Hello, I'm trying to use a 10k thermistor on a attiny85 with a 433mhz transmitter on a coin cell. 99.9% of the time it's on sleep mode and when Transmitting then back to sleep. The problem I have sense Everything is on 3v I do not know the resistor needed for the thermistor to run at that voltage. I know it's different for 3.3v and 5v but at 3v I do not know. Can someone help me to figure which resistor I need for 3v please?
Joseph
Resistor and code is exactly the same on 3.3volt as on 5volt.
Most Arduinos don't measure voltage by default. They return a ratio of their supply.
So the A/D returns a ratio of the thermistor and resistor, which is supply voltage independent.
Leo..
Hello wawa, So a 10k on 3v same with 3.3v and 5v should be okay then?
Joseph
Thank you.
Joseph
Yes.
10k fixed and a 10k thermistor at 25C (77F) output 2.5volt on a 5volt supply and 1.5volt on a 3volt supply. But the ratiometric A/D of the Attiny85 compensates for that ratiometric source.
It returns 512 with 2.5volt on a 5volt supply and the same 512 with 1.5volt on a 3volt supply.
Next question is if the 10k fixed resistor is the right value.
For the highest resolution (counts per degree C/F), the fixed resistor must have about the same value as the resistance of the thermistor at the temperature of interest. So 10k fixed would be ok if you measure room temp with a 10k thermistor. 10k fixed and a 10k thermistor would not be that good if you measure the temperature of a hot car engine.
Leo..
I would need to measure from 10F to 105f. The voltage is 3v.
Joseph
Yes, that's the range you can use the same value fixed resistor as the resistance of the thermistor.
The most common thermistor value is 10k at 25C (77F).
Is that the thermistor from your dead weather station?
You could measure it. It should be that value at about 77F.
For a low power project (you mentioned coin cell) you could also use a 100k thermistor with 100k resistor. Or, better, switch power to the resistor/thermistor voltage divider on/off with a spare pin of your Attiny85.
Leo..
Hello wawa, What I do have is only 10k thermistors i have 5 of them. I can not afford to get new stuff So I'm trying to use what I have now.
Joseph
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