NTC temperature sensor

tobru

Thanks a lot for the explanation. But I don't get it yet =(

I connected 3V3 to the thermistor on one end, G3 (P16) and the 10k resistor to the other end of the termistor. GND is connected to the 10k resistor (other end).

                      10K Thermistor

+-----------+          +-------+
|           |          |       |
|       3V3 +----------+       +----+
|           |          +-------+    |
|           |                       |
|       G3  +-----------------------+
|           |                       |
|           |          +-------+    |
|           |          |       |    |
|       GND +----------+       +----+
+-----------+          +-------+

                      10K Resistor

All I get is the value 4095 when I read it with the following code:

import machine
import time

adc = machine.ADC() 
apin = adc.channel(pin='P16')

for i in range(10):
    val = apin()
    print(val)
    time.sleep(1)

What did I understand wrong?

tobru

Ok, looks like the 10K resistor doesn't fit here with 3V3. Nevertheless I'm a bit lost and not sure how to chose the correct resistor and calculate the temperature from the measured value.

slavik73

"The 10k resitor" should be the same value as your termistor resistance (it can be different than 10k, I used the one with 50k resistance). This schematic worked for me with esp8266 (on LUA), so I don't think it have a problem with 3.3V

Shaun

@tobru The problem is that the ADC is saturating.
Try change the ADC input attenuation, which will increase the ADC full scale conversion voltage.
attn = (0 = 0db : 1.1V max), (1 = 2.5db : 1.4V max), (2 = 6db : 1.9V max), (3 = 11db : 3.2V max)
apin=adc.channel(attn=3, pin='P16') # should work for you

Edit:
Be warned that the ESP32 ADC is nonlinear and rather noisy.
To reduce the jitter in readings, I take 5 readings at 100us cadence, discard the highest and lowest and average the remaining 3.

The non linearity has to be dealt with using a lookup table or conversion function for now.

tobru

@Shaun said in NTC temperature sensor:

apin=adc.channel(attn=3, pin='P16') # should work for you

Great, this changes the bevahiour and looks much better now. Where do I find documentation about this? The official documentation about ADC doesn't talk f.e. about the attn parameter.

tobru

@tobru said in NTC temperature sensor:

Where do I find documentation about this?

I answer myself: Here in the forum under LoPy ADCs. Next step: How to calculate the temperature from the measured value. Any hints appreciated.

robert-hh

At least you need a known mapping between the temperature and the NTC value. If you have a data sheet of that part, you'll find it there.
From the adc value you can calculate back the resistor value of the NTC, and with the mapping between resistor value and the temperature the temperature.
Assuming that the ADC's range is set to 3.2 V, and the Range is 12 bit, and the connection is as shown below,

R_NTC = R1 * (ADC_FULL_RANGE/adc_reading - 1)

R1 = 10000, and ADC_FULL_RANGE = 4096

Line said below, you might take the average of a few readings, and you might have to interpolate between values not in your mapping table.

slavik73

You can find some Arduino code here: https://github.com/adafruit/Thermistor-test/blob/master/thermistortest.pde. Hope you can convert it to Python. Note that our ADC full range is 4096, so you should use 4095 instead of 1023

tobru

I've tried to convert the arduino code to MicroPython, but I'm not sure if I've done this right (especially the calculation part):

import machine
import time
import math

# which analog pin to connect
THERMISTORPIN = 'P16'
# resistance at 25 degrees C
THERMISTORNOMINAL = 10000
# temp. for nominal resistance (almost always 25 C)
TEMPERATURENOMINAL = 25
# how many samples to take and average, more takes longer
# but is more 'smooth'
NUMSAMPLES = 5
# The beta coefficient of the thermistor (usually 3000-4000)
BCOEFFICIENT = 3950
# the value of the 'other' resistor
SERIESRESISTOR = 10000

# take N samples in a row, with a slight delay
adc = machine.ADC(0)
adcread = adc.channel(attn=3, pin=THERMISTORPIN)
samples = [0.0]*NUMSAMPLES
for i in range(NUMSAMPLES):
    samples[i] = adcread()
    print("sample %u: %u" %(i, samples[i]))
    time.sleep(1)
  
# average all the samples out
average = 0
for i in range(NUMSAMPLES):
    average += samples[i]
    average /= NUMSAMPLES

print("average sample: %u" %average)

# convert the value to resistance
resistance = 4095 / average - 1
resistance = SERIESRESISTOR / resistance

print("resistance: %u" %resistance)
  
steinhart = resistance / THERMISTORNOMINAL # (R/Ro)
steinhart = math.log(steinhart) # ln(R/Ro)
steinhart /= BCOEFFICIENT # 1/B * ln(R/Ro)
steinhart += 1.0 / (TEMPERATURENOMINAL + 273.15) # + (1/To)
steinhart = 1.0 / steinhart # Invert
steinhart -= 273.15 # convert to C

celsius = ((steinhart - 32) / 1.8) - 1

print("celsius: %u" %celsius)

To test it, I've connected a 10k resistor instead of the NTC thermistor. The output is:

sample 0: 1879
sample 1: 1857
sample 2: 1863
sample 3: 1878
sample 4: 1863
average sample: 466
resistance: 1284
celsius: 25

This looks good at first sight. But then I connected a 1k resistor instead of the NTC thermistor, this should output around 85c, but the output is this:

sample 0: 181
sample 1: 211
sample 2: 199
sample 3: 211
sample 4: 212
average sample: 52
resistance: 130
celsius: 75

The value isn't really correct. Any ideas what I'm doing wrong? I'm also wondering about the comment # convert to C in the Steinhart equation, the output looks more like Farenheit to me, this is why I added the conversion to celsius (hopefully correct).

For the record: I'm using the following NTC thermistor: http://shop.boxtec.ch/temperatursensor-ntc-mit-stahlkappe-p-42342.html

robert-hh

Hello @tobru. I see a small glitch in you code. Line 32 should not be indented. It should read:

average /= NUMSAMPLES
without the indents, otherwise the division is done in every loop.
and line 38 has to read
resistance = SERIESRESISTOR * resistance

The Steinhart-Hart equation calculates Kelvin. I found another example here, which may be the same you were using: https://gist.github.com/100ideas/1119533.

slavik73

I also think the problem was with average. I did not used your latest line (celsius = ((steinhart - 32) / 1.8) - 1) in my code and I think I got adequate results, but I didn't check it without thermistor connected. As of line 38 - it is actualy "/", not "*" - it make 1/x

tobru

Line 32 should not be indented

Ouch. This of course should not have happened! Thanks a lot to @robert-hh for spoting this! Looks much better now... But: It seems that the measurement is not really accurate, connecting the 10k resistor instead of the NTC thermistor should give me a value around this (I know that it won't be 100% perfect), but it looks like this:

sample 0: 1827
sample 1: 1869
sample 2: 1823
sample 3: 1843
sample 4: 1845
average sample: 1841
resistance: 8170

the code:

import machine
import time

THERMISTORPIN = 'P16'
NUMSAMPLES = 5
SERIESRESISTOR = 10000

adc = machine.ADC(0)
adcread = adc.channel(attn=3, pin=THERMISTORPIN)
samples = [0.0]*NUMSAMPLES
for i in range(NUMSAMPLES):
    samples[i] = adcread()
    print("sample %u: %u" %(i, samples[i]))
    time.sleep(1)
  
average = 0
for i in range(NUMSAMPLES):
    average += samples[i]

average /= NUMSAMPLES

print("average sample: %u" %average)

resistance = SERIESRESISTOR / ((4095 / average) - 1)

print("resistance: %u" %resistance)

Could it be that the ADC isn't very accurate? What can I do to get a more accurate result of the measurement? Or could it be that the equation for calculating the resistance is wrong?

slavik73

@tobru I think the problem could be also caused by non-exactly-accurate assumption about an ADC range. Even if we assume it's a 3.2V, it's still not the same as the one you are giving to thermister. I think it could cause some non-linear behavior. (In arduino I think they are using external Vref in this mode.)

jmarcelino

@tobru
There is some known non-linearity in the ESP32 ADC which is not (yet) being compensated on the ESP-IDF (and thus neither in micropython)

https://twitter.com/ba0sh1/status/815759185816129536/

For example up to 0.1v it registers 0.0. So at the least you need to account for that.

You may be able to compensate these in software. Personally I'd just use a much more accurate dedicated external ADC - connected over I2C for example - for NTC measurements. For example a ADS1115 breakout can be had for less than $2.

robert-hh

I'm still thinking how you have connected the thermistor a) between Vcc and ADC (like sketched below) or b) between ADC and GND. From your code, it looḱs like b)
And yes, there were quite a few notes about the ADC's poor performance. If you need more precise values, you should compensate at least for the actual range.

tobru

@robert-hh said in NTC temperature sensor:

I'm still thinking how you have connected the thermistor a) between Vcc and ADC (like sketched below) or b) between ADC and GND. From your code, it looḱs like b)

It's currently connected like this:

3V3
 +
 |
 |
+-+
|-|
|-| 10k resistor (instead of NTC)
+-+
 |
 |
 +--------------+ P16
 |
 |
+-+
|-|
|-| 10k resistor
+-+
 |
 |
 +
GND

Do you think the code doesn't fit this scheme?

@jmarcelino said in NTC temperature sensor:

use a much more accurate dedicated external ADC - connected over I2C for example

I've ordered a Velleman MM112 external ADC with I2C interface, I can then compare the values.

robert-hh

I think it should. If you NTC is connected between Vcc and ADC, this line:

resistance = SERIESRESISTOR / ((4095 / average) - 1)

should be:

resistance = SERIESRESISTOR * ((4095 / average) - 1)

For instance:
at 50°C, the value of the NTC is 3588 Ohm. That should give an ADC reading of:

4095 * (10000/(3588 + 10000)) = 3014

Put that into your formula, and you get 27881 Ohm
Put that into the other formula, and you get 3587 Ohm.
Regards, Robert

tobru

@robert-hh said in NTC temperature sensor:

resistance = SERIESRESISTOR * ((4095 / average) - 1)

Yes, that looks better, I use the following test code now:

import machine
import time

THERMISTORPIN = 'P16'
NUMSAMPLES = 5
SERIESRESISTOR = 10000

adc = machine.ADC(0)
adcread = adc.channel(attn=3, pin=THERMISTORPIN)
samples = [0.0]*NUMSAMPLES
for i in range(NUMSAMPLES):
    samples[i] = adcread()
    print("sample %u: %u" %(i, samples[i]))
    time.sleep(1)
  
average = 0
for i in range(NUMSAMPLES):
    average += samples[i]

average /= NUMSAMPLES

print("average sample: %u" %average)

resistance = SERIESRESISTOR * ((4095 / average) - 1)

print("resistance: %u" %resistance)

Vin = 3.2
buffer = average * Vin
Vout = buffer / 4095
buffer = (Vin / Vout) - 1
R2 = 10000 * buffer

print("resistance2: %u" %R2)

and the output is:

sample 0: 1850
sample 1: 1836
sample 2: 1837
sample 3: 1845
sample 4: 1829
average sample: 1839
resistance: 12262
resistance2: 12262

Nevertheless, the ADC accuracy seems to be very bad so measuring the temperature this way seems useless. I'll try it with an external ADC as soon as it arrived.

robert-hh

I think you could stil use the built-in ADC, if you requirement for precision is not extreme (like with 1% error). But you have to compensate the errors of the ADC.
a) the zero point. look at which input voltage the output is different to zero. According to a comment below, this is somewhat like 0.1 V
b) the scaling: Look at which input voltage the output just reaches the maximum value.
c) nonlinearity. You could make a compensation table, or ignore it, if it is small enough and outside the range of interest.
d) Noise
a) and b) is easily compensated for. d) is reduced with taking an average of many samples, like you did. You do not have to wait a second between the samples. A few ms is sufficient, if at all, and a small capacitor like 10 nF with short leads directly at the ADC helps.

Another observation: You connect the NTC to 3.2 V (are you sure it's not 3.3V?). The ADC max range is at 3.6 V, the reading at 3.2 V would be 3631. Instead of 4095, you should therefore use 3631 as the virtual maximum value. Then, the resistor value is 9793 (10412 for 3.3 V), which is 2% off the nominal value, and depending of the precision class of the resistor that could be the real value.

tobru

@robert-hh said in NTC temperature sensor:

if you requirement for precision is not extreme (like with 1% error)

There is no exterme precision requirement. 1% would be completely fine.

a) the zero point. look at which input voltage the output is different to zero. According to a comment below, this is somewhat like 0.1 V
b) the scaling: Look at which input voltage the output just reaches the maximum value.
c) nonlinearity. You could make a compensation table, or ignore it, if it is small enough and outside the range of interest.
d) Noise
a) and b) is easily compensated for. d) is reduced with taking an average of many samples, like you did. You do not have to wait a second in between, and a small capacitor like 10 nF with short leads directly at the ADC helps.

Thanks for the inputs!

Another observation: You connect the NTC to 3.2 V (are you sure it's not 3.3V?).

It is connected to 3V3, so it is of course 3.3V. I've chosen 3.2V for Vin in the second equation because of the parameter attn=3. I've read that in this threads: https://forum.pycom.io/topic/226/lopy-adcs and here https://forum.pycom.io/topic/602/ntc-temperature-sensor/6

The ADC max range is at 3.6 V, the reading at 3.2 V would be 3631. Instead of 4095, you should therefore use 3631 as the virtual maximum value. Then, the resistor value is 9793 (10412 for 3.3 V), which is 2% off the nominal value, and depending of the precision class of the resistor that could be the real value.

Interesting. I must have missed this information in the documentation, I thought 3.3V is the maximum.

Explore Pybytes | Official Documentation | Report a Firmware Bug/Issue | GitHub

NTC temperature sensor

Pycom on Twitter