Using the SMT100 with Python

August 21, 2019 - Reading time: 11 minutes

When I was young I started programming with punch cards, then moved to assembler which is still my favourite way of programming. From time to time I get interested in high level languages which are so popular these days. Python is among them and so I decided to make first step towards reading out the SMT100 soii moisture sensor from my PC or Raspberry Pi. The hardware required is a SMT100 in RS-485 ASCII version and a USB to RS-485 converter cable. A RS485 shield for the Raspberry Pi would be fine as well, but I simply started with the converter cable which provides power (5V), GND, RS-485A and RS-485B.

As a first step I developed this simpy Python code. Choose the correct COM port for your system.

import serial
import time

def GetTemperature():
    ser.readline()                                               # remove any unwanted characters
    command = bytes("GetTemperature!\r\n", 'utf-8')              # set command here!
    ser.write(command)                                           # write to serial port
    line = ser.readline()                                        # read from serial port
    line = line.decode('utf-8')                                  # convert bytes object to string
    line = line[:-2]                                             # remove CR+LF from right side of string
    if line == '':
        print("no response from sensor")  
        print("Temperature: "+ line+" °C")
def GetWatercontent():
    command = bytes("GetWatercontent!\r\n", 'utf-8')                                
    line = ser.readline()
    line = line.decode('utf-8')
    line = line[:-2]
    if line == '':
        print("no response from sensor")                                        

        print("Watercontent: "+ line+" %")
port = 'COM3'                                                         # set comport here!
ser = serial.Serial( port,
         baudrate=9600,                                               # set baudrate here!

    if(ser.isOpen() == False):
        print ("error open serial port: " + port)                    

    if ser.isOpen():
        print("Please enter your command: ", end = '')
        INPUT = input()
        if INPUT == "GetTemperature!":
        if INPUT == "GetWatercontent!":
        print ("error communticating")                               



Please enter your command: GetTemperature!

Temperature: 24.7 °C

Please enter your command: GetWatercontent!

Watercontent: 96.46 %

Please enter your command:

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