Geoelectrical surveying works by injecting a controlled electric current into the ground through one pair of electrodes, labeled A and B, and measuring the resulting voltage with a second pair, M and N. Because different earth materials conduct electricity differently, the measured response reveals the spatial distribution of electrical resistivity beneath the surface. This simple principle underpins an astonishing range of applications: locating groundwater aquifers, tracking contamination plumes and landfill leachates, characterizing soil for civil engineering foundations, exploring for minerals, monitoring dams and levees, studying archaeological sites, and even watching dynamic subsurface processes unfold over time through electrical resistivity tomography. The technique is non-invasive, relatively fast, and, with the right instrumentation, remarkably precise.

The problem, the researchers argue, is that the right instrumentation has historically been expensive and closed. Commercial resistivity meters prioritize robustness and automation, but their proprietary electronics and firmware make it nearly impossible for scientists to modify the architecture or adapt it to unusual experimental needs. Open-hardware initiatives such as OhmPi have begun to change that landscape, demonstrating a growing appetite for accessible, reproducible, and customizable instruments. PRISM pushes this trend further by pairing an unusually high-resolution analog-to-digital converter with automatic polarity reversal and a modern browser-based control interface, all under permissive open-source licenses: GPL-3.0 for the software and CERN-OHL-S-2.0 for the hardware.

At the heart of the instrument sits the LTC2440, a 24-bit delta-sigma analog-to-digital converter chosen for its high resolution and low noise. In geoelectrical work, both the injected currents and the measured potential differences can be vanishingly small, so conversion resolution matters enormously. A 24-bit converter reduces quantization effects and allows the system to resolve fine variations in signal that a conventional 16-bit converter would blur into noise. To keep the conversion honest, the designers paired the ADC with an LT1236-5 precision voltage reference providing a stable 5.000 volts with minimal thermal drift, and they built a virtual ground circuit around an LTC2051 operational amplifier that shifts the signal’s DC level to 2.5 volts, allowing bipolar measurements in a single-supply system without violating the converter’s input limits.

The measurement chain continues with an LT1007 low-noise operational amplifier configured as a high-impedance voltage follower for the voltage coming from the ground, complete with a 10-kilohm multiturn potentiometer for offset compensation, and a resistive divider that can attenuate signals by a factor of five when excitation levels climb. Current is measured indirectly through a precision 22-ohm shunt resistor with a tolerance of just 0.1 percent, switched into the circuit by a relay only when needed to prevent parasitic effects. An Arduino Pro Micro board based on the ATmega32U4 microcontroller orchestrates everything, coordinating relay switching, ADC readout over SPI, and communication with the user’s computer at 115200 baud.

Perhaps the most elegant feature is the automatic polarity reversal mechanism. Every measurement point is sampled twice: once with current flowing in the forward direction and once in reverse. The voltage measured at the electrodes is the superposition of the true resistive response and the spontaneous potential that the ground naturally generates. When current flows one way, the measured voltage equals the response plus the spontaneous potential; reversed, it equals the response minus the spontaneous potential. Summing the two readings cancels the spurious component entirely, yielding twice the true resistive voltage. Crucially, the system reverses the polarity of the potential electrodes simultaneously with the current electrodes, guaranteeing that the final recorded value is always positive regardless of injection direction.

The team validated the instrument rigorously in the laboratory. Calibration curves were constructed for voltage and current using a Siglent SDM3055 digital multimeter as reference, a precision LT1021-based voltage source, and a Newport Model 505 laser diode current source, with linear regression coefficients of 0.9999 across all ranges. Statistical testing with 10,000 consecutive measurements of a 0.999091-volt reference produced a mean of 0.999139 volts, a standard deviation of just 2.57 microvolts, and an effective number of bits, or ENOB, of 19, meaning real-world noise degrades the nominal 24-bit resolution to a still-extraordinary 19 bits. Signals on the order of 100 microvolts sit comfortably above the noise floor, and the error distribution followed a clean Gaussian profile, confirming that fluctuations stem from random thermal noise rather than systematic drift.

The head-to-head comparison with commercial hardware is where the story becomes striking. Measuring precision resistors spanning 10 ohms to 68 kilohms under conditions mimicking field surveys, PRISM kept its maximum measurement error to just 0.95 percent, below the nominal 1 percent tolerance of the test resistors themselves, with a mean relative error of 0.39 percent. The commercial PASI MOD. 16GL-N, by contrast, stayed below 1 percent error only up to roughly 2200 ohms, then degraded progressively, reaching approximately 27 percent error at 68 kilohms. The researchers are careful to note that the advantage cannot be credited to the 24-bit converter alone; it emerges from the complete measurement chain, including the low-noise reference, analog conditioning, printed circuit board design, firmware, and polarity-reversal technique working in concert.

The operating envelope suits field practice well. The instrument measures bipolar voltages from 100 microvolts to 10 volts and currents from 100 microamps to 100 milliamps, the latter capped by the 2.5-volt ADC input limit across the 22-ohm shunt. Injected currents in typical geoelectrical surveys rarely exceed 100 milliamps, so the range covers practical conditions. The browser-based graphical user interface, built with HTML, CSS, and JavaScript and communicating through the Web Serial API, lets users configure Wenner, Schlumberger, or Dipole-Dipole electrode arrays, computes resistance, geometric factor, and apparent resistivity in real time, and exports data to CSV for inversion processing. The firmware even discards the first ADC conversion after each relay switch to avoid transient artifacts and waits for the soil’s electrical response to stabilize before sampling.

The authors are candid about limitations: the system is designed exclusively for direct-current resistivity and cannot perform AC impedance or induced polarization surveys, its acquisition speed favors precision over rapidity, and field validation under real survey conditions remains future work. Still, the implications extend well beyond geophysics. The same four-point probe architecture can characterize the resistivity of graphene and other two-dimensional materials, perovskite thin films, and thermoelectric compounds such as bismuth telluride. With complete design files, bills of materials, assembly instructions, and firmware hosted openly on the Open Science Framework, PRISM invites a global community of researchers, educators, and tinkerers to replicate, modify, and improve it. In a field where a single commercial instrument can cost as much as a car, a 360-dollar, 19-effective-bit, fully open alternative may prove to be one of the most consequential pieces of scientific hardware published this year.

Subject of Research: An open-source, high-resolution analog-to-digital converter-based data acquisition system for geoelectrical resistivity prospecting

Article Title: High-resolution analog-to-digital converter-based data acquisition system for geoelectrical prospecting

Article References: Jiménez, M. L., Ruiz, A. G., Martínez, P. P., & Navarro, J. Á. (2026). High-resolution analog-to-digital converter-based data acquisition system for geoelectrical prospecting. HardwareX, 28, Article e00837. https://doi.org/10.1016/j.ohx.2026.e00837

Image Credits: AI Generated

DOI: 10.1016/j.ohx.2026.e00837

Keywords: geoelectrical prospecting, electrical resistivity, open-source hardware, analog-to-digital converter, 24-bit ADC, PRISM instrument, subsurface imaging, resistivity meter, polarity reversal, HardwareX, low-cost instrumentation, electrical resistivity tomography

Writing in the open-access journal HardwareX, Bas Stijnen, Joseph Thompson, Ryan Paetzold, Eoghan Somers, and David McKeown present a fully open-source, camera-based trifilar pendulum system designed to measure the mass moment of inertia of CubeSat-class objects with impressive accuracy. The complete hardware and software package, from 3D-printed platform tiles to Python analysis code, is released under permissive licenses including CERN-OHL, CC-BY-4.0, and the MIT License, and the total cost ranges from roughly 55 euros for the bare pendulum platform to about 550 euros for a full setup with support frame and camera. The system has even earned open-source hardware certification under OSHWA UID IE000005, a formal stamp of reproducibility that few laboratory instruments can claim.

The trifilar pendulum itself is a beautifully simple piece of physics. A platform is hung from three equal-length wires and given a gentle twist. Because the platform’s centre of mass sits directly beneath the suspension point, it oscillates about the vertical axis with a period that depends on its rotational inertia, its mass, the suspension radius, and the length of the wires. The classical relation, derived under the assumptions of small-angle motion, rigid bodies, and negligible friction, links the measured oscillation period directly to the moment of inertia. What has traditionally made such setups expensive is not the pendulum but the instrumentation: precision rotary encoders or inertial sensors must be physically attached to the oscillating platform, adding mass, friction, and damping that corrupt the very quantity being measured.

The Dublin team’s key innovation is to remove contact entirely. Instead of sensors, they print two paper fiducial markers and tape them to the underside of the platform. These are not ordinary targets but N-fold markers, one with four-fold and one with five-fold rotational symmetry, selected using the MarkerLocator framework. The coprime symmetry orders allow the image-processing software to distinguish the two markers unambiguously and estimate rotational pose reliably from a single camera, even as the platform twists back and forth. A webcam or action camera mounted below the platform records the oscillation, and open-source Python software built on OpenCV, NumPy, and PyQt6 extracts the oscillation period, applies the trifilar equation, and reports the moment of inertia in kilogram metres squared, complete with an estimated measurement error.

The platform itself is assembled from nine triangular 3D-printed PLA tiles joined with brass threaded inserts and M4 screws, forming an equilateral triangular footprint roughly 407 millimetres on a side. The validated configuration handles test articles up to approximately two kilograms, a limit set not by the suspension hardware but by the stiffness of the printed platform, which can flex under concentrated loads and alter the effective suspension geometry. An optional support frame built from aluminium extrusion holds the pendulum and mounts the camera, making the system portable enough for ISO 8 CubeSat assembly cleanrooms, though the team found that suspending the platform directly from a rigid ceiling generally yields better results.

Validation was thorough and revealing. Using calibration masses with analytically known inertias, the researchers tested nine different moment of inertia values spanning from 0.15 to 6.1 times ten to the minus three kilogram metres squared, repeating every measurement five times. The results fell into three clear regimes. For inertias above three times ten to the minus three kilogram metres squared, errors stayed below five percent regardless of camera choice. In the intermediate range, errors ranged between five and fifteen percent, still acceptable for CubeSat characterisation. Below ten to the minus three, accuracy degraded sharply, sometimes exceeding forty percent with the support frame, because the inertia of the object becomes small compared with that of the platform itself, and the final answer emerges from subtracting two large, similar numbers.

Two practical findings stand out for anyone planning to build the system. First, the suspension material matters more than one might expect. Replacing steel cables with braided Dyneema fishing line, chosen for its negligible mass and bending stiffness, cut measurement errors dramatically, bringing even the lowest-inertia test case down to about 6.6 percent error. Second, camera quality matters mainly at the low end: a GoPro Hero 7 Black at fifty frames per second outperformed a basic Logitech C270 webcam by roughly ten percent for small inertias, thanks to better tracking resolution, while the two cameras performed nearly identically for larger objects. Camera distance, between fifteen and thirty centimetres below the platform, proved almost irrelevant, though the GoPro’s wide-angle fish-eye distortion introduced slight errors when markers drifted toward the frame edges.

The software also tackles a common experimental headache: imperfect centring. The trifilar equation assumes the test object’s centre of mass sits exactly over the platform centre, but real satellites are rarely so cooperative. The team implemented an optional correction based on the parallel axis theorem, subtracting the term mass times offset squared from the measured value. Verification tests with calibration masses displaced by five to twenty millimetres showed the software’s corrections matched theoretical predictions to within one part in a million of a kilogram metre squared. A free-decay experiment further confirmed that damping is negligible: the logarithmic decrement of 0.0162 corresponds to a damping ratio of just 0.00257, and the oscillation period shifted by only 0.38 percent over fifty seconds of decay.

The most convincing demonstration came with a representative CubeSat mock-up, an aluminium frame carrying four PCB-based solar panel simulators and integrated calibration masses. The pendulum measured a moment of inertia of 6.738 times ten to the minus three kilogram metres squared, within 3.7 percent of the CAD prediction of 6.996. The small discrepancy was attributed to exactly the kinds of details that make experimental measurement valuable in the first place: tape, fasteners, T-slot hardware, and assembly tolerances that no model captures perfectly. For university CubeSat programmes and small research groups, the message is clear. With a desktop 3D printer, a webcam, a kitchen scale, and freely downloadable design files and software, laboratory-grade mass property measurement is now within reach of virtually any team, and the era of guessing a satellite’s inertia may finally be drawing to a close.

Subject of Research: An open-source camera-based trifilar pendulum for measuring the mass moment of inertia of small satellites

Article Title: An open-source camera-based trifilar pendulum setup for measuring mass moment of inertia of small satellites

Article References: Stijnen, B., Thompson, J., Paetzold, R., Somers, E., & McKeown, D. (2026). An open-source camera-based trifilar pendulum setup for measuring mass moment of inertia of small satellites. HardwareX, Article e00821. https://doi.org/10.1016/j.ohx.2026.e00821

Image Credits: AI Generated

DOI: 10.1016/j.ohx.2026.e00821

Keywords: mass moment of inertia, trifilar pendulum, CubeSats, open-source hardware, fiducial markers, optical tracking, 3D printing, attitude control, small satellites, PocketQubes, HardwareX, low-cost instrumentation

The new platform, described in the open-access journal HardwareX by Jerry Febrico and Basari of Universitas Indonesia, is built around two inexpensive, widely available integrated circuits: the STM32F407VGT6 microcontroller and the AD5933 impedance converter. EIT works by injecting a small, safe alternating current into a subject or phantom through an array of surface electrodes and measuring the faint voltages that appear at the boundary. Because different tissues conduct electricity differently, the pattern of these boundary voltages encodes information about the internal distribution of conductivity, which a computer algorithm can then reconstruct into a cross-sectional image. For the lungs, this is particularly powerful: as air flows in and out during breathing, the local impedance of lung tissue changes dramatically, allowing EIT to track regional ventilation at the bedside without a single X-ray. Diseases such as chronic obstructive pulmonary disease and pulmonary edema, which demand repeated monitoring rather than one-off snapshots, are precisely the conditions where this continuous, non-ionizing approach holds the greatest clinical promise.

What distinguishes the new design from earlier open-source efforts is its deliberately modular analog front-end. The heart of the system is the AD5933, a chip that combines a direct digital synthesizer signal generator and a 12-bit impedance converter on a single die. In this implementation, the AD5933 is configured to output a sinusoidal excitation signal at 50 kilohertz, which is fed into a voltage-controlled current source that injects a constant current of roughly 1.44 milliamperes peak-to-peak into the imaging domain. On the receive side, weak differential voltages picked up from the electrodes are amplified by an instrumentation amplifier and routed back into the AD5933, whose on-chip digital signal processor applies a discrete Fourier transform to extract the real and imaginary components of the measured signal. The STM32 microcontroller orchestrates the entire sequence over an I2C bus, computes impedance magnitudes, and streams the results over USB to a personal computer running Python-based reconstruction software.

The modularity is not an abstract design philosophy; it is physically built into the circuit board. The single custom PCB, measuring 177.8 by 155.83 millimeters, carries two alternative voltage-controlled current source topologies, a Load-in-the-Loop design and a Mirrored Howland current pump, alongside two instrumentation amplifier options, a discrete INA118P chip and an AD620 module. Manual DIP switches on the board let researchers route the signal through any combination of these circuits, enabling direct, empirical comparison of four distinct analog front-end configurations under identical operating conditions. The board is assembled with IC sockets, female headers, and through-hole resistors specifically so that users can swap components, replace damaged parts, and experiment with their own circuit modifications without fabricating a new board. Every component is listed in a bill of materials with purchase links from global suppliers such as DigiKey and AliExpress, and the complete design files, from EasyEDA schematics to PCB Gerber files, are released under CERN-OHL, MIT, and CC BY 4.0 open-source licenses.

Data acquisition follows the classic adjacent four-terminal sensing pattern used across the EIT field. Four 16-channel CD74HC4067 analog multiplexers, steered by the microcontroller, dynamically connect the current source and the measurement amplifier to a ring of 16 stainless steel electrodes mounted on a 3D-printed polylactic acid container. Current is injected through one adjacent electrode pair while differential voltages are measured sequentially across all remaining neighboring pairs; the injection pair then rotates around the ring, and the process repeats. One complete sweep yields 208 impedance measurements, which the firmware collects in just two seconds. The authors tested alternative injection patterns, including opposite and diagonal schemes that push current deeper into the domain, but found in their comparative experiments that the adjacent pattern best preserved the shape and location of target inclusions, while cross and opposite patterns introduced excessive noise that obscured objects entirely.

Validation of the hardware was thorough and refreshingly candid. Signal-to-noise ratio measurements, taken across 50 consecutive frames in a saline-filled phantom with a conductivity of 966 millisiemens per meter, ranged from 21.5 to 43.7 decibels depending on the front-end configuration. The Mirrored Howland paired with the AD620 achieved the highest peak SNR of 43.7 decibels, a figure the authors note is comparable to the clinically validated Sheffield Mk 3.5 system, which operated at roughly 40 decibels. The Load-in-the-Loop configurations delivered more uniform channel-to-channel performance, around 37 to 38 decibels, because this topology does not depend on precisely matched resistor networks, whereas the Howland pump’s high output impedance degrades rapidly when built with standard 1 percent tolerance components. Precision testing with standard resistors from 100 to 1000 ohms revealed exceptionally tight repeatability, with standard deviations between 0.54 and 6.33 ohms across all configurations and coefficients of determination reaching as high as 0.9997.

Perhaps the most instructive finding concerns absolute accuracy. The system exhibited a systematic positive offset, producing relative errors that ranged from 0.60 percent to as much as 37.82 percent for the lowest resistor values. In most measurement contexts, such errors would be disqualifying. But EIT, as practiced here, relies on time-difference imaging: a baseline reference dataset is captured in a homogeneous medium, and images are reconstructed from the changes that occur relative to that baseline. Static hardware offsets, parasitic capacitances, and multiplexer channel resistances are mathematically subtracted during this differential process, which means repeatability, not absolute accuracy, is the metric that matters. By this standard, the platform performs admirably, and the authors are transparent that the fixed additive error, traced through linear regression to positive y-intercepts between 11.2 and 99.4 ohms, is effectively cancelled by the reconstruction algorithm.

Imaging experiments confirmed the system’s practical capability. Using cylindrical phantom targets, one highly conductive stainless steel tube and one non-conductive plastic tube, placed at various positions inside the saline tank, the team reconstructed 2D images using three algorithms available in the open-source pyEIT library: Back-Projection, the Jacobian-based JAC method, and GREIT. Back-Projection was favored for its speed and low computational cost. Quantitative comparison against simulated ground-truth images, scored with the Structural Similarity Index Measure and Root Mean Square Error, showed strong reconstruction fidelity, with the HOW-INA118 configuration achieving a peak SSIM of 0.9242 and the HOW-AD620 configuration delivering the lowest RMSE of 28.29. The reconstructed inclusions showed the characteristic boundary smearing and spatial deformation inherent to adjacent-drive EIT with linear reconstruction, particularly near the center of the domain, but the system reliably detected and localized both conductive and non-conductive anomalies across all four hardware configurations and all tested positions.

The team is explicit that this prototype is intended strictly for education and non-human phantom experiments; it lacks the medical-grade patient isolation required by the IEC 60601-1 safety standard, and no commercial analog currently exists at this price point. Yet the significance of the work extends well beyond the laboratory bench. Existing open-source EIT systems that support standard 16-electrode arrays typically cost between 250 and 330 dollars, while cheaper designs sacrifice electrode count or capability. By delivering a full 16-electrode, 50-kilohertz system with an end-to-end open-source software pipeline, including a Tkinter-based graphical interface, STM32 firmware, 3D-printable container files, and even a step-by-step video calibration tutorial, for under 100 dollars, the researchers have lowered the barrier to entry for students and early-career researchers to a fraction of its previous level. Future plans include upgrading to next-generation impedance converters with multi-frequency capability, compressing acquisition below one second for true real-time respiration monitoring, boosting SNR with active-shielded cabling, and exploring generative deep-learning models such as conditional variational autoencoders and diffusion models to push image quality beyond the physical limits of conventional reconstruction. For a field whose clinical value has long been constrained by hardware cost and complexity, a 95-dollar, fully open imaging platform is a genuinely democratizing development.

Subject of Research: A portable, low-cost, open-source electrical impedance tomography system with a modular analog front-end based on an STM32 microcontroller and AD5933 impedance converter.

Article Title: A portable, low-cost, and open-source electrical impedance tomography system with a modular analog front-end based on STM32 and AD5933

Article References: Febrico, J., & Basari (2026). A portable, low-cost, and open-source electrical impedance tomography system with a modular analog front-end based on STM32 and AD5933. HardwareX, Article e00836. https://doi.org/10.1016/j.ohx.2026.e00836

Image Credits: AI Generated

DOI: 10.1016/j.ohx.2026.e00836

Keywords: electrical impedance tomography, open-source hardware, STM32, AD5933, lung imaging, biomedical engineering, analog front-end, pyEIT, medical imaging, respiratory monitoring, low-cost instrumentation, phantom experiments