Connecting an Electrical Conductivity Sensor to a Raspberry Pi
Raspberry Pi Electrical Conductivity Sensor
If you have a saltwater pool and are using a chlorinator to provide most of the chlorine then knowing the amount of dissolved salt in the water is crucial to maintaining your pools health and safety.
Electrical Conductivity sensors provide a reading in microsiemens, which is a measure of conductance (as opposed to resistance). Using this we can calculate a salinity reading in parts per million(ppm).
The Atlas Scientific Electrical Conductivity sensor is an industrial grade sensor that works well with the Raspberry Pi, it is fully submersible up to the BNC connector in saltwater. The sensor can work in serial or via I2C protocol, for this project you will be configuring the sensor to use the I2C interface on the RPi.
To configure the RPi I am assuming that you are running the latest version of Raspian and have the ability to connect to your Pi either through SSH with putty and FTP with filezilla, or directly with a keyboard and monitor, if you haven’t set-up your RPi yet then check out my getting started section.
The first thing we need to do is enable the I2C interface on the RPi. This is done by entering the following at the command prompt to start the configuration tool
select option 9 – Advanced Options
select option A7 – I2C
select “Yes” for all the questions and reboot the RPi
After the reboot connect to the command prompt and enter
sudo apt-get install i2c-tools
i2cdetect -y 1
This should produce the following without the sensor attached.
Now that we have our I2C module working correctly we can go ahead and connect our EC sensor. The following materials will be needed to get started:
- Raspberry Pi
- Atlas Scientific Electrical Conductivity sensor kit
- Jumper Wires
- Adafruit T-Cobbler Plus (optional)
When describing the physical pin connections I will be following the GPIO pin numbering convention show below.
Firstly we need to get the EC circuit into the correct mode, when delivered the EC circuit will be in UART (serial) mode, the EC circuit has to be manually switched from UART mode, to I2C mode. When this is done the EC circuit will have its I2C address set to 100 (0x64 Hexadecimal).
Using your breadboard perform the following actions
- Cut the power to the device
- Disconnect any jumper wires going from TX and RX to the RPi
- Short the PRB pin to the TX pin
- Power the device
- Wait for LED to change from Green to Blue
- Remove the short from the probe pin to the TX pin
- Power cycle the device
The device is now I2C mode.
The RPi and EC circuit are now configured so we can go ahead and connect it all together.
Assuming that all of the parts are now mounted on your breadboard
- Connect the GND pin of the EC circuit to the ground pin of your RPi.
- Connect the TX(SDA) pin of the EC circuit to GPIO pin 2.
- Connect the RX(SCL) pin of the EC circuit to GPIO pin 3.
Note: Do Not Use jumper wires for these connections or your readings will not be accurate.
- The 2 PRB pins should be connected via your breadboard to the center and shield pins of your BNC connector.
Note: It makes no difference which way around the pins on the E.C. circuit are connected to the two BNC pins.
- Finally power your EC circuit by connecting the Vcc pin to the +3.3V pin.
You can now run a quick test to prove that we are setup correctly, from the command prompt enter the following:
you should see the following response, if not then check you connections, ensure the light on the EC circuit is blue and reboot your RPi.
In the image above I have 3 sensors connected to my RPi, the EC sensor connection is indicated by Hex value 64. The factory pre-set address for the EC sensor is 100 or 64 in hexadecimal as mentioned above, if you have more than 1 EC circuit connected then you will need to specify a different value. To do this we need to add some python code to our RPi.
Atlas Scientific provide the python code that I will be using here for interfacing with the EC Circuit.
We start by importing the required python modules
import io # used to create file streams import fcntl # used to access I2C parameters like addresses import time # used for sleep delay and timestamps import string # helps parse strings
Next we add the class code to interface with the EC circuit (or any other Atlas Scientific circuit for that matter)
class atlas_i2c: long_timeout = 5 # the timeout needed to query readings and calibrations short_timeout = 5 # timeout for regular commands default_bus = 1 # the default bus for I2C on the newer Raspberry Pis #certain older boards use bus 0 default_address = 100 # the default address for the EC sensor def __init__(self, address=default_address, bus=default_bus): # open two file streams, one for reading and one for writing # the specific I2C channel is selected with bus # it is usually 1, except for older revisions where its 0 # wb and rb indicate binary read and write self.file_read = io.open("/dev/i2c-" + str(bus), "rb", buffering=0) self.file_write = io.open("/dev/i2c-" + str(bus), "wb", buffering=0) # initializes I2C to either a user specified or default address self.set_i2c_address(address) def set_i2c_address(self, addr): # set the I2C communications to the slave specified by the address # The commands for I2C dev using the ioctl functions are specified in # the i2c-dev.h file from i2c-tools I2C_SLAVE = 0x703 fcntl.ioctl(self.file_read, I2C_SLAVE, addr) fcntl.ioctl(self.file_write, I2C_SLAVE, addr) def write(self, string): # appends the null character and sends the string over I2C string += " 0" self.file_write.write(string) def read(self, num_of_bytes=31): # reads a specified number of bytes from I2C #then parses and displays the result res = self.file_read.read(num_of_bytes) # read from the board response = filter(lambda x: x != 'x00', res) # remove the null characters to get the response if(ord(response) == 1): # if the response isnt an error char_list = map(lambda x: chr(ord(x) & ~0x80), list(response[1:])) # change MSB to 0 for all received characters except #the first and get a list of characters # NOTE: having to change the MSB to 0 is a glitch #in the raspberry pi, and you shouldn't have to do this! return "Command succeeded " + ''.join(char_list) # convert the char list to a string and returns it else: return "Error " + str(ord(response)) def query(self, string): # write a command to the board, wait the correct timeout #and read the response self.write(string) # the read and calibration commands require a longer timeout if((string.upper().startswith("R")) or (string.upper().startswith("CAL"))): time.sleep(self.long_timeout) elif((string.upper().startswith("SLEEP"))): return "sleep mode" else: time.sleep(self.short_timeout) return self.read() def close(self): self.file_read.close() self.file_write.close()
Finally we will add our main program
def main(): device = atlas_i2c() # creates the I2C port object, specify the address # or bus if necessary print ">> Atlas Scientific sample code" print ">> Any commands entered are passed to the board via I2C except:" print (">> Address,xx changes the I2C address the Raspberry Pi " "communicates with.") print (">> Poll,xx.x command continuously polls the board every " "xx.x seconds") print (" where xx.x is longer than the %0.2f second " "timeout." % atlas_i2c.long_timeout) print " Pressing ctrl-c will stop the polling" # main loop while True: myinput = raw_input("Enter command: ") # address command lets you change which address # the Raspberry Pi will poll if(myinput.upper().startswith("ADDRESS")): addr = int(string.split(myinput, ',')) device.set_i2c_address(addr) print ("I2C address set to " + str(addr)) # contiuous polling command automatically polls the board elif(myinput.upper().startswith("POLL")): delaytime = float(string.split(myinput, ',')) # check for polling time being too short, # change it to the minimum timeout if too short if(delaytime < atlas_i2c.long_timeout): print ("Polling time is shorter than timeout, setting " "polling time to %0.2f" % atlas_i2c.long_timeout) delaytime = atlas_i2c.long_timeout # get the information of the board you're polling info = string.split(device.query("I"), ",") print ("Polling %s sensor every %0.2f seconds, press ctrl-c " "to stop polling" % (info, delaytime)) try: while True: print device.query("R") time.sleep(delaytime - atlas_i2c.long_timeout) except KeyboardInterrupt: # catches the ctrl-c command, # which breaks the loop above print "Continuous polling stopped" # if not a special keyword, pass commands straight to board else: try: print device.query(myinput) except IOError: print "Query failed" if __name__ == '__main__': main()
All of this python code is available for both 2.x and 3.x on my HydroPi GitHub repository.
We now transfer our code to our chosen folder on the RPi using an FTP client and then run the program.
The screenshot above shows that we are ready to start sending commands to our EC circuit, to confirm that sensor is now fully functioning we will enter the following command
This will poll the sensor every 5 seconds and return the result until a ctrl-c command is entered as shown below, to stop the program enter ctrl-c again.
With the sensor now working there are also a series of other commands that we now have available to us to configure our probe.
The EC circuit actually outputs 4 different values in a CSV string, Electrical Conductivity(EC), Total Dissolved Solids(TDS), Salinity(S) and Specific Gravity(SG) as can be seen above. For the purposes of determining the salinity of a pool we will only be using the EC value. The additional commands that can be seen in the print out are explained just below under the title Removing Output Parameters.
Enable/disable the LED on the EC Circuit:
L,1 -LED enable
L,0 – LED disable
L,? – Query the LED
Take a single reading:
R – Returns a single reading
Set the Probe Type:
K,(Any value between 0.1 and 10) – Should be set to the value of your probe (0.1, 1, 10), for this project K1.0 is the correct choice.
K,? – Report the current K value
To ensure accurate results the probe needs to know the temperature of the liquid it is measuring in °C, factory default is 25°C
T,22.5 – Set the temperature offset value
T,? – Query the set temperature
The EC circuit can be calibrated using a single point or double point reference
Cal,dry – This should always be the first reference point you configure. Probe must be completely dry, as any liquid will affect the result.
Single Point Calibration
Cal,one,n – Used for single point calibration, n is any EC value expressed in microsiemens
Double Point Calibration
Cal,low,n – Used for the double point calibration, n is the low test EC value expressed in microsiemens
Cal,high,n – Used for the double point calibration, n is the high test EC value expressed in microsiemens
Cal,clear – Clears all calibration data
Cal,? – Query the calibration
Removing output parameters:
The EC circuit outputs 4 different values in a CSV string, if you don’t want all of these parameters then some will have to be removed from the output.
O,(parameter),(1 or 0) – Parameter can be any one of the four listed above(EC, TDS, S, SG).To enable the parameter set the second value to 1, to disable set it to 0.
O,? – Displays the enabled/disabled state of each of the parameters.
See image above for examples.
Circuit Address Change:
I2c,n – n is the new decimal address
Changes to the address of the circuit will cause a loss of connectivity until the python script is restarted with the new address.
Info, Status, Low Power and Factory Reset:
I – Device information
STATUS – Reports the reason for the last reboot and the Vcc voltage
FACTORY – Factory reset. This will not change the communications protocol back to UART.
SLEEP – Enter low power sleep state.
If you would like to see how I have implemented the EC sensor why not check out my Hydropi overview page which has all the articles related to my own personal project.
For more information on configuration of the Atlas Scientific EC circuit read this.
There we have it, you have now configured your RPi to interface with the Atlas Scientific EC Sensor.
If you have any thought’s about this article, improvements or errors let me know in the comments below and if you found this helpful, why not share it with others.