⚡ Spot Welder Pro

🇺🇦 Arduino-based spot welder controller for battery pack welding — featuring an OLED display, INA226 power monitoring, dual-pulse mode, live weld-resistance measurement, foot switch support, and persistent EEPROM settings.

Features

Dual-pulse welding — configurable short pre-pulse + main pulse for better weld quality
Auto & Manual modes — automatic pulse via touch sensor or manual trigger via foot switch / encoder button
Real-time monitoring — battery voltage and weld current via INA226 over I2C
Live weld-resistance measurement — asynchronous, non-blocking voltage-sag sampling calculates the welding-circuit resistance (mΩ) on every pulse, shown on the main screen
Voltage compensation — pulse duration automatically adjusted for battery voltage variation
OLED UI — 128×64 SSD1306 display with menu navigation via rotary encoder
Protection system — low/high battery voltage alarms, over-temperature alarm (NTC), NTC/INA226 fault detection, watchdog timer (250 ms)
User-overridable alarms — battery voltage and NTC faults can be bypassed from the encoder button when a sensor is missing or a threshold needs to be overridden, and auto-reset once conditions are back to normal
Electrode contact check — manual (foot switch) trigger is blocked with an on-screen warning if the electrodes aren't making contact
EEPROM persistence — settings auto-saved every 30 s (CRC-16 integrity check), plus an instant force-save when leaving the settings menu
Weld counter — cumulative weld count stored in EEPROM
Standby mode — auto standby after configurable inactivity timeout
Sound feedback — piezo buzzer on weld events and alarms
Screen invert — 0°/180° OLED rotation option
Debug output — Serial diagnostic mode (_DEVELOPMENT_ build flag, 115200 baud)
Flash-optimized — custom lightweight number formatting removes the sprintf dependency, freeing 2+ KB of program space

Firmware Variants

The repository ships two .ino entry points that share the same headers, state machine, display and EEPROM code:

File	Status	Description
`Spot-Welder-Pro-Fix.ino`	✅ Recommended	Current build. Includes the full fix set described in Changelog: non-blocking current/voltage sampling, resistance measurement, NTC/INA226/voltage fault handling with bypass, contact check, instant EEPROM save, flash optimization.
`Spot-Welder-Pro.ino`	⚠️ Legacy	Original entry point kept for reference. Lacks the fixes above and will reboot-loop if the INA226 is not detected.

Important: the Arduino IDE compiles every .ino file in the sketch folder as a single sketch. Having both entry points present at once causes a duplicate setup()/loop() compile error. Before building, either delete/move Spot-Welder-Pro.ino out of the folder, or rename Spot-Welder-Pro-Fix.ino to Spot-Welder-Pro.ino — keep only one.

Hardware Requirements

Component	Specification
MCU	Arduino (ATmega328P), e.g. Arduino Nano / Uno
OLED display	128×64 SSD1306, I2C
Current/voltage sensor	INA226, I2C (addr `0x40`)
Rotary encoder	Incremental, with push button
Weld pulse switch	MOSFET gate or SSR driven from digital pin 5
Foot switch	Normally-open, pull-up
Auto-pulse sensor	Touch / proximity, active-high on pin 3
Buzzer	Passive piezo
Temperature sensor	NTC thermistor on A3
Current sensor (weld)	Hall effect sensor SS49E on A0
Shunt resistor	0.01 Ω / INA226 path
Power source	LiPo / Li-ion pack (supported range: 3.5 V – 15 V)

Pin Mapping

Signal	Arduino Pin	Notes
Encoder CLK	D2	INT0 — hardware interrupt
Encoder DT	D8	Phase detection
Encoder SW	D6	Pull-up, active-low
Foot switch	D7	Pull-up, active-low
Auto-pulse sensor	D3	Active-high
Weld pulse output	D5	To MOSFET gate / SSR
Buzzer	A1	Passive piezo
Temperature (NTC)	A3	Voltage divider to GND
Hall sensor (weld current)	A0	SS49E — sampled asynchronously mid-pulse
OLED + INA226 (I2C)	A4 / A5	SDA / SCL

I2C addresses:

SSD1306 OLED: 0x3C
INA226: 0x40

Firmware Architecture

The firmware is built around an event-driven state machine:

ISR (INT0) ──► mEvent (volatile) ──► stateMachine()
Timers      ──► mEvent               │
Button poll ──► mEvent               ▼
                                 14 States × 12 Events
                                     │
                          ┌──────────┴──────────┐
                          │                     │
                     Display layer         EEPROM layer
                     (b_display.ino)  (c_eeprom_sound.ino)

States (14): STANDBY, MAIN_SCREEN, MAIN_SCREEN_CNT, MENU_SCREEN, SUB_MENU_1/2, BATTERY_HIGH, TEMP_HIGH, SYSTEM_SCREEN, SYSTEM_MENU, REBOOT_MENU, MAXWELD_SCREEN, INVERT_SCREEN, and more.

Events (12): NONE, BTNDN, BTNUP, ENCUP, ENCDN, BOOTDN, STBY_TIMEOUT, BATT_LV, BATT_HV, TEMP_HIGH, EEUPD.

All access to the shared mEvent volatile is wrapped in atomic helpers (atomicReadEvent(), atomicReadAndClearEvent(), atomicSetEvent()) — never touched directly.

During sendWeldPulse(), the current ADC conversion (A0) is triggered asynchronously at the pulse midpoint (ADCSRA |= (1 << ADSC)) and read back only after the pulse ends, so measurement never extends the pulse duration. INA226 bus-voltage sag is sampled immediately before and after the pulse (with averaging temporarily forced to 1 sample) to compute the welding-circuit resistance.

Library Dependencies

Install via Arduino Library Manager or manually:

Library	Purpose	Install name
Adafruit SSD1306	OLED display driver	`Adafruit SSD1306`
INA226	Voltage/current sensing	`INA226` by Rob Tillaart
EEPROM	Settings persistence	Built-in (AVR)
Wire	I2C bus	Built-in (Arduino)
avr/wdt.h	Watchdog timer	Built-in (AVR)

Getting Started

1. Clone the repository

git clone https://github.com/MYMDO/Spot-Welder-Pro.git

2. Pick one firmware entry point

Keep only Spot-Welder-Pro-Fix.ino (recommended) in the folder — remove or rename Spot-Welder-Pro.ino, otherwise the sketch will fail to compile (see Firmware Variants).

3. Open in Arduino IDE

Open the remaining .ino file — the IDE will automatically include all other .ino tab files (a_state_machine.ino, b_display.ino, c_eeprom_sound.ino) and Spot-Welder-Pro.h.

4. Install dependencies

Use Sketch → Include Library → Manage Libraries and install Adafruit SSD1306 and INA226.

5. Configure build flags (optional)

Edit the top of Spot-Welder-Pro.h before flashing:

#define _DEVELOPMENT_    // Enable Serial debug output (115200 baud)
// #define _BOOTSYS_     // Force boot into system menu (for testing)
// #define _TESTING_     // Ignore low-battery alarm

6. Select board and flash

Board: Arduino Nano (ATmega328P) or compatible
Programmer: USB (standard bootloader)
Upload via Sketch → Upload

7. First boot

On first boot with blank EEPROM, the firmware loads defaults automatically. Hold the encoder button during power-on to enter the system menu. If the INA226 isn't detected, the display shows SENSOR ERROR instead of reboot-looping.

Configuration

All parameters are adjustable at runtime via the on-device menu and persisted to EEPROM. Key compile-time limits defined in Spot-Welder-Pro.h:

Define	Default	Description
`_SERIAL_BAUD_`	`115200`	Serial debug baud rate
`INA_MAX_CURRENT_A`	`8`	INA226 full-scale current (A)
`INA_SHUNT_RESISTANCE_OHM`	`0.01`	Shunt resistor value (Ω)
`INA_AVERAGING_MODE`	`4`	INA226 averaging samples (restored after each pulse)
`STANDBY_TIME_OUT`	`640000` ms	Idle-to-standby timeout
`EEPROM_UPDATE_T`	`30000` ms	EEPROM background auto-save interval
`WP_RETRIGGER_DELAY`	`50` (×10 ms)	Min delay between welds
`FS_TRIGGER_DELAY`	`200` ms	Foot switch debounce delay

Operating Modes

Auto-pulse mode

Weld triggered automatically when the auto-pulse sensor (D3) detects electrode contact. After the configurable delay (autoPulseDelay), the pulse fires without manual input. On entering the main screen, if the sensor is already closed, auto-trigger is suppressed until contacts are released first — this prevents accidental double-firing.

Manual mode

Weld triggered by pressing the foot switch (D7) or the encoder button. Before firing, the auto-pulse sensor state is checked; if the electrodes aren't touching the workpiece, the pulse is blocked, a warning beep sounds, and NO CONTACT is shown on screen.

Dual-pulse sequence

When shortPulseTime > 0, each weld fires a short pre-pulse (% of main pulse time) followed by the full main pulse. This cleans the nickel strip surface and improves weld penetration.

Weld resistance readout

After every pulse, the welding-circuit resistance (mΩ) — derived from the voltage sag under load and peak current — is shown on the main screen beneath the weld counter.

Menu System

Navigate with the rotary encoder; press to confirm.

Main Screen
│
├── [Pulse Set]        — Weld pulse duration (1–100 ms)
├── [Shrt Pulse]       — Short pre-pulse (0–100% of pulse time)
├── [Mode]             — Auto / Manual
├── [Delay]            — Auto-pulse delay
├── [Batt Alarm]       — Low / High battery voltage alarms
├── [Sound]            — Weld sound on/off
├── [Exit]             — Return to main screen (settings saved instantly)
│
└── System Menu (hold button on boot)
    ├── Settings
    │   ├── Max Pulse  — Maximum allowed pulse time
    │   ├── Vnom       — Nominal voltage for compensation
    │   └── Calibrate Battery
    ├── Display
    │   └── Screen Orientation — Normal / Inverted
    └── Boot
        ├── Reboot
        ├── Safe Reset — Reload defaults, keep weld count
        └── Full Reset — Clear all EEPROM

Default Parameters

Parameter	Default	Range
Pulse time	5 ms	1–100 ms
Max pulse time	100 ms	1–100 ms
Short pulse	0 %	0–100 %
Auto-pulse delay	2000 ms	500–5000 ms
Low battery alarm	7500 mV	3500–15000 mV
High battery alarm	15000 mV	3500–15000 mV
Nominal voltage	8500 mV	—
High-temp alarm	65 °C	—
Auto-pulse	Enabled	—
Weld sound	Enabled	—
OLED orientation	Normal	Normal / Inverted

Fault Handling & Safety Overrides

Condition	Display	Recovery
INA226 not detected at boot	`SENSOR ERROR`	Firmware continues booting instead of reboot-looping; reconnect sensor and reboot
NTC open (`ADC > 900`) or shorted (`ADC < 15`)	`NTC ERROR / Sensor open/short`	Press encoder button to bypass (high-temp alarm at 65 °C remains enforced); auto-clears when the sensor is reconnected
Battery voltage outside low/high alarm thresholds	`BATT LOW` / `BATT HIGH`	Press encoder button to temporarily bypass and resume welding; bypass auto-resets once voltage returns to range
Electrodes not touching workpiece on manual trigger	`NO CONTACT / Press electrodes`	Make contact with the electrodes and retry

Project Structure

Spot-Welder-Pro/
├── Spot-Welder-Pro-Fix.ino  # Recommended entry point: setup(), loop(), ISR (with fixes)
├── Spot-Welder-Pro.ino      # Legacy entry point — kept for reference only
├── Spot-Welder-Pro.h        # Config, pin defines, macros, structs
├── a_state_machine.ino      # Full event-driven state machine, sendWeldPulse(), fault logic
├── b_display.ino            # All OLED display routines
├── c_eeprom_sound.ino       # EEPROM load/save, valStr() formatter, buzzer
├── printf.h                 # printf() support for AVR
├── implementation_plan.md   # Technical analysis & improvement roadmap
├── walkthrough.md           # Detailed changelog of the Fix build (UA)
├── AGENTS.md                # Codebase notes for AI coding agents
├── .github/FUNDING.yml      # Sponsorship links
└── LICENSE                  # GPL-3.0

Safety Notes

⚠️ This device controls high-current welding pulses. Incorrect wiring or software misconfiguration can cause fire, battery damage, or personal injury. Build and operate at your own risk.

Always verify MOSFET/SSR wiring before first power-on.
The foot switch is checked at startup — if pressed during boot, the device halts and requires a clean restart.
The watchdog timer (250 ms) will force a hardware reset if the main loop stalls.
Battery voltage is measured every 2 s; audible and visual alarms activate outside the configured thresholds and can be bypassed manually — only bypass if you understand the risk.
Temperature is sampled every 10 s; the welder disables pulse output above 65 °C (default) until cooling, and this alarm cannot be bypassed even when the NTC fault override is active.
Remove power before modifying hardware connections.

Changelog

Fix build (current)

Non-blocking current/voltage sampling — ADC and INA226 reads no longer extend pulse duration.
Weld-circuit resistance measurement — computes and displays mΩ per weld using pre/post-pulse voltage sag; sensor averaging is temporarily disabled during capture to avoid the pulse being masked by 128-sample averaging.
NTC fault detection with bypass — open/short thermistor shows a dedicated warning instead of the normal temperature screen, overridable via the encoder button, with automatic recovery.
Battery voltage alarm bypass — low/high alarms can be overridden from the encoder button and auto-reset when voltage returns to range.
No more INA226 reboot loop — a missing/failed sensor now boots to a SENSOR ERROR screen instead of retrying reboot() indefinitely.
Corrected compensation default — nominal voltage default aligned to 8.5 V so pulse-time compensation scales in both directions instead of clamping low.
Fixed sound menu display bug — the sound toggle screen was reading the auto-pulse flag instead of the sound flag.
Instant EEPROM save — settings are force-written on menu exit instead of waiting for the 30 s background timer.
Auto-pulse re-trigger guard — prevents a false weld if the auto-pulse contact is already closed when the main screen is entered.
Electrode contact check on manual trigger — blocks the pulse and warns if the electrodes aren't in contact.
Boot menu entry fix — button state read is latched at the very start of setup() so encoder interrupts during boot delays/beeps can no longer overwrite the boot-menu request.
Flash footprint reduction — replaced sprintf_P-based value formatting with a lightweight custom formatter, freeing 2+ KB of program memory.

See walkthrough.md for the full technical write-up (Ukrainian) and implementation_plan.md for the underlying issue analysis.

Contributing

Pull requests and issues are welcome.

Fork the repository
Create a feature branch: git checkout -b feature/my-improvement
Commit your changes: git commit -m "Add: description"
Push and open a Pull Request

Please respect existing code style and document any new #define constants in Spot-Welder-Pro.h.

License

This project is licensed under the GNU General Public License v3.0 — see LICENSE for details.

Made in Ukraine 🇺🇦 · Sponsor

Name		Name	Last commit message	Last commit date
.github		.github
AGENTS.md		AGENTS.md
LICENSE		LICENSE
README.md		README.md
Spot-Welder-Pro-Fix.ino		Spot-Welder-Pro-Fix.ino
Spot-Welder-Pro.h		Spot-Welder-Pro.h
Spot-Welder-Pro.ino		Spot-Welder-Pro.ino
a_state_machine.ino		a_state_machine.ino
b_display.ino		b_display.ino
c_eeprom_sound.ino		c_eeprom_sound.ino
implementation_plan.md		implementation_plan.md
printf.h		printf.h
walkthrough.md		walkthrough.md

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⚡ Spot Welder Pro

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