OHMPI: AN OPEN-HARDWARE RESISTIVITY METER THAT IMAGES WHAT IS UNDER THE GROUND
An instrument that images what is under the ground by measuring how soil conducts electricity, built for about €1,500 and published in a peer-reviewed paper.
by Rémi Clement, Arnaud Watlet, Guillaume Blanchy, Yannick Fargier, Nicolas Forquet, Vivien Dubois, Helene Guyard, Olivier Kaufmann and colleagues
ScienceOpen-hardware
Built withRaspberry Pi
- difficulty
- ●●●●○
- time
- several weekends
- license
- GPL-3.0 (software), CERN-OHL-S-2.0 (hardware)
- repo
- repo ACTIVE0 stars
●●●●○ · several weekends · GPL-3.0 (software), CERN-OHL-S-2.0 (hardware) · 0 stars · repo ACTIVE
WHAT YOU’LL NEED
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COMPAREE VERDICT
OhmPi v2024 is a complete electrical resistivity tomography system for geoscience fieldwork, built by researchers who needed flexible, repairable equipment for long-term monitoring. Electrical resistivity tomography works by injecting current into the ground through two electrodes and measuring voltage between two others; repeat that across an array and inversion software reconstructs a 2D or 3D image of subsurface electrical properties. Geoscientists use it to watch water move through soil, track contamination plumes, monitor landslides and follow biogeochemical processes. The latest version is a ground-up redesign: a new measurement board around an ADS1115 ADC with signal conditioning, an H-bridge injection unit with current sensing, multiplexer boards with 32 relays each that can be configured for eight or sixteen electrodes, a DPH5005 power module supplying up to 50 V, and a Raspberry Pi running rewritten Python software with a web interface, integrated server and MQTT for network integration. It can record full waveforms, which opens the door to time-domain induced polarisation analysis. The hardware, software and an assembly guide are all published, and the paper puts a 16-electrode system at under 1,500 euros. The one thing most likely to go wrong is underestimating the assembly: this is a multi-board measurement system with crimped ribbon cables, and it must be tested before field deployment. The paper also warns the instrument can deliver a lethal current, so treat the electrode array with respect. If you need a resistivity meter for a teaching lab, a student project, or long-term environmental monitoring and you have the patience for a proper electronics build, this is the published, tested design. If you want plug-and-play fieldwork next weekend, it is not.
IN THE REPO
GOOD TO KNOW
- —Full hardware design files (measurement board, multiplexer board, injection unit schematics) archived on Zenodo and developed on GitLab.
- —Complete software stack (Python control software, web interface, MQTT integration) in the repository under GPL-3.0.
- —Step-by-step assembly guide with photographs published in the HardwareX paper, from cable preparation to fitting the finished instrument into a Stanley toolbox.
- —Hardware is CERN-OHL-S-2.0, which is a strongly reciprocal licence: modifications must be released under the same licence. Not a barrier for personal or academic use; check with your institution if distributing modified hardware commercially.
- —The cost figure of about 1,500 euros for a 16-electrode system comes from the paper itself; larger arrays need extra multiplexer boards, electrodes and cable.
- —Assumes intermediate skill with PCB assembly (SMD and through-hole), ribbon cable crimping, and Raspberry Pi setup. The measurement electronics are not plug-and-play.
Parts to buy
12 itemsFrom our check of the build. Exact quantities and part numbers are in the creator’s BOM.
- Raspberry Pi 4Find
- ADS1115 ADCFind
- DPH5005 buck-boost module (0-50 V)Find
- Custom PCBs for measurement board and multiplexer boardsfrom the repo files
- OMRON G5LE-1-VD 12 VDC relays (32 per MUX board)Find
- H-bridge driver componentsFind
- Passive signal conditioning componentsFind
- Ribbon cable and IDC connectorsFind
- Stainless steel electrodesFind
- Electrode cable (the authors used single-wire cables)Find
- Stanley toolbox or equivalent field enclosureFind
- Power supply (12 V recommended)Find
Can I build this?
Build at your own risk. Projects involve tools, electronics and sometimes mains voltage — follow the creator’s safety notes.
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Start here
Navigation into the creator’s own docs — we don’t rewrite the guide, we route you to the source.
- 1.Read the HardwareX paper (The paper includes the full assembly guide with photographs, system architecture, testing results and field validation. Start here before ordering parts.)
- 2.Download the hardware design files from Zenodo (Gerbers for the measurement board and multiplexer boards, schematics, and the full BOM are archived here.)
- 3.Clone the software repository (Python control software, web interface and documentation. Check the README for Raspberry Pi setup instructions and software dependencies.)
- 4.Order PCBs and components(Use the Gerbers from Zenodo and the BOM for the component order. The relay boards are the long-lead item; check stock before committing to a build timeline.)
- 5.Follow the assembly guide in the paper(The paper walks through cable preparation, PCB assembly, enclosure fitting and initial testing with photographs at each step.)
KNOWN ISSUES
- Underestimating the assembly time. This is a multi-board measurement system with crimped power cables, IDC ribbon connections between boards and bench testing against analogue test circuits. The paper's assembly guide is thorough but it is not a quick build. Plan for several sessions, not one weekend.
- Ordering the wrong relays. Each mux.2024 board uses 32 OMRON G5LE-1-VD 12 VDC power relays driven by two MCP23017 I/O expanders; check the BOM carefully and match the coil voltage and footprint before substituting.
- Skipping bench testing. Before going to the field, check the assembled system against a known test circuit, as the authors did with ground analogue circuits, and run a small test array; follow the soldering and placement guide on ohmpi.org for the boards.
- Not checking the CERN-OHL-S-2.0 licence terms. The hardware is strongly reciprocal: if you distribute modified versions, they must be under the same licence. Not a problem for personal or academic use, but check with your institution if you are modifying the design for a product.
- Assuming you can debug the software without reading the code. The Python control software is well-structured but if something does not work, you will need to read the class definitions and MQTT handlers. The web interface is a convenience, not a black box.
- Not budgeting for field deployment costs separately. The €1,500 figure is for the instrument. Electrodes, cable reels, and a field laptop or tablet for control add to that, as does transport to a field site if you are not already there.
What is electrical resistivity tomography and why would I want to build this?
Electrical resistivity tomography (ERT) measures how easily the ground conducts electricity at different points, then reconstructs a 2D or 3D image of the subsurface from those measurements. Geoscientists use it to watch water move through soil, track contamination plumes, monitor landslides, find buried structures and follow biogeochemical processes. You would build this if you need a flexible, repairable resistivity meter for teaching, research or long-term environmental monitoring and you do not have the budget or institutional access for a commercial system.
How does the 16-electrode system scale if I need a larger array?
The mux.2024 boards stack in multiples of eight electrodes (the 16-electrode Lite build uses two boards), and the paper describes a modular design scaling from 8 to 256 electrodes. For a 32- or 48-electrode array, budget for additional MUX boards and longer ribbon cables. The software already supports larger configurations.
Can I use this for induced polarisation measurements, or just DC resistivity?
The v2024 redesign can record full waveforms, not just averaged voltages, which the authors say opens new possibilities for time-domain induced polarisation analysis (the decay of voltage after the current is switched off). The paper explains the IP method, but its published results focus on resistivity, so treat IP as a capability to explore rather than a validated feature.
What software do I use to turn the raw measurements into a subsurface image?
The OhmPi software collects and logs the measurements, but the actual inversion (turning resistance values into a subsurface image) requires separate software. The authors used ResIPy, an open-source tool for geoelectrical inversion; other open-source packages such as pyGIMLi also exist. The inversion step is not trivial and requires some understanding of the method.
Is this compatible with older OhmPi boards, or do I have to build the v2024 version?
The software is backward compatible with older OhmPi hardware, so if you already have a v1 or v2 board, the new control software will still work. The v2024 hardware is a complete redesign and not a drop-in replacement. If you are building from scratch, build v2024; the improved measurement board and MUX architecture are worth it.
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Discussion1
FROM THE COMPAREE TEAM
A 16-electrode system costs under 1,500 euros and the entire design is published in a peer-reviewed paper — but you still need to assemble a multi-board measurement system and run inversion software to get a subsurface image. If you were setting this up for a teaching lab or field monitoring site, what would you build the first test array to measure?
Rémi Clement, Arnaud Watlet, Guillaume Blanchy, Yannick Fargier, Nicolas Forquet, Vivien Dubois, Helene Guyard, Olivier Kaufmann and colleagues
OhmPi was built by a group of European geoscientists who needed flexible, repairable equipment for long-term field monitoring and teaching. The high cost and limited configurability of commercial ERT systems was restricting access to the method, particularly for small research groups and field courses. The v2024 redesign incorporates years of field experience and user feedback into a more robust and capable instrument.
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CompareeTEAM1mo agoedited
Practical notes from our verification: the hardware design files are archived on Zenodo with a DOI and actively developed on GitLab, the documentation site at ohmpi.org is live with user guides and application examples, and the build instructions are published in the open-access HardwareX paper (DOI 10.1016/j.ohx.2026.e00811), with a soldering and component-placement guide maintained on ohmpi.org. The CERN-OHL-S-2.0 hardware licence is strongly reciprocal: modifications must be shared under the same licence, which is fine for academic or personal use but worth checking with your institution if you are modifying the design for distribution. The figure of under 1,500 euros is the paper's own number for a 16-electrode system and does not include field deployment costs (electrodes, cable, transport). The single biggest time sink is assembling and testing the measurement and multiplexer boards — budget several weekends, not one. Correction (4 October 2026): we re-checked this page line by line against the project's own repository, documentation and videos, and fixed errors in earlier versions.