DIY (de)soldering station for $25

DIY (de)soldering station with heat gun for $25

In this article, a simple design of a soldering station is proposed. It consists of a few low cost components such as an arduino, an LCD display, an encoder and a homemade breakout board (2 opamps, 2 transistors, 1 TRIAC with a driver). The soldering station is open source Software and Hardware. So everyone can download and learn, study, edit, modify.

1.Intro 

Pic.overall view

I explain how to make and calibrate a simple soldering station. The soldering station is controlled by an arduino. 

Main features 

1) the soldering station's display showing both temperatures for the soldering iron and the heat gun also showing the fan's speed of the heat gun;

2) separate temperature control for the soldering iron and the heat gun;

(thermostats are implemented by using on-off controller);

3) control of the fan's speed of the heat gun.

Fig. structural block diagram of the soldering station

Pic. inside the box

Pic. Display menu

Pressing the knob (encoder) selects the active menu item (the soldering iron's temp, the heat gun's temp, and the fan's speed). Turning the knob changes the item's value.

2. Role of breakout board / Schematic

This is a board designed for controlling a soldering iron and a heat gun.
This board is helpful for connecting digital systems (like a 5V microcontroller or an Arduino) to a high-voltage system.

OpAmps are used for temperature measurements. They are implemented into the circuit as a pair of non-inverting amplifiers, which transform the thermocouples' voltages to the appropriate range for the arduino's ADC. Power MOSFETs are used as power switches. When the first MOSFET turns on/off, high current starts/stops to flow through the soldering iron's heating element. The second MOSFET is used for controlling speed of the heat gun's fan. A TRIAC with a zero-crossing driver are used for delivering high AC voltage to the heat gun's heating element.

Fig. Schematic of the breakout board

Fig. PCB layout

The schematics and PCB layout can be downloaded here (EagleCAD' files) 

3.Wiring diagrams

Pic. heat gun (hot air gun) pinout

Check out my video where I explained how to find out the pinout for the heat gun handle.

Pic. soldering iron handle pinout

Check out my video where I explained how to find out the pinout for the soldering iron handle.

Pic. encoder & display

Pic. breakout board

4. Program (a sketch for arduino)

Here you can find firmware for the arduino.

5. Calibration

Pic. Calibration

if a thermocouple is located near a heating element, it produces a voltage proportional to a temperature of the heating element (sensitivity is about 41uV/°C for a type K thermocouple). However, in practice we want to convert this voltage to the actual temperature of the soldering iron's tip that we use. So we need to calibrate our soldering iron station. This means we need to find a function that maps the given voltage to the temperature. Our arduino already maps the thermocouple's voltage to some number by design. Why? See the schematics and the wiring diagrams. We use the opamp to amplify the thermocople's voltage then we use the arduino's ADC to convert it to a number.


To calibrate the soldering station I use an external thermocouple with a multimeter that can measure temperatures. (Although other devices could be used)

The procedure consists of the following steps.

1. Rotate the soldering station's encoder to set a temperature value. Wait until the actual temperature stabilizes (the actual temperature = the set temperature). Let Xi be the set temperature.

2. Using an external thermocouple measure the temperature of the soldering iron's tip. Let Yi be the temperature measured by an external device.

3. Using different set temperatures repeat 1-2 steps a few times . i=1,2,...n

4. Now we have the set of points (X1,Y1),(X2,Y2),....,(Xn,Yn)

5. Find a function that maps Xi to Yi for all i=1,..,n.
If we have only 2 points (X1,Y1),(X2,Y2), then we can draw the line (Y=a*X+b) between the points. This line will be the solution to the problem.  If we have more than 2 points, we should use the method of least squares.

6. Use the function to convert ADC's numbers to actual temperatures.



Example.

We've followed the steps 1-4.
Now we have the following table

i	Xi set temperature	Yi temperature measured by an external device
1	40	52
2	80	95
3	100	125
4	160	195

We can use an online service to find the function.
Let's open wolframalpha.com. Then we should type the string "linear fit {40,52},{80,95},{100,125},{160,195}" in the edit box.

And here we are!

The solution for our problem is y= 1.202 x+2.56.

I used this function in the arduino's sketch to convert ADC's numbers to actual
temperatures.

Hints

1)Do the calibration in the vicinity of your working temperature.

2)If you don't have an external device to measure temperatures you can calibrate anyway (although with low resolution). For example you can use the melting point of a flux (rosin flux), a solder (for example, 60/40 Sn-Pb - 183°C),  the boiling point of water as reference points.

3)The soldering iron should be recalibrated after changing the iron, or replacing the heating element or tip.

4)The tip temperature will vary according to the shape of the tip. A less accurate method involves adjusting the temperature control knob according to the adjustment value for each tip.

6. Misc (some thoughts)


I don't like to drill extra holes in a fiberglass copper board. So I used SMD components but you can use full size components (or even assemble the station on a breadboard).

Instead of the MOSFETs you can use power BJT (Darlington transistors) for the same voltage range. But keep in mind then you probably should use additional heat-sinks for BJT to dissipate heat.

I recommend to use an encoder having threads and a fitted nut to easily mount it on the soldering station' front panel.

Use 1nF capacitors for the encoder's pins to filtrate switching noise (otherwise you should modify firmware to treat debouncing...).



There is a correlation between input DC Voltage and heating speed of the soldering iron. So if you want DC Voltage to be at 24V you should make additional DC 5V..15V for the arduino. Why? Because the arduino's linear regulator (in my case it was a LM1117) doesn't tolerate 24V. (For example, It can be done by using a DC-DC buck (or SEPIC) converter.)

Always cool down the hot air gun with the internal fan before turn it off. Its heating element will last longer.

For safety purposes, you should use a fuse in series with the heat gun's heating element.


7. Components

table of the components

devices	links
Arduino nano v3 clone (ATmega328P/ch340)	ebay aliexpress
IRFZ44N (MOSFET) *	ebay aliexpress
MOC3043 (TRIAC driver) *	ebay aliexpress
BT139 (TRIAC) *	ebay aliexpress
LM358D (Opamp)	ebay aliexpress
Hot air gun handle	ebay aliexpress
Soldering iron handle (Hakko 907 clone)	ebay aliexpress
Soldering tips (900M series)	ebay aliexpress
Encoder (having threads and a fitted nut)	ebay aliexpress
LCD 2x16 Display (Hitachi HD44780 clone)	ebay aliexpress

* Be careful with power devices. I had some unpleasant experience with broken MOSFETs (probably they were refurbished or counterfeit) which I bought from a high rated ebay seller...  maybe it's better to buy power devices from a trusted (re)seller or at your local trusted store where you can easily replace/exchange them. There are some other reports.

other devices

device	links
External device to measure temperature	ebay aliexpress
DC-DC SEPIC (buck/boost) converter	ebay aliexpress

8. Demo

 

Links

[1] This project on GitHub

[2] If you are interested in making a PID controller for this soldering station,
you should read this article: "PID without a PhD" by Tim Wescott

[3]Youtube videos:
       Soldering iron handle pinout
       Heat gun handle pinout
       How to make rework station/hot air gun for $15