Precision LR Meter · JEP‑Electronics

🔍 Overview

The Precision Auto‑Ranging LR Meter is a professional‑grade instrument designed by JEP‑Electronics to measure resistance (10 Ω – 2 MΩ) and inductance (80 µH – 30 mH) with outstanding accuracy. Built on the ubiquitous Arduino UNO platform, it combines a custom analog shield with sophisticated firmware to deliver results rivaling much costlier bench equipment.

Two fundamental measurement principles are employed:

Resistance – auto‑ranging voltage divider using four precision 0.1% reference resistors (2k, 20k, 200k, 1M).
Inductance – LC ring‑down method with a fixed 2.0 µF film capacitor and an LM393 comparator to convert the decaying sine wave into a square wave for pulse width measurement.

The device is controlled locally via two tactile buttons (Test / Mode) and a 16×2 I2C LCD, or remotely over USB using a full SCPI command set. Every design decision – from the 0.1% metal‑film resistors to the non‑blocking debounced button handling – is documented and traceable to the System Requirements Specification (SRS‑LRMETER‑002 v2.1).

⚙️ Firmware version 2.1 – fully compliant with SRS, featuring 16× oversampled ADC, auto‑ranging with fallback, 8‑pulse averaged inductance measurement, and real‑time non‑blocking operation.

📊 Specifications

Resistance range

10 Ω – 2 MΩ

autoranging with 4 sub‑ranges

Resistance accuracy

±1.5% / ±2 Ω

whichever greater (after calibration)

Inductance range

80 µH – 30 mH

using 2.0 µF tank capacitor

Inductance accuracy

±5%

of reading

📋 Detailed characteristics

Parameter	Value
Microcontroller	Arduino UNO (ATmega328P, 5 V, 16 MHz)
Reference resistors (measured & calibrated)	2005.0 Ω, 20030.0 Ω, 199870.0 Ω, 1001200.0 Ω (examples)
LC tank capacitance	2.0 µF film (WIMA MKS2, ±5% , low ESR)
Display	16×2 I2C LCD (PCF8574, address 0x27 or 0x3F)
Remote interface	SCPI over USB‑Serial (9600 baud)
Supply voltage	5 V from USB or barrel jack (shield <150 mA)
Measurement time	resistance <200 ms, inductance <3 s

⚠️ Error detection

Condition	Display message
Open circuit / no DUT	`OPEN / NO PART`
Short circuit (<10 Ω)	`SHORT CIRCUIT` + buzzer
Out of range (>2 MΩ)	`OUT OF RANGE`
No LC oscillation	`NO OSCILLATION`

📐 Schematic

Below is the complete circuit diagram of the LR Meter shield. All components are clearly marked and correspond to the hardware construction report (HCR‑LRMETER‑002).

⬆️ Preview — high‑resolution files available in Downloads.

Key functional blocks: auto‑ranging voltage divider (R2‑R5), LC tank (C2, J1), LM393 comparator with clamping diode, I2C LCD connector, and two push‑buttons with internal pull‑ups.

🛠️ Hardware construction

The LR Meter is built on a custom double‑sided Arduino shield. Every component is carefully selected to meet the stringent requirements of SRS‑LRMETER‑002. Below are highlights from the hardware construction report (HCR‑LRMETER‑002 v1.1).

🔧 Precision references

R2 (2 kΩ), R3 (20 kΩ), R4 (200 kΩ), R5 (1 MΩ) – 0.1% metal‑film, 25 ppm/°C (Vishay RNMF series). Actual values measured during commissioning and stored in firmware.
C2 (2.0 µF) – WIMA MKS2 film capacitor, low ESR, ±5% tolerance, excellent temperature stability.

🔀 Comparator interface

IC1A (LM393) – converts decaying LC sine to square wave. Open‑collector output pulled up with 330 Ω (R7) for fast edges.
D1 (1N4148) – clamps negative excursions, protecting LM393 input.
R6 (150 Ω) – series current limit.

🔌 Pin assignment (shield to UNO)

Arduino pin	Function
D4	Test button (active LOW, internal pull‑up)
D5	Mode button (active LOW, internal pull‑up)
D6	Inductor charge output (OUT_L_TEST_PIN)
D7	Divider supply enable (APPLY_VOLTAGE_PIN)
D8	2 kΩ range select (active LOW)
D9	20 kΩ range select (active LOW)
D10	200 kΩ range select (active LOW)
D11	1 MΩ range select (active LOW)
D12	Pulse input from LM393
A2	Voltage sense (divider midpoint)
A4/A5	I2C (SDA/SCL) to LCD
A3	Buzzer (optional)

📄 Full netlist, component justification and thermal calculations are available in the hardware report.

💻 Firmware & SCPI

The firmware (LR_meter_final_vol2b.ino) is a single‑file, purely procedural implementation compliant with SW‑REQ‑001 to SW‑REQ‑017. It features a cooperative state machine, non‑blocking debounce, and a fully featured SCPI parser.

⚙️ Core algorithms

Auto‑ranging resistance: iterates over the four reference resistors, reads oversampled ADC (16×), selects range with ADC closest to 512 (within [80…944]), falls back to extreme if needed. Formula: Rx = Rk * (5.0 / Vout - 1.0).
Inductance measurement: 8‑pulse averaging. Charge inductor 5 ms, wait 150 µs, measure half‑period with pulseIn(). Frequency f = 1e6/(2*T½), then L = 1/(C·(2πf)²).
Display formatting: automatic unit scaling (Ω/kΩ/MΩ, µH/mH/H) using dtostrf().

📡 SCPI command set (USB‑Serial, 9600 baud)

Command	Response example
`*IDN?`	`Arduino UNO,LR_Meter,V2.1`
`MEAS:RES?`	`R : 1234.567890 Ohm`
`MEAS:IND?`	`L: 0.001234 H`
`*CLS`	`OK`
`SYST:ERR?`	`No error`

📱 UI modes (cycled by MODE button)

0 – Ready (idle)
1 – Resistance Auto (press TEST to measure)
2 – Inductance (press TEST to measure)
3 – Help/Pinout
4 – ADC Test (continuous A0 monitor)

All time‑critical sections use millis() timers; no delay() longer than 10 ms in the main loop (SW‑REQ‑014).

📥 Downloads & documentation

All files are fully versioned and compliant with the project documentation requirements (DOC‑REQ‑003).

📄 Firmware (.ino) v2.1 📄 System Requirements Specification (TXT) 🔧 Hardware Construction Report (TXT) 💻 Software Construction Report (TXT) 📐 Schematic (PDF) 🖼️ Schematic (PNG) 📊 PCB Design & Production Report (TXT) 📄 Product Description (TXT)

📋 Direct file references (from upload)

https://github.com/janengelbrecht/LRMeter

File	Description
`LR_meter_final_vol2b.ino`	Firmware source, fully commented (SW‑REQ‑005).
`Hardware Construction Report.txt`	Detailed shield design, component selection, netlist.
`Software_Construction_Report.txt`	Architecture, module tests, traceability matrix.
`System Requirements Specification.txt`	Master SRS document, v2.1.
`LRMeterSchematics.png / .pdf`	Complete circuit diagram.
`Github files.`