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Topic: Troubles with analog temp sensors (Read 1 time) previous topic - next topic

jpadie

@outsider
i am sure about my code stipulating the 1.1v internal reference.   i'm not using an arduino but an atmega328p. 
i've tried a few analog sensors.  i'm just getting garbage from all of them. 

i've wired a few more up on the boards, directly powering Vin with AA cells (2) and having no other components on the board.  still getting garbage on the output (sometimes 300mVish and sometimes 150mVish).  it's almost as if there is nothing coming out at all and the multimeter is just displaying arbitrary rubbish as it might without the probes touching anything.

ive also wired a lmt94 up without a board.  same rubbish. 

very minor variations with temp changes (fridge, hands, heatgun) (i.e. less than 50mv).

so either it happens that every sensor i've got from mouser (lmt94 and the 94022s) are bust or by soldering or reflowing I'm killing them. 

I'm giving up on this.  I'll investigate digital sensors and redesign the board if I find something suitable.  the tmp36 needs more voltage than i have planned for at the low end.  these will run on 2aaa bats and i'd like them to keep going until under 2.5v.  something like a tmp102 should be ok.

wvmarle

Another simple solution would be a thermistor. That works at any voltage. Use Vcc as reference voltage for the ADC and it's independent of actual voltages. Use a 100k nominal version for lower current.

Running the ATmega at that low voltage is another issue, though. You'll have to bring down the oscillator frequency to 1 MHz (but it does save a lot on power that way).
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

jpadie

i think at 2.5v i can eke out 8Mhz.  I may be able to get away with a lower frequency but that may interact negatively with the SPI comms with the radio.  I will have a look.  In any event, the plan is that at 2.7v the device sends out a lo-batt warning and then slows down its sampling rate even further; and adds hysteresis before sending the new value across the radio. 

thermistor may also be an idea.  I can't remember now why I decided not to use them originally.  Some research to do over the weekend. 

thanks again
Justin

wvmarle

For the speed you're right. Just checked the data sheet, it's rated 10 MHz at 2.7V so 8 MHz at 2.5V sounds reasonable (minimum voltage 1.8V is still good for 4 MHz). I think I mixed it up with the ATtiny.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

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