Universal Soldering Station

I created this prototype because I thought that my Yihua soldering station was broken. When that happened I started to reverse engineer that soldering station of which I made a video called “Reverse Engineered Yihua 937D Soldering station”. In parallel I decided to prototype my own soldering station based on the Yihua design which resulted in this Instructable.

This universal soldering station has some extra features compared to my Yihua soldering station and is capable of handling soldering iron handles from different manufacturers. In short the main features are:

• Variable temperature from 200 degrees Celsius to 450 degrees Celsius
• Two programmable temperature preset buttons
• Shows the set and actual temperature including a temperature reached indication
• Easy calibration without the need for a thermometer
• Error detection when sensor is broken or heater is broken
• Powered by an external DC power supply
• Simple and inexpensive design

I finished this working prototype completely but because I could repair my Yihua soldering station I did not create a final product. Due to the simple design you can easily build this design on a breadboard.

Due to a mistake in something I ordered I ended up with two different soldering iron handles, one from Hakko and one from Yihua. I discovered that they have different specifications which makes it impossible to use them on the same soldering station even when you leave out the difference in connectors.

The differences are in the resistance values of the sensor and the heater and in the used connector:

· The Yihua 907a: Sensor 10-20 Ohm, Heater 1-2 Ohm, female connector

· The Hakko 907: Sensor 43-58 Ohm, Heater 2.5 - 3.5 Ohm, male connector

The connector connection differences are given in the picture.

This universal soldering station is capable of handling both soldering iron handles, connectors excluded.

The schematic diagram shows the design of the universal soldering station.

You need the following electronic components for this project:

• 1 * PIC microcontroller 16F1829
• 1 * operational amplifier LM358
• 1 * voltage regulator LM7805
• 1 * MOSFET IRF4905
• 3 * 7-segment displays, common cathode
• 1 * electrolytic capacitor 10 uF/16V
• 4 * ceramic capacitors 100nF
• 1 * potentiometer 10k
• Resistors: 3 * 10k, 1 * 4k7, 1 * 3k3, 1 * 2k2, 6 * 1k, 1 * 470 Ohm, 3 * 100 Ohm
• 1 * LED, high brightness
• 1 * BC547 transistor or equivalent
• 3 * push button switch
• Optional: 1 fuse 3A / Slow

You can build the circuit on a breadboard and does not require much space, as can be seen in the picture of the prototype. The electronics – without the heater – draws a current of 20 mA. Although a 78L05 voltage regulator would be able to create the 5 Volt supply voltage, it is better to use the mentioned 7805 due to the high input voltage which causes a heat dissipation of about 0.4 Watt when the supply voltage is at its maximum of 24 Volt.

Note that you can also use a lower supply voltage than 24 Volt which will lower the heating power. I used the prototype with a 12 Volt power supply, created from an old PC switch mode power supply, which worked quite well. The heater was then initially drawing 2 Ampere so the power dissipation in the heater was about 24 Watt. The heater in this soldering handle is capable of providing an output of 48 Watt.

I also tried the power supply of my laptop to test the operation of this universal soldering station. This power supply has an output voltage of 19 Volt and can easily deliver the 2 Ampere. When the heating element is cold, it draws about 2.4 Ampere at 19 Volt but when the heating element warms up the current drops to about 1.5 Ampere.

The software provides the following functions:

• Variable temperature control between 200 degrees Celsius and 450 degrees Celsius
• LED indication when the soldering station is warming up, cooling down or when the set temperature is reached
• Temperature display showing the set temperature and the actual temperature
• Storing and retrieving temperatures using 2 preset buttons. One additional button is available to switch back to the variable mode in which the potentiometer sets the temperature
• Storing the last selected mode (variable, preset 1 or preset 2) and restoring it at power up
• Calibration of the soldering station. The calibration setting are retrieved at power up
• Error detection when sensor is broken or when the heater is broken
• To prevent any malfunctioning of the soldering station, the watchdog of the PIC is used. The software has to trigger the watchdog frequently as to prevent that it generates a reset

For temperature control the integrated internal Analog to Digital Converter of the PIC is used which measures the voltage of the sensor and the voltage of the potentiometer.

Something worth mentioning is the multiplexing the 7-segment displays. Often 7-segment displays are multiplexed per display which requires one resistor per segment so 8 resistors (decimal point included) in total. In this design the display is not multiplexed per 7-segment display but per individual segment per display. The advantage of this way of multiplexing is that you only need 1 resistor per display so 3 in total. This also reduces the power consumption of the electronics since only 1 segment is on at the same time.

When the temperature is changed, either by selecting another preset or by changing the position of the potentiometer, the new set temperature will be displayed. After 3 seconds the display will switch back to showing the actual temperature.

When the soldering iron is heating up the heating LED will be turned on. When the set temperature is reached, the LED will start blinking. If the set temperature is lowered while the actual temperature is higher, the LED will be turned off. During warming up or cooling down the actual temperature is displayed. Once the set temperature is reached, the display will show the set temperature. This is done because the actual temperature is constantly changing and that can be quite disturbing on the display.

When a certain temperature is set with the potentiometer it can be stored by pressing one of the preset buttons for at least 2 seconds. When pressing the preset button shorter, the preset temperature of that setting will be retrieved. Note that when a preset temperature is used, changing the setting of the potentiometer does not have any effect until the variable button is pressed again.

The universal soldering station can show various messages on the display:

• The set and actual temperature in degrees Celsius
• The message ‘CAL’ is shown when the soldering station enters the calibration mode, see the section on calibration
• The message ‘C-L’ is shown in calibration mode to indicate that the low calibration setting has to be set, see the section on calibration
• The message ‘C-H’ is shown in calibration mode to indicate that the high calibration setting has to be set, see the section on calibration
• The message ‘E-S’ is shown when the temperature sensor is broken. This can be a short circuit of the sensor or an open connection of the sensor
• The message ‘E-H’ is shown when the heating element is broken. This is detected when the heating element is turned on but the heating iron temperature does not increase.
• The message ‘E-C’ is shown when the calibration is not done correctly, e.g. when the high calibration setting is lower than the low calibration setting

When a sensor error and heating error occurs, it can be reset by pressing the variable key. If the error still persists, it will be shown again on the display.

The software for the PIC is programmed with the JAL programming language. The Intel Hex file for programming the PIC are attached. Due to an error reported during uploading I was not - yet - able to upload the zip file with all JAL files that are needed for this universal soldering station.

It is possible to calibrate the universal soldering iron without a temperature meter. The calibration process is started when the station is powered up for the first time and can be re-started by pressing the 2 preset buttons at power up. During the calibration cycle the display will show the values of the ADC and not temperatures. This is also indicated by the fact that on all 7-segment display the decimal points will be on until the calibration cycle is complete after which the temperature is shown again on the display.

This calibration method assumes that the temperature sensor in the soldering iron has a linear behavior with the increase in temperature which seems to be more or less correct.

The calibration process is as follows:

• For calibration we need two measuring points one for 20 degrees Celsius and one for 300 degrees Celsius.
• The calibration cycle starts with the message ‘CAL’ after a few seconds followed by the message ‘C-L’
• For the lower calibration value we start with a room temperature of 20 degrees Celsius and we store the current ADC value shown on the display as the low calibration value (C-L) by pressing the variable button. The low calibration setting is then stored in the EEPROM of the PIC Microcontroller. No further action is required – nor possible - here
• After this the message ‘C-H’ appears which means that we need to enter the high calibration value.
• For this we first need to heat up the soldering iron by pressing preset 2 button until the solder (lead based) melts nicely on the soldering iron. We call this the 300 degrees Celsius point. This point gives us the second calibration point (C-H).
• Pressing the variable button again will store the high calibration value in the EEPOM of the PIC Microcontroller. Calibration is now complete. Temperatures higher than 300 degrees Celsius are calculated by the device using the two calibration points.

A remark about the 300 degrees as reference temperature. On my Yihua soldering station, lead based solder melts nicely at 300 degrees Celsius. Silver based solder melts nicely at 350 degrees on my Yihua. Of course you can use temperature meter to measure the actual calibration high temperature and then store that as the 300 degrees reference.

The final result can be seen in the video. The video explains the project and shows how the device works including calibration.

If you are interested in using the PIC microcontroller and the programming language JAL – a Pascal like programming language – visit the JAL website.

Have fun making this Instructable and looking forward to you reactions and results.