Low Power Temperature Sensor
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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.
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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 -
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 -
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.
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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.
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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.
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..
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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.
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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
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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.
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also check HTU21D....
