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.. _2.2.2_c_mcp3008:
2.2.2 Thermistor(MCP3008)
============================
.. note::
.. image:: img/mcp3008_and_adc0834.jpg
:width: 25%
:align: left
Depending on your kit version, please identify whether you have **ADC0834** or **MCP3008** and proceed with the matching section.
Introduction
------------
Just like photoresistor can sense light, thermistor is a temperature
sensitive electronic device that can be used for realizing functions of
temperature control, such as making a heat alarm.
Required Components
------------------------------
In this project, we need the following components.
.. image:: img/list2_2.2.2_thermistor.png
Principle
---------
A thermistor is a thermally sensitive resistor that exhibits a precise
and predictable change in resistance proportional to small changes in
temperature. How much its resistance will change is dependent upon its
unique composition. Thermistors are the parts of a larger group of
passive components. And unlike their active component counterparts,
passive devices are incapable of providing power gain, or amplification
to a circuit.
Thermistor is a sensitive element, and it has two types: Negative
Temperature Coefficient (NTC) and Positive Temperature Coefficient
(PTC), also known as NTC and PTC. Its resistance varies significantly
with temperature. The resistance of PTC thermistor increases with
temperature ,while the condition of NTC is opposite to the former In
this experiment we use NTC.
.. image:: img/image325.png
The principle is that the resistance of the NTC thermistor changes with
the temperature of the outer environment. It detects the real-time
temperature of the environment. When the temperature gets higher, the
resistance of the thermistor decreases. Then the voltage data is
converted to digital quantities by the A/D adapter. The temperature in
Celsius or Fahrenheit is output via programming.
In this experiment, a thermistor and a 10k pull-up resistor are used.
Each thermistor has a normal resistance. Here it is 10k ohm, which is
measured under 25 degree Celsius.
Here is the relation between the resistance and temperature:
R\ :sub:`T` =R\ :sub:`N` exp\ :sup:`B(1/TK – 1/TN)`
**R\ T** is the resistance of the NTC thermistor when the temperature is
**T\ K**.
**R\ N** is the resistance of the NTC thermistor under the rated
temperature **T\ N**. Here, the numerical value of **R\ N** is 10k.
**T\ K** is a Kelvin temperature and the unit is K. Here, the numerical
value of **T\ K**\ is 273.15 + degree Celsius.
**T\ N** is a rated Kelvin temperature; the unit is K too. Here, the
numerical value of **T\ N** is 273.15+25.
And **B**\ (beta), the material constant of NTC thermistor, is also
called heat sensitivity index with a numerical value 3950.
**exp** is the abbreviation of exponential, and the base number e is a
natural number and equals 2.7 approximately.
Convert this formula
T\ :sub:`K`\ =1/(ln(R\ :sub:`T`/R\ :sub:`N`)/B+1/T\ :sub:`N`) to get
Kelvin temperature that minus 273.15 equals degree Celsius.
This relation is an empirical formula. It is accurate only when the
temperature and resistance are within the effective range.
Schematic Diagram
-----------------
.. list-table::
:widths: 30 30 30 30
:header-rows: 1
* - T-Board Name
- physical
- WiringPi
- BCM
* - SPICE0
- pin24
- 10
- 8
* - SPIMOSI
- pin19
- 12
- 10
* - SPIMISO
- pin21
- 13
- 9
* - SPISCLK
- pin23
- 14
- 11
.. image:: img/schematic_2.2.2_thermistor_mcp3008.png
Experimental Procedures
-----------------------
**Step 1:** Build the circuit.
.. image:: img/july24_2.2.2_thermistor_mcp3008.png
For C Language Users
^^^^^^^^^^^^^^^^^^^^
**Step 2:** Go to the folder of the code.
.. raw:: html
.. code-block::
cd ~/davinci-kit-for-raspberry-pi/c/2.2.2-2/
**Step 3:** Compile the code.
.. raw:: html
.. code-block::
gcc 2.2.2_Thermistor.c -o Thermistor -lwiringPi -lm
.. note::
-lm is to load the library math. Do not omit, or you will make
an error.
**Step 4:** Run the executable file.
.. raw:: html
.. code-block::
./Thermistor
With the code run, the thermistor detects ambient temperature which will
be printed on the screen once it finishes the program calculation.
.. note::
If it does not work after running, or there is an error prompt: \"wiringPi.h: No such file or directory\", please refer to :ref:`install_wiringpi`.
**Code**
.. code-block:: c
#include
#include
#include
#include
#define SPI_CHANNEL 0 // CE0
#define SPI_SPEED 1000000 // 1MHz
int read_ADC(int channel) {
if (channel < 0 || channel > 7) return -1;
unsigned char buffer[3];
buffer[0] = 1; // Start bit
buffer[1] = (8 + channel) << 4; // Single-ended mode + channel
buffer[2] = 0;
wiringPiSPIDataRW(SPI_CHANNEL, buffer, 3);
int value = ((buffer[1] & 3) << 8) | buffer[2];
return value;
}
int main(void) {
int analogVal;
double Vr, Rt, temp, cel, Fah;
if (wiringPiSetup() == -1) {
printf("setup wiringPi failed!\n");
return 1;
}
if (wiringPiSPISetup(SPI_CHANNEL, SPI_SPEED) == -1) {
printf("SPI setup failed!\n");
return 1;
}
while (1) {
analogVal = read_ADC(0); // Read from CH0
// MCP3008 is 10-bit ADC (0–1023)
Vr = 3.3 * analogVal / 1023.0; // Assume Vref = 3.3V
Rt = 10000.0 * Vr / (3.3 - Vr); // Voltage divider, 10k resistor
temp = 1 / ((log(Rt / 10000.0) / 3950.0) + (1 / (273.15 + 25.0)));
cel = temp - 273.15;
Fah = cel * 1.8 + 32;
printf("Celsius: %.2f C Fahrenheit: %.2f F\n", cel, Fah);
delay(1000);
}
return 0;
}
Code Explanation
---------------------------
.. code-block:: c
#include
#include
#include
#include
These header files include libraries for GPIO control (``wiringPi.h``), SPI communication (``wiringPiSPI.h``), standard I/O operations (``stdio.h``), and math functions (``math.h``) in C.
.. code-block:: c
#define SPI_CHANNEL 0
#define SPI_SPEED 1000000
Define constants for the SPI channel and SPI communication speed. Here, SPI channel 0 (CE0) and a clock speed of 1 MHz are used.
.. code-block:: c
int read_ADC(int channel)
This function reads analog data from a specified channel of the MCP3008 ADC.
.. code-block:: c
buffer[0] = 1;
buffer[1] = (8 + channel) << 4;
buffer[2] = 0;
These lines format the SPI command according to the MCP3008 protocol: a start bit, configuration for single-ended mode, and the channel number.
.. code-block:: c
wiringPiSPIDataRW(SPI_CHANNEL, buffer, 3);
Transfer the SPI command and receive the 10-bit ADC data from MCP3008.
.. code-block:: c
int value = ((buffer[1] & 3) << 8) | buffer[2];
Extract and combine the 10-bit ADC result from the returned SPI buffer.
.. code-block:: c
if (wiringPiSetup() == -1) { ... }
if (wiringPiSPISetup(SPI_CHANNEL, SPI_SPEED) == -1) { ... }
These lines initialize WiringPi and configure SPI. If initialization fails, the program exits.
.. code-block:: c
analogVal = read_ADC(0);
Reads the analog signal from MCP3008 channel 0, where the thermistor voltage divider is connected.
.. code-block:: c
Vr = 3.3 * analogVal / 1023.0;
Convert the digital ADC value into an analog voltage. The ADC range is 0–1023 with 3.3V reference voltage.
.. code-block:: c
Rt = 10000.0 * Vr / (3.3 - Vr);
Calculate the resistance of the thermistor using the voltage divider formula. A 10kΩ resistor is assumed in series with the thermistor.
.. code-block:: c
temp = 1 / ((log(Rt / 10000.0) / 3950.0) + (1 / (273.15 + 25.0)));
Use the B-parameter equation to convert the thermistor resistance to temperature in Kelvin.
**T(K) = 1 / [ln(Rt/Râ‚€)/B + 1/Tâ‚€]**, where
- R₀ = 10kΩ
- B = 3950
- T₀ = 25°C = 298.15K
.. code-block:: c
cel = temp - 273.15;
Convert the temperature from Kelvin to degrees Celsius.
.. code-block:: c
Fah = cel * 1.8 + 32;
Convert the Celsius temperature to Fahrenheit.
.. code-block:: c
printf("Celsius: %.2f C Fahrenheit: %.2f F\n", cel, Fah);
Display the temperature in both Celsius and Fahrenheit on the terminal with 2 decimal places of precision.
For Python Language Users
^^^^^^^^^^^^^^^^^^^^^^^^^
**Step 2:** Set up the SPI interface and install the ``spidev`` library (see :ref:`spi_configuration` for detailed instructions). If you have already completed these steps, you can skip this.
**Step 3:** Go to the folder of the code.
.. raw:: html
.. code-block::
cd ~/davinci-kit-for-raspberry-pi/python
**Step 4:** Run the executable file
.. raw:: html
.. code-block::
sudo python3 2.2.2-2_thermistor.py
With the code run, the thermistor detects ambient temperature which will
be printed on the screen once it finishes the program calculation.
.. warning::
If there is an error prompt ``RuntimeError: Cannot determine SOC peripheral base address``, please refer to :ref:`faq_soc`
**Code**
.. note::
You can **Modify/Reset/Copy/Run/Stop** the code below. But before that, you need to go to source code path like ``davinci-kit-for-raspberry-pi/python``. After modifying the code, you can run it directly to see the effect.
.. raw:: html
.. code-block:: python
#!/usr/bin/env python3
# -*- coding: utf-8 -*-
import spidev
import time
import math
import RPi.GPIO as GPIO
# Set GPIO mode
GPIO.setmode(GPIO.BCM)
# Initialize SPI for MCP3008 (Bus 0, CE0)
spi = spidev.SpiDev()
spi.open(0, 0) # Bus 0, Device 0 (CE0)
spi.max_speed_hz = 1000000 # 1 MHz
def read_adc(channel):
"""
Read analog value from MCP3008 channel (0–7)
"""
if channel < 0 or channel > 7:
return -1
adc = spi.xfer2([1, (8 + channel) << 4, 0])
value = ((adc[1] & 0x03) << 8) | adc[2]
return value
try:
while True:
# Read analog value from CH0 of MCP3008
analogVal = read_adc(0)
# Convert to voltage (assuming 3.3V reference)
Vr = 3.3 * analogVal / 1023.0
# Calculate thermistor resistance (R2 in voltage divider is 10kΩ)
Rt = 10000.0 * Vr / (3.3 - Vr)
# Steinhart–Hart calculation
tempK = 1.0 / (((math.log(Rt / 10000.0)) / 3950.0) + (1.0 / (273.15 + 25.0)))
# Convert to Celsius and Fahrenheit
Cel = tempK - 273.15
Fah = Cel * 1.8 + 32
# Print the result
print('Celsius: %.2f °C Fahrenheit: %.2f °F' % (Cel, Fah))
time.sleep(0.2)
except KeyboardInterrupt:
pass
finally:
spi.close()
GPIO.cleanup()
**Code Explanation**
#. This section imports required libraries:
- ``spidev`` for SPI communication with MCP3008
- ``time`` for delay functionality
- ``math`` for logarithmic operations in the Steinhart–Hart temperature formula
- ``RPi.GPIO`` for initializing and cleaning up GPIO (included for structural completeness)
.. code-block:: python
#!/usr/bin/env python3
# -*- coding: utf-8 -*-
import spidev
import time
import math
import RPi.GPIO as GPIO
#. Initializes the GPIO mode as BCM and configures the SPI interface on bus 0 and device 0 (CE0), with a speed of 1 MHz.
.. code-block:: python
# Set GPIO mode
GPIO.setmode(GPIO.BCM)
# Initialize SPI for MCP3008 (Bus 0, CE0)
spi = spidev.SpiDev()
spi.open(0, 0) # Bus 0, Device 0 (CE0)
spi.max_speed_hz = 1000000 # 1 MHz
#. Defines a function ``read_adc(channel)`` to read analog values from a specified MCP3008 channel (0–7). It sends a 3-byte SPI command and receives a 10-bit analog result (0–1023).
.. code-block:: python
def read_adc(channel):
"""
Read analog value from MCP3008 channel (0–7)
"""
if channel < 0 or channel > 7:
return -1
adc = spi.xfer2([1, (8 + channel) << 4, 0])
value = ((adc[1] & 0x03) << 8) | adc[2]
return value
#. Main loop: Reads analog voltage from a thermistor on channel 0, converts it to resistance, then uses the Steinhart–Hart equation to estimate temperature in Celsius and Fahrenheit. Updates are printed every 0.2 seconds.
.. code-block:: python
try:
while True:
# Read analog value from CH0 of MCP3008
analogVal = read_adc(0)
# Convert to voltage (assuming 3.3V reference)
Vr = 3.3 * analogVal / 1023.0
# Calculate thermistor resistance (R2 in voltage divider is 10kΩ)
Rt = 10000.0 * Vr / (3.3 - Vr)
# Steinhart–Hart calculation
tempK = 1.0 / (((math.log(Rt / 10000.0)) / 3950.0) + (1.0 / (273.15 + 25.0)))
# Convert to Celsius and Fahrenheit
Cel = tempK - 273.15
Fah = Cel * 1.8 + 32
# Print the result
print('Celsius: %.2f °C Fahrenheit: %.2f °F' % (Cel, Fah))
time.sleep(0.2)
#. The ``finally`` block ensures graceful shutdown. It closes the SPI interface and performs GPIO cleanup to release all hardware resources.
.. code-block:: python
except KeyboardInterrupt:
pass
finally:
spi.close()
GPIO.cleanup()