Coding the Thermometer – A blog by RC

In this post, I will describe the code I have developed to run the thermometer, log the data and save it in such a way that it can be used to carry out some (limited) analytics. At this point, the thermometer has already been installed

Part of the code I am showing here was taken from the Ardumotive - How to use the DHT22 I have already referenced. I then expanded on it to add some simple logging.

Libraries

To run the code, only three libraries are needed: the Adafruit_DHT library, time library (for sleep), and the datetime library for better date/time utilities.

sleep is needed to slow down the program, so that the sensor is able to gather the information (in fact, it cannot be accessed more than once every two seconds).

# Library for reading from the DHT sensor
import Adafruit_DHT as dht
from time import sleep
from datetime import datetime as dt

The main loop

The main loop of the progam features a while True loop, which will let the program run until it gets interrupted by the user. As soon as the program runs, the log file is opened in append mode, so that it is possible to add more lines without deleting previous readings (this is pretty useful when debugging!). The value of DHT_data is set to be the pin that is connected to the DATA output of the DHT22 sensor.

Next, the while loop is executed. In each iteration of the loop, the timestamp of the iteration is saved in now, then the sensor is read. The values of temperature and humidity are printed on screen, then put together with the timestamp in variable data_line and added to the log file.

The loop is wrapped in a try-except block, so that it is possible to interrupt it “gracefully” using Ctrl+C. When this happens, the log file is closed and the program terminates.

#Set DATA pin (this is the pin I will be connecting the DATA output of the chip to)
DHT_data = 4 

# Open the log file in append mode, so that previous readings will not be deleted 
# when the program restarts.
fw = open('log.txt', 'a')

try: 
    # Run forever, until the program is killed through Ctrl+C
    while True:
        # Getting timestamp for the current measurement
        now = dt.now()
        # Reading Humidity and Temperature from DHT22
        h,t = dht.read_retry(dht.DHT22, DHT_data)

        # Print Temperature and Humidity on Shell window
        print('Time={0}\tTemp={1:0.1f}*C\tHumidity={2:0.1f}%'.format(now, t, h))

        # Preparing the datetime object for logging, splitting date and time for convenience.
        this_date = now.date()
        this_time = now.time()
        # Writing the data in CSV format.
        data_line = "{},{},{},{}\n".format(this_date, this_time, t,h)
        fw.write(data_line)

        sleep(30) #Wait 30 seconds and read again

except KeyboardInterrupt:
    fw.close()
    print('Ending the program.')

When all is said and done, this is a possible output of the program:

Time=2022-07-12 00:57:15.430657 Temp=25.0*C     Humidity=58.2%
Time=2022-07-12 00:57:56.118202 Temp=25.0*C     Humidity=58.3%
Time=2022-07-12 00:58:29.214395 Temp=25.0*C     Humidity=58.3%
Time=2022-07-12 00:58:59.776892 Temp=25.0*C     Humidity=58.5%
Time=2022-07-12 00:59:30.340131 Temp=25.1*C     Humidity=58.5%
Time=2022-07-12 01:00:13.552538 Temp=25.1*C     Humidity=58.3%
Time=2022-07-12 01:00:46.646864 Temp=25.1*C     Humidity=58.4%
Time=2022-07-12 01:01:17.210069 Temp=25.1*C     Humidity=58.4%
Time=2022-07-12 01:01:52.816956 Temp=25.1*C     Humidity=58.5%

Note that there is a bug in the line

print('Time={0}\tTemp={1:0.1f}*C\tHumidity={2:0.1f}%'.format(now, t, h))

which is caused by the temperature sensor returning invalid data: this raises an exception in the print function and interrupts the program. I was not able to reproduce it (after admittedly not a lot of testing) to add a fix, but a simple solution to the problem is simply deleting the offending line. At least, until I figure out a fix.

So there we go! This is how it is possible to code a simple log of temperature and humidity by using a Raspberry Pi and a DHT22 sensor. In a future post, I will be play around with some data visualisation and plot the temperature and humidity over time.

Thanks for reading!