OhmPi low-cost, open hardware resistivity meter

[j570]

Very interesting project.
https://reversaal.gitlab.irstea.page...rst/Ohmpi.html

[BillNEWAST]

I and a friend are currently starting on the OhmPi. It's supposed to cost around $500.00 +++ your time.

It is supposed to take up to 32 probe data sites.

In a Werner array at 1m intervals it should be able to do a VES of 0.6 of widest spread of electrodes, ie 19.2m or 62.99ft DEEP. For most water table and Archaeological Sites this is Very Adequate.

The 8 multiplexer boards that pick the set of electrodes to measure in what makes this meter so INTERESTING.
It means you only have to set the line up once and then move it to the next 1m setting.

OR you should be able to do 4 rows of 8 pins to cover a 3D grid? but not sure till we get it assembled and working.
I plan on testing it out on MY BACKYARD to find the Septic Tank & the Drain Field.

[ivconic]

Looks interesting indeed.
What I don't like to se are those Traco power modules and relays.
Bare multiplexer is alright... come to think; it never ocuured me to read about internal resistance at those!?
For measuring the soil resistance a high currents are not must, so bare multiplexers should do the job just fine if the internal resistance at ON state is close to zero.
Relays... most often used but a source of terrible spikes and need a settle down time before measuring anything. Also not durable enough.
Also never occured to me to analyze deeper the solid state relays.
This is always interesting topic for further improvements.

[BillNEWAST]

The following website is the 'Official' site for the ongoing project!
https://ohmpi.org/
The first site is from 2010. I was told to use the above site as it is current.
I have 4 2023 measurement boards half filled with components and waiting for my new orders to come in.
all you have to do is take the Fritzing file and drop it into your favorite board manufacture and then wait for it to arrive.
There is a Detailed Pictorial Assembly page for each of the boards that is awesome! I like the way it is done as I always know where I am in the process.

[BillNEWAST]

here is the partially populated boards, still have some components to solder on the need to test if the Raspberry PI 3 will work and record to the Micro-SD card.

[j570]

Now I started with my device.

[BillNEWAST]

Great j570, Where did you get the MUX boards? How much did they cost?
when I tried last year they were going to cost me $40.00 Per Board Plus Parts.
I started with the v.2023 Measurement Board as they only cost $20 each, and I had 5 made.
when I'm done with them and they are tested I keep 3 and sell the other 2 to someone!!

[j570]

I ordered MUX boards 5 pieces and Measurement Board 5 pieces on PCBWAY. I paid for everything around $200. I need 1 Measurement board and four MUX board.
I bought the electronic parts on Aliexpress.
IRF640 transistors ($16 for 100 pieces) instead of STP16NF06L ($44 for 100 pieces) and SRD-12VDC-SL-C relays ($28 for 100 pieces) instead of Omron G5LE-1A (minimum $ 100 for 100 pieces)
Relay SRD has 5 pins but Omron G5LE 4 pins. You have to cut one pin ( NC) because is not used and not exist in G5LE, and PCB has four holes. Electrical connections and dimensions are identical between two relays.

Additionally from Aliexpress:
Instead Traco Power THD 15-1211 DC-DC 5V,1V 15W ( 3000mA) 2 pieces $100 (+delivery) I ordered URB2405YMD-20WR3 5VDC/4000mA 2 pieces $12 with delivery.
Instead Traco Power THD 15-1222N DC DC CONVERTER +/-12V 15W (2x 625mA) $52 ( + delivery) I ordered 2 pieces VRB1212LD-10WR3 12V/833mA - 2 pieces $12 with delivery.
I will make additional PCB with Isolated power supply because their dimensions are different than Traco Power and I did not find dual voltage converter. I will use two single voltage.I tested the purchased modules and in my opinion worked perfect.
I spent $24 instead $150.

[BillNEWAST]

Which project are you working on?
The 2024 version or the 2023 version?

The two Measurement Boards are completely different!

I picked the 2023 version because it has one not two Traco power supplies AND
it showed it as being powered by a 12vdc deep cycle battery and it can power up 64 electrodes at once!
I started with the Measurement Board so I could test them and make sure they are all working right!!

[j570]

I make 2024 version of measurement board and 2024 version of MUX board and 32 electrodes. I was waiting for this version because it has the ability to work with 50V. Old version in my opinion was not so good in dry soil. The Ohmpi team examines sewage and wet soils that do not require high voltage.

[BillNEWAST]

j570, sounds like you have been working on this project for a while!
I just read about it in April of 2024.

[j570]

I was interested in the project since the beginning of 2023 but I was waiting for the promised version with a voltage of 50V. At the end of 2024 I ordered the boards and I am slowly assembling the device.

[rwl518p]

Very interested in this, although I would like to make this with 2 fixed probes and 2 movable probes as used on Frobisher etc. I guess I could just use it without the MUX boards. Anyone any ideas on this configuration?

[pheomis]

Hi j570, Have you finished the setup? How are the tests going?

[j570]

I have minor issues with parts. Plugs, cables, and final assembly. This takes a lot of time. I slightly modified the power supply. It's not ready yet. Regarding the configuration, I spoke with GPT CHAT, and he promised to help me configure it. The initial conversation showed that he understood the issue and seemed to be able to do it.