Earth Pit Resistance Tester with Digital Readout (Fall-of-Potential Method)

An earth pit resistance tester with digital readout is a portable instrument that measures how well an earth electrode conducts fault current into the soil, using the fall-of-potential method. It injects a small AC test current between the electrode under test and a remote current spike, measures the voltage drop at a potential spike placed at about 62% of the distance, and computes resistance as R = V/I on an onboard microcontroller, displaying the result in ohms on an LCD. Earthing is the silent safety backbone of every electrical installation: a high-resistance earth pit lets fault voltage linger on equipment bodies instead of tripping protection. Yet most students and small contractors still verify earthing by guesswork or by borrowing a commercial earth tester. This project closes that gap with a battery-powered handheld tester built from an Arduino-class microcontroller, a precision ADC and analog signal conditioning — demonstrating AC measurement theory, the 62% rule, and honest instrument design in one build.

Concept
The fall-of-potential method rests on one quotable mechanism: drive a known current into the earth between the electrode under test (E) and a remote current spike (C), then measure the resulting voltage with a second probe (P) placed where the potential curve has flattened — at roughly 62% of the E–C distance. At that point the measured V/I equals the true earth resistance of E, nearly independent of small placement errors. A DC test current would polarize the electrode-soil interface and let stray DC in the ground bias the reading, so this build injects AC at about 128 Hz (design target): far from the 50 Hz mains frequency, letting a narrow band-pass filter reject mains hum and stray pickup. The microcontroller samples the filtered voltage, computes its RMS value, divides by the known test current, and updates the LCD — a complete AC-measurement instrument in a handheld box.

End to end, the tester works like this: the user disconnects the electrode under test from the installation, drives the current spike into the soil 10–40 m away (design target span), and places the potential spike at 62% of that distance in a straight line, wiring all three to the E, C and P terminals. On push-to-test, the microcontroller switches on a ~128 Hz AC test current of a few milliamps between E and C; the resulting voltage between E and P is picked up, band-pass filtered around the test frequency to reject 50 Hz mains hum, amplified and digitized. Firmware computes the RMS voltage, divides by the known test current to get R = V/I, and shows the resistance on the LCD within about one second (design target). Because the potential spike sits where the potential-gradient curve flattens, small placement errors barely move the reading — the classic reason the 62% rule is trusted in the field.

1. Disconnect the electrode under test from the installation (dead test only) and clip the E lead to it; drive the current spike 10–40 m away (design target) in a straight line and connect it to C.
2. Place the potential spike at 62% of the E–C distance and connect it to P; in dry soil, pour water around the spikes to improve contact (field practice, approximate).
3. Power on and select the range with the rotary knob; the LCD shows the standby screen with battery status.
4. Press push-to-test: the unit injects a ~128 Hz AC test current of 2–20 mA (design target) between E and C for about one second.
5. The sense channel filters, amplifies and digitizes the E–P voltage; the microcontroller computes RMS and divides by the calibrated test current.
6. The LCD displays the earth resistance in ohms with auto-ranging; repeat the reading twice and average for the logbook.

Tech stack:

• Arduino Nano (ATmega328P) — measurement and display firmware in C/C++
• ADS1115 16-bit ADC — sense-channel digitization
• HD44780 16×2 LCD with I2C backpack — digital readout
• LM358 op-amp band-pass filter + gain stage — signal conditioning
• MCU-generated ~128 Hz test tone + RC low-pass — AC excitation source
• 2S 18650 Li-ion pack with TP4056 charger — portable power
• ABS enclosure with E/P/C binding posts — field housing
• KiCad — schematic capture and PCB layout

Requirements

Hardware

• Arduino Nano (ATmega328P), ADS1115 ADC module, 16×2 LCD with I2C backpack
• LM358 dual op-amp, resistors and capacitors for the band-pass filter and gain stage
• 2 × 18650 Li-ion cells, TP4056 charger module, 5 V boost converter
• 3 binding posts (E/P/C), rotary selector switch, push-button, ABS enclosure
• 2 galvanized-steel auxiliary spikes (~30 cm, design target), 3 coiled test leads with crocodile clips
• Digital multimeter (buyer's own) for the calibration check

Software

• Arduino IDE (firmware), KiCad (schematic/PCB), serial terminal for calibration

The Earth Pit Resistance Tester with Digital Readout is built around four capabilities: a true fall-of-potential 3-point measurement that places the potential spike at the 62% point of the E–C span so the reading equals the electrode's real earth resistance; an AC test source at about 128 Hz with narrow band-pass filtering that rejects 50 Hz mains hum and avoids the electrode polarization a DC test current would cause; a direct digital readout covering 0.01 to 2000 Ω (design target) with auto-ranging across three scales; and a portable battery-powered enclosure with E, P and C binding posts, a push-to-test button and coiled test leads with crocodile clips. The firmware samples the filtered sense voltage, computes RMS, divides by the calibrated test current and updates the LCD in about one second. Two galvanized auxiliary spikes and a placement ruler card ship with the unit so a first field test runs without extra shopping.

Implemented features

• [Fall-of-potential 3-point measurement] Implements the classic E–P–C method: current is driven between the electrode under test and a remote current spike while the potential spike samples voltage at the 62% point, so R = V/I gives the true earth resistance.
• [AC test source at ~128 Hz] A microcontroller-generated AC test tone avoids electrode polarization and stray-DC bias; a narrow band-pass stage around the test frequency rejects 50 Hz mains hum before digitization.
• [Digital LCD readout with auto-ranging] A 16×2 LCD shows the resistance in ohms across three auto-selected scales, covering 0.01 to 2000 Ω (design target) without manual range switching.
• [Push-to-test one-second cycle] One button press runs the full cycle — inject, filter, sample, compute RMS, divide by test current, display — completing in about one second (design target).
• [Battery-powered portable enclosure] A 2-cell Li-ion pack in an ABS box with E, P and C binding posts, rotary selector and carry-ready test leads makes the unit genuinely usable away from mains power.
• [Calibrated test current] The test current is set against reference resistors during the build and stored as a firmware constant, with zero-offset compensation measured at every power-on.
• [Complete field kit] Two galvanized auxiliary spikes, three coiled test leads with crocodile clips and a 62%-rule placement card ship with the unit, so the first field measurement needs nothing extra.

Configurable

• Measurement averaging count and LCD backlight timeout are firmware constants the buyer can change and reflash.

Optional customization (not in the base build)

• Data logging to SD card or Bluetooth readout to a phone; 4-point soil-resistivity (Wenner) mode via a firmware and wiring change; serial plot of the potential profile on a laptop.

Future scope (not implemented)

• Clamp-on (stakeless) measurement attachment; GPS-tagged test logging with a companion mobile app.

• Assembled and tested earth-pit resistance tester unit
• 2 galvanized auxiliary spikes, 3 coiled test leads with crocodile clips
• Spike-placement quick card with the 62% rule diagram
• Firmware source code (Arduino C++) with calibration constants documented
• Circuit schematic and wiring diagram (KiCad/PDF)
• Component list with part numbers and alternates
• Project report PDF (theory, design calculations, build photos, test procedure)
• PPT presentation for final review
• Viva Q&A preparation document (fall-of-potential theory, AC measurement, calibration)
• Project abstract PDF

Timeline

• Week 1–2: circuit design, filter and gain-stage calculations, PCB layout
• Week 3–4: PCB assembly, firmware for test-tone generation and RMS computation
• Week 5: enclosure fitting, terminals, calibration against reference resistors
• Week 6: field trials on 2–3 earth pits, report and documentation finalization
• Approximate build schedule; the actual plan is confirmed at order time.

Support

• Video-call walkthrough of the field test procedure at delivery
• Guidance on viva questions around the 62% rule and AC measurement
• Firmware and document updates for the ordered configuration within the support window