Tim's Precision 4-Wire Low Resistance Meter

🔧 Tim’s 4‑Wire Low‑Ohms Resistance Meter

A precision Kelvin‑probe instrument using an ESP8266, ADS1115, and a custom motherboard.

This project is a compact, high‑accuracy 4‑wire (Kelvin) low‑ohms meter designed for measuring resistances from milliohms up to several ohms with excellent stability. It uses an ADS1115 16‑bit ADC, a precision current source, and an ESP8266 to provide both a local OLED display and a full real‑time web interface over Wi‑Fi.

✨ Features

True 4‑wire Kelvin measurementPrecision current source (100 mA / 10 mA ranges)ADS1115 differential measurement for high resolutionESP8266 Wi‑Fi connectivityReal‑time web dashboard with graphs and averagingLocal OLED displayCalibration mode for shunt accuracyConfigurable OLED pinout using solder jumpersLow‑profile or socketed assembly optionsOpen‑source firmware and documentation

The custom PCB is the core of the instrument. It provides:Correct Kelvin routingStable current‑source geometryClean module placementReduced wiring noiseA compact, professional layout

There is a Full Instructable:

Tim's Precision 4‑Wire Low‑Resistance Meter

All components used on the PCB are standard modules:

ESP8266 NodeMCU

ADS1115 ADC module

0.96" I²C OLED

Range switchPrecision shunt resistor (R1)Kelvin probe connector

PCB

From:

Here is the link to my shared project: Tim's Low Ohms Resistance Meter

PCBWay Home

🖥️ OLED Pinout Compatibility (Solder Jumpers)

OLED modules from different suppliers often use different pin orders:GND and VCC swappedSDA and SCL swappedAll four pins rearrangedTo ensure compatibility, each OLED pin is routed through a 3‑pad solder jumper.You simply bridge the correct pads to match your OLED’s pinout.This prevents accidental reverse‑power damage and makes the board future‑proof.

📐 Measurement Overview

The PCB handles all routing internally:I²C bus shared between ADS1115 and OLEDDifferential ADS1115 inputs for DUT voltage and shunt voltageCurrent source feeding the DUT through TP1/TP4Sense lines connected directly across the DUT via TP2/TP3Range switch selecting 100 mA or 10 mANo external wiring is required beyond the Kelvin probe connector.

🧮 How the Measurement Works

The meter uses a precision current source, a differential ADC, and Ohm’s Law:R = V_DUT / IWhere:V_DUT is measured between TP2 and TP3I is calculated from the voltage across the shunt resistor R1The firmware continuously samples both values, applies averaging, and updates:The OLEDThe web interfaceThe live graph

🎛️ Calibration

To achieve instrument‑grade accuracy, the value of the shunt resistor (R1) must be calibrated.Enable calibration mode in the firmwareInsert an ammeter in series with the DUTRead the actual currentRead the shunt voltage from the OLEDCompute:R1 = V_shunt / I_actualUpdate CURRENT_SHUNT_R1 in the firmwareDisable calibration mode and re‑uploadCalibration only needs to be done once unless R1 is changed.

🌐 Wi‑Fi and Web Interface

On boot, the ESP8266 connects to your Wi‑Fi network.The IP address is shown in the Serial Monitor at 115200 baud.The web interface provides:Live voltage, current, and resistanceAveragingReal‑time graphLow‑latency updates via WebSockets

🔌 Kelvin Probe Usage

The instrument uses four probes:

This eliminates lead resistance and contact errors, enabling accurate milliohm measurements.

🛠️ Assembly Options

Option A — Socketed Modules

• Use female headers
• Modules plug in and are replaceable
• Beginner‑friendly

Option B — Low‑Profile Build

• Solder modules directly using pin headers
• Compact, rigid, professional
• This is the version shown in the images
• Both methods work perfectly.