Maker moekoe’s kind of soldering iron

Basic idea of this very simple tool is slightly different in comparison to any other soldering method. Here, the heat is supplied from the bottom of the PCB and not directly to the soldering pads and pins of the components.

The idea of building something like this came in my mind as my mom said to me, that I have to clean the garage and that all the scrap laying around there since several years has to leave this garage. Well, sometimes it is hard for me to break up with electronic scrap, but sometimes, like in this case, it is worth it to keep something, like that old flat iron which I found.

Parts you will need

  • PCB
  • An old iron
  • Wemos D1mini
  • Solid state relay 25A
  • Thermocouple sensor (MAX6675)
  • Thermocouple K Type
  • 128×64 pixels oled display
  • 5V DC power supply
  • Button, potentiometer 10k, 0603 10k resistor, 0603 0,1µ capacitor and wires
  • Piece of wood as a base and some screws

Something similar is the Reflow Skillet from SparkFun: They tested several methods to solder SMD components to PCBs and it turns out that their Skillet is the best solution for makers with a small budget. Inspired by this, my first thoughts about this old flatiron I found in the garage were to build a similar hot plate thing for soldering. And it turns out that this is working like a charm.

Concept of this is that the Wemos D1 mini controls the solid state relay based on temperature informations from the thermocouple sensor, which I placed with some heat resistant kapton tape directly onto the iron (actually the sensor should be placed onto the PCB for preciser temperature readings). If you have a really fast iron than maybe you should use something like a PID controller. For me and my very sluggish iron a normal threshold based control is enough for now. I added my simple threshold code to this repository.

It takes about 3 minutes for the whole soldering process, which is very fast, compared to some professional solutions like reflow ovens. Maximal temperature is something about 200-240 degrees, depending on several things like used solder paste, PCB thickness, …

– Works like a charm
– Very low cost (I found everything at home)
– Very fast soldering
– Simple to build
– Small frame size, suitable in every maker lab
– Works just for one side of a PCB
– Sluggish heat control (especially for the cool down)
– PCB has to be removed after soldering process has finished

Building your own

When connecting the 230V power supply to your board be careful not to switch some wires, like I did 😀

The three terminals are labeled with SSR for solid state relay, DC5V for the Wemos power supply and Thermo for the thermocouple sensor. When switching the thermocouple sensor wires, the temperature on the display will get smaller when the plate gets hot. If this is the case, simply switch the Thermo wires.

Schematics of the PCB

For the PCB I desoldered the MAX6675 thermocouple IC from the small breakout board and used it directly on the PCB to safe a little bit of space. All other components are just connected via pin headers to the PCB. The 5V power supply, the thermocouple and the solid state relay on GPIO 8 are connected via those screw terminals. Value of R! is 10k, value of C1 is 0.1µ and potentiometer is 10k as well. Of course, you don’t need this PCB, you can get it working with just this few components and some wires. My first version of the soldering iron was exactly like this, but you know, I love creating PCBs. 🙂

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