just wondering if anyone had a sketch for connecting a K type thermocouple to the mysensors network or could point me in the right direction?

I received a coupe today and want get them hooked up

http://www.ebay.com.au/itm/400798924042?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

[Sparkman]

@m26872 I've seen the same with real ones. Two sitting right next to each other and their temperature readings are off by over 1°C.

Cheers
Al

[ahhk]

also check HTU21D....

[riataman]

Check the bmp180. Its a presure sensor, but it includes a fairly accurate temperature sensor and is rated down to 1.8v.