Fun with NTCs


I'm toying with WS2812B muti colored LEDs and wanted to do a "smart" thermometer with them.

At first i used a DHT11/12 i2C temperator/humidity sensor, but they are far from perfect.

They use the i2c bus (or a weird custom protocol) , the accuracy is far from good and they are not so cheap (2$).

Then i realised : I'm using a bluepill with a fairly good ADC converter (as seen on the DSO shell series), let's directly use a NTC resistor.

Good thing i had a broken hotend printer head from the i3, so i extracted the thermistor named '3950/100k'

After looking around, here is what i've found :

* The "3950" part is the beta coefficient , actually it is -3950 as it is a NTC
* The 100k is the resistance at 25 degrees C

The formula is

  Beta= TxTref x ln (Rref/R) / (T-Tref)
         
which can be inverted as

alpha= -beta / ln( Rref/R)
Temp = alpha *(K+25)/(K+25+alpha) -K

with K=273.15

So we have beta = -3950, Rref=100k, Tref=25

How do we measure R ?

Simple we use a resistor divider with a known value , for example 200k



The Adc will measure value V at PB1 and the resistance value is
R= 200 * V/(4095-V)

You can use other resistance value, you just have to know the value accurately

Since it is a relative value, we don't care about the actual Vcc.


And the result ?

Good ! Only one resistor, fairly accurate (+- 0.5 degree) and dirt cheap
So overall, much better than the original DHT1x sensor. The only thing we lose is humidity.

NB : Price = ~ 1$ for 5 NTC thermistor on ebay, Ebay link


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