V2_00.rst

OhmPi V 2.xx (64 or 128 electrodes)

Note

In this version, we have developed two new board types that allow the assembly of Ohmpi v2.00, a measurement board and a multiplexer board. This new version is made up of:

1. A measurement board for four-point measurement
2. 4 multiplexer cards
3. A box

The philosophy of Ohmpi

The philosophy of Ohmpi V2.00 is to offer a new DIY multielectrode resistivity meter. It is a resistivity meter with 64 electrodes , which can be upgraded to 128 electrodes. It is limited to low-current injection,but suitable for small laboratory experiments and small field time-lapse monitoring. Ohmpi, is developed by a team that seeks to share all its experience and wishes to improve and offer a more and more robust tool to the community.Ohmpi V2.00 is a completely different version from the previous one. We will stop the development on the previous version, to dedicate our efforts on this new version.

Technical data

Parameter	Specifications V1	Units	Specifications V2	Units
Electrodes	32		64 to 128
Operating temperature	-0 to 50	°c	-25 to 50	°c
Power consumption of CPU and control system	18.5	W	18.5	W
Voltage injection	12	V	12	V
Battery	9	V	12	V
Current	0 to 40	mA	0 to 40	mA
Min pulse duration	150	mS	150	mS
Input impedance	80	Mohm	80	Mohm
Data storage	micro SD card		micro SD card
Resolution	O.O1	ohm	O.O1	ohm

STEP n°1 : Raspberry Pi configuration

OS installation

Required components	Quantity
Raspberry Pi 4 Model B	1
Micro SD 32 Go	1
HDMI Cable	1
Computer mouse	1
Computer Keyboard	1

The first step is to start up the Raspberry Pi board, including installation of an OS (operating system). For this step, the installation instructions are well described on the Raspberry website

1. Watch the vidéo "how to set up your raspberry Pi" (https://www.youtube.com/watch?v=wjWZhV1v3Pk)
2. The authors recommend installing the latest stable and complete version of Raspbian by using NOOBS (a simple-to-use operating system installer).
3. or you can visit this website : (https://projects.raspberrypi.org/en/projects/raspberry-pi-setting-up)

Note

All the development tests were performed on Raspberry Pi 3 Model B, we used the following version of Raspbian:

Warning

Once the OS has been installed, 1-wire, spi and GPIO remote option must be deactivated via the Raspbian GUI settings menu. Failure to carry out this task may cause damage to the relay shield cards during measurements.

3. When the relays are connected to the GPIO, make sure that all the GPIOs are in the low position when the raspberry starts up. If not, the relays will activate unexpectedly. To ensure that the GPIOs are in Low position, you will need to modify the /boot/config.txt file.

Run the terminal, and write

cd /boot/

4. Open config.txt with GNU nano editor

sudo nano config.txt

5. At the end of the file write :

gpio=8=op,dl
gpio=7=op,dl

6. Press Ctrl +O to save the modifications and press enter
7. Press Ctrl +x to escap and return to the terminal
8. Close the terminal

Virtual Environnement and packages

A virtual environment is a way to have multiple, parallel instances of the Python interpreter, each with different package sets and different configurations. Each virtual environment contains a discrete copy of the Python interpreter, including copies of its support utilities. It also protects your system in case of problems with the packages.

All dependencies are specified in requirements.txt

Note

All instructions below should be typed in the terminal

It is first necessary to ensure that the libatlas-base-dev library is installed:

sudo apt-get install libatlas-base-dev

We strongly recommend users to create a virtual environment to run the code and installed all required dependencies. It can be done either in a directory gathering all virtual environments used on the computer or within the ohmpy directory.

Create the virtual environment:

python3 -m venv ohmpy

Activate it using the following command:

source ohmpy/bin/activate

Install packages within the virtual environment. Installing the following package should be sufficient to meet dependencies:

pip install RPi.GPIO adafruit-blinka numpy adafruit-circuitpython-ads1x15 pandas

Check that requirements are met using

pip list

You should run you code within the virtual environment to leave the virtual environment simply type:

deactivate

Activate virtual environnement on Thonny (Python IDE) (on Rapberry Pi)

If you decided to use a virtual environment, it is necessary to setup Thonny Python IDE the first time you use it.

1- Run the Thonny Python IDE software, Click on raspebrry acces menu > programming> Thonny pythonIDE

2- Thonny's interface opens, Python runs on the Root (Python 3.7.3 (/usr/bin/python3))

3-Click on Run>select interpreter, a new window opens click on interpret

4-On the new open windows select alternative Pyhton3 or virtual environnement

5- New buttons appeared, selected "locate another python executable "

6- A new window opens, find the folder where there is the python 3 file in the virtual environment folder previously created /home/pi/ohmpi/bin/python3.

7- In the known interpreter tab the path of the virtual environnementshould appear

8- Close the window by clicking on ok.

9- Close thonny to save modifications

STEP n°2: Assembly of the measurement board

Required components

Description

Figure shows the general schematics for the electronic measurement board developed. We have developed a complete "plug and play" measurement board. To measure electrical resistivity with Raspberry Pi. two ADS1115 were used,one for the voltage measurement one for the current measurement, as proposed by Florsch [7]. The ADS1115 is a 16-bit ADC (Analog-to-Digital Converter), with an adaptable gain. The advantage of ADS1115 is that the input signal value could lie between - to + 6.114 V. For the current measurement we have directly integrated the INA282 component, which allows to realize precise current measurement around a shunt resistor. The assembly steps are described in the following steps:

1
	Installation of female header, 1 by10 pins, for ADS1115

2
	Soldering of 4 dual-in-line- socket (2 by 2 pins) for optical relay, AQY211EH.

3
	Soldering of 1 dual-in-line- socket (2 by 9 pins) for INA282.

4
	Soldering of 1 dual-in-line- socket (2 by 4 pins)

5
	Soldering of 2 dual-in-line- socket (2 by 4 pins)

6
	Traco Power Supply 12V to 5V, TDH15 - 1211N

7
	Four screw terminals for 12V inpout et 5V outpout

8
	Traco power Supply 12V to 24V, TRN-1215

9
	Two screw terminals electrodes M and N

10
	Four screw terminals, for 12V power supply and for electrodes A and B

11
	Soldering three capacitors (100nF)

12
	2 ohms shunt resistor pre-adjustment

13
	Soldering 2 ohms shunt resistor

14
	1 Mohms resistors

15
	Soldering tow 1 Kohms resistors

16
	Soldering tow 1.5Kohms resistors

17
	Soldering tow 1.5Kohms resistors

18
	Female header 2 by 20 pins for Raspberry Pi connection

19
	View of the female header 2 by 20 pins installation for Raspberry Pi connection

20
	Fixing MCP23008 component

21
	Mounting optical relay, AQY211EH

22
	Mounting LM158 operational amplificator

23
	Mounting ADS115 board

24
	Fixing the INA282

STEP n°3: Start-up of the measurement board.

Required components

1
	Shutdown the raspberry Pi and unplug the power supply

2
	Mounting the bottom spacers on the Raspberry Pi (male/female, 11mm, M3)

3
	Mounting the upper spacers on the Raspberry Pi (female/female, 11mm, M3)

4
	Mounting the Ohmpi's measurement board on the Raspberry Pi

5
	Mounting the Ohmpi's measurement board on the Raspberry Pi

6
	Plug the power supply into a socket and connect it to your Raspberry Pi's power port.

STEP n°4: Measurement board validation

Run the terminal, and write

i2cdetect -y 1

1
	If everything is working, three I2C addresses should appear on the screen.

Setting up an equivalent electronic circuit, for this you will need:

• 4 1kohm resistor (R2)
• 1 220 ohm resistor (R1)
• 1 small padboard
• Spool of solder

2
	Schematic of equivalent electronic circuit test

3
	Préparer le circuit équivalent

4
	Préparer les cables de connexion aux batterie et les cosses

5
	Souder les câbles et les cosses

6
	Installer les cables rouges sur la borne +12V et cable noir sur la borne ground.

7
	Installer le circuit de référence

Run the Thonny IDE and load the code Ohmpy4elec.pi

STEP n°5: Assembly of the MUX board

STEP n°6: Check Mux Board

STEP n°7: Assemble Ohmpi

STEP n°8: Boxing Ohmpi

STEP n°9: Field measurement