🔧 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 measurement

Precision current source (100 mA / 10 mA ranges)

ADS1115 differential measurement for high resolution

ESP8266 Wi‑Fi connectivity

Real‑time web dashboard with graphs and averaging

Local OLED display

Calibration mode for shunt accuracy

Configurable OLED pinout using solder jumpers

Low‑profile or socketed assembly options

Open‑source firmware and documentation

The custom PCB is the core of the instrument. It provides:

Correct Kelvin routing

Stable current‑source geometry

Clean module placement

Reduced wiring noise

A compact, professional layout

There is a full Instructable:

https://www.instructables.com/Tims-Precision-4Wire-LowResistance-Meter/

All components used on the PCB are standard modules:

ESP8266 NodeMCU

ADS1115 ADC module

0.96" I²C OLED

Range switch

Precision shunt resistor (R1)

Kelvin probe connector

🖥️ OLED Pinout Compatibility (Solder Jumpers)

OLED modules from different suppliers often use different pin orders:

GND and VCC swapped

SDA and SCL swapped

All four pins rearranged

To 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 OLED

Differential ADS1115 inputs for DUT voltage and shunt voltage

Current source feeding the DUT through TP1/TP4

Sense lines connected directly across the DUT via TP2/TP3

Range switch selecting 100 mA or 10 mA

No 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 / I

Where:

V_DUT is measured between TP2 and TP3

I is calculated from the voltage across the shunt resistor R1

The firmware continuously samples both values, applies averaging, and updates:

The OLED

The web interface

The live graph

🎛️ Calibration

To achieve instrument‑grade accuracy, the value of the shunt resistor (R1) must be calibrated.

Enable calibration mode in the firmware

Insert an ammeter in series with the DUT

Read the actual current

Read the shunt voltage from the OLED

Compute:

R1 = V_shunt / I_actual

Update CURRENT_SHUNT_R1 in the firmware

Disable calibration mode and re‑upload

Calibration 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 resistance

Averaging

Real‑time graph

Low‑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.