Soldering Iron Driver design overview

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Prototype PCBs are available in the free PCB drawer.

You can get a [project PCB for $10.00], including worldwide shipping.

The Soldering Iron Controller was designed by Arhi (Bogdan Kecman) and shared under public domain in our forums. With his consent we developed the PTH version 1.5 based on the 3rd generation of his driver.

From the get go the Soldering Iron Driver was conceived to drive Hakko 900M (clones) and Solomon irons. As an added bonus it was discovered Weller PES51 irons were compatible as well.

The key to this universal design is the analog front-end which is capable of reading both Thermocouple and PTC temperature sensors. The SID features:

  • Compatible with any PTC or TC soldering iron.
  • PID temperature control.
  • USB data transfer, and firmware update.
  • UART data transfer.
  • HD44780 2x16 character LCD.
  • external NTC temperature sensor, for calibration and better cold junction compensation of TC type sensors.
  • Low noise, zero crossing AC heater control.



  • History

Soldering Iron Driver works by using PID calculations to maintain the soldering iron tip at a constant pre-set temperature. This is accomplished usurping the analog front end to convert the sensor value into a 0-5V range which corresponds to 0-500 degrees C.

From there the microcontroller uses and internal ADC to read that value. It is then run through a PID algorithm to calculate the power that is needed to supply to the heater to achieve or maintain the preset temperature.

The LCD and buttons are used to calibrate, and set the wanted tip temperature, as well as to display the current tip temperature.



Click for a full size schematic image. Schematic and PCB were designed with the freeware version of Cadsoft Eagle, download the latest project files from our Google Code project page.



The Soldering Iron Driver uses the PIC18F2550 microcontroller (IC1). It is powered through a 5V DCDC converter. The power pin (VDD) is decoupled using a 0.1uF capacitor (C1), while the internal 3.3V USB power supply gets two 0.1uF capacitors (C2 and C3).

For stable USB operation a 20 MHz quartz crystal (XTAL 1) is used with 27pF load capacitors (C5 and C6).

For debugging, and future development purposes, RX and TX pins were broken out to the UART header.

There are two indicator LEDs. One is for power supply (PWR), and the other (HCL) indicates the current heater operation.

The ICSP programing connection is brought to a header for easy development. The MCLR (RESET) programming pin is connected to the power supply through a diode (D1) and 10K pull-up resistor (R1). The diode stops the 13volt supply using during programming from damaging the rest of the circuit.

Analog Front-end

File:SID HWO analog.jpg

Power Supply

File:SID HWO PS.jpg

Heater Control

File:SID HWO PS.jpg


File:SID HWO LCD.jpg


File:SID HWO CON.jpg



We used the freeware version of Cadsoft Eagle to make the schematic and PCB. Download the latest designs and firmware from the project Google Code page.

  • PCB and placement notes
  • soldering advise



Click for a full size placement image.

heading heading
cell cell
cell cell


The firmware is written in C and compiled with the free Microchip C18 compiler. You can download the latest files from our Google Code project page.

  • Operating modes
  • extra software required

We used the Microchip USB stack to run the 18F2550 as a virtual serial port. Microchip's code is open but not redistributable. If you want to compile the source, download the stack from Microchip, then drag the source code into the install directory. See the detailed instructions in the PIC compiler how-to.

.inf installation

The virtual serial port (CDC) is an open standard, it should work on any modern operating system.

You don't need a driver, but you will need a .inf file to tell Windows how to use the device. A suitable .inf is included in the project archive.



File:SID HWO boot.jpg

Source and license

Taking it further

Here's some ideas for the future:

  • List

We'll post the most recent firmware updates on our blog. You can also join the discussion in the [forum].

Get one!

You can [get one for $00], including worldwide shipping.

Your purchases at Seeed Studio keep the open source project coming, we sincerely appreciate your support!