Automotive Relay Tester

Arduino Nano-based relay tester built for automotive diagnostics. Tests 4-pin and 5-pin relays, measuring coil resistance, contact resistance, and activation time. Displays results on a 20x4 I2C LCD with green/red pass/fail LEDs.

Overview

Commercial relay testers are expensive and slow to use in a shop environment. This tester gives you everything you need at a glance -- coil resistance, activation time in milliseconds, and a clear pass/fail result. Built for daily use as an automotive mechanic.

Features

4-pin and 5-pin relay modes
Single test (x1) and 10-cycle stress test (x10) modes
Measures NC and NO contact resistance
Measures relay activation time in milliseconds
Green/red pass/fail LED indicators
20x4 I2C LCD display
Serial output for logging

Test Modes

Mode	Description
4-pin x1	Single NO contact test
4-pin x10	10-cycle stress test -- each cycle triggers immediately on contact closure for realistic stress testing, reports averages
5-pin x1	Single NC and NO contact test
5-pin x10	10-cycle NC and NO stress test -- each cycle triggers immediately on contact closure for realistic stress testing, reports averages

Hardware

Component	Details
Microcontroller	Arduino Nano
Display	20x4 I2C LCD (address 0x27)
Pass indicator	Green LED
Fail indicator	Red LED
Sense circuit	Voltage divider with 1kohm reference resistor (measure actual value with a multimeter for best accuracy)

Pin Definitions

Pin	Function
A2	Circuit sensor (voltage divider)
3	4/5 pin mode select button
4	Relay coil activation
5	NO contact ground
6	NC contact ground
7	Test trigger button
8	Fail LED (red)
9	Pass LED (green)
10	x1/x10 mode select button
SDA/SCL	I2C LCD

Schematic

See schematics/relay-tester-schematic.svg for the full wiring diagram.

How It Works

The sense circuit is a voltage divider -- a known reference resistor (nominally 1k ohm) in series with the relay contact being tested. Measure your actual resistor value with a multimeter and update RREF in the code for accurate resistance readings.

For x10 stress tests, the relay is cycled 10 times and the results are averaged -- useful for catching intermittent failures that a single test would miss. Each cycle triggers the next activation as soon as contact closure is detected rather than waiting a fixed delay, simulating real-world rapid switching conditions and putting genuine stress on the relay contacts and coil.

Libraries

LiquidCrystal_I2C -- install via Arduino Library Manager

Flashing

Open src/relay-tester.ino in Arduino IDE
Install LiquidCrystal_I2C via Library Manager if not already installed
Select Arduino Nano as the board
Select your COM port
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