Auto Cut-Off System for Soldering Iron

AC-powered soldering irons without built-in temperature control are widely used because they are more affordable than adjustable-temperature models. However, these irons remain powered continuously, leading to unnecessary power consumption, overheating, and potential safety hazards if left unattended.

This project allows users to set two timers to automatically turn the soldering iron on and off, indirectly regulating its temperature and helping maintain a consistent heat level.

Additionally, the system includes stand detection logic using a simple electrical continuity method. If the iron is placed in the stand and remains unused for more than five minutes, the system overrides the timer and turns it off automatically. (This feature is only applicable for soldering irons with an earthed body)

An LCD display provides real-time status updates, including timer values, ON/OFF status, and stand detection. The system also stores the previously set timer values in EEPROM, ensuring they remain saved even after a power reset.

With this smart auto cut-off system, you can use a basic soldering iron more efficiently and safely —without the need for an expensive temperature-controlled station!

Note: This project is primarily designed for AC powered soldering irons without built-in temperature control. However, if your iron already has temperature control but lacks an auto cut-off feature, you can still take inspiration from this design.

• Microcontroller: Arduino Nano

• Relay Module: Single-channel mechanical relay / solid-state relay (recommended; at least 2A)

• Display: 16x2 LCD with I2C interface / 0.96" OLED (recommended)

• Rotary Encoder: KY-40

• Power Supply: Hi-Link 5V AC-DC module (5W recommended)

• 3-pin AC Wall Socket

• Diodes: 2x 1N4007 PN junction diodes

• Resistor: 1x 1 KΩ

• Jumper Wires (10cm, female-female)

First, check whether your soldering iron's body is earthed. If it isn’t, the stand detection logic won’t work.

If your soldering iron’s power plug has no earth pin, it is definitely not earthed. If it does have an earth pin, use a multimeter in continuity mode to verify. Place one probe on the earth pin and the other on the metal part of the iron that heats up. If the multimeter beeps, the iron is earthed, and the detection logic will function correctly.

To be honest, this project didn’t work for me initially, but I found the issue along with a solution. If you're planning to build this, I recommend checking this out before proceeding.

Issue:

The system occasionally resets when turning on the soldering iron via the relay. This occurs due to a sudden current surge when the relay connects the iron to the live wire, causing a voltage dip that resets the microcontroller. However, when no load is connected, the system operates perfectly fine.

Solutions:

• Use a Solid-State Relay (SSR) (≥2A) – More stable, efficient, faster, and durable than mechanical relays.

• Upgrade to a 5V, 5W Power Supply – Handles current surges better than a 5V, 3W module.

• Add a 100μF Electrolytic Capacitor – Connect across the power supply terminals to smooth voltage fluctuations.

To download the code, visit my GitHub repository:

🔗 GitHub Link

Possible Upgrades: For more practicality and versatility, consider adding an adjustable soldering iron-in-stand timer so users can modify the 5-minute timer using the encoder.

However, the 16x2 LCD lacks space to display this setting. A 0.96" OLED display is a better alternative as it provides more display headroom, allows flexible customization of displayed information, enables cool startup animations, and enhances readability. Additionally, it is almost the same price as the 16x2 LCD display.

Connection Diagram: The full circuit diagram is available in my GitHub repository, but I’m also including it here for convenience.

Important Notes: In the diagram, a push button is used to simulate the iron-in-stand detection logic. Two 1N4007 diodes should be used in forward bias—one after pin D6 and one before GND with a 1 KΩ resistor. This detection logic works because my soldering iron is earthed, but if yours is not, it will not be applicable. The diodes help block AC interference, ensuring stable operation.

Always test the system on a breadboard with an AC load, such as a light bulb or the soldering iron itself, before finalizing the build.

For testing purposes, you can connect the system to the soldering iron stand by soldering a wire between the detection pin of the Arduino Nano and the metal stand, ensuring it is electrically connected to the soldering iron when in contact.

I designed a PCB for this project but used a perf board for my prototype. The Gerber files are available on my GitHub for anyone who wants to fabricate a PCB. I have also attached here a true-scale PDF of the PCB design.

🔹 Tip: Take a modular approach so you can modify the system easily in the future.

I used a 4x4 inch PVC electrical box for the enclosure. The most challenging part was cutting out the area for the LCD display and the AC wall socket. Using a small OLED display is a better option as it has a smaller footprint, making installation easier while also improving the overall look.

For a cleaner design, consider using a 3D-printed case. Ensure all components fit properly—mine was a tight fit.

Before finalizing, ensure everything works as expected.

For the soldering stand, make sure the metal heating element of the soldering iron is in direct contact with the metal stand. If the stand has paint where the iron rests, scrape it off to establish proper electrical contact.

Connecting the system to the metal stand can be tricky. I used two bolts—one fixed onto the project enclosure and properly connected to the detection pin on the Arduino Nano from the inside, and another secured onto the soldering stand by screwing it into an existing hole with a nut. I made sure there was no paint between the bolt and the stand to ensure a solid connection. To link the stand and the system, I used a wire with crocodile clips on both ends, allowing for easy disconnection and modularity.