The Soldering Iron Controller for Hakko T12 Tips © LGPL

Per-tip calibration and high-frequency temperature check by Timer1 for a more accurate soldering iron.

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Components and supplies

Necessary tools and machines

09507 01
Soldering iron (generic)

Apps and online services

About this project

The Project

This is the second generation of software sketch for soldering iron controller for Hakko T12 tips. This version supports per-tip temperature improved calibration by 3 points (200, 300 and 400 degree of Celsius), Timer1 interrupts driven high-frequency PWM and interrupts driven high-speed temperature checking procedure (10 times per second).

The Soldering Tips

The Hakko T12 soldering tips are so amazing tools that should be widely used. They can keep the required temperature within the small interval, are compact and are heating extremely fast. Usually it takes about 12-15 seconds to reach the desired temperature. In this project I will show how you can build own non-expensive digital soldering controller based on Arduino and invest several interesting details. The main feature of the controller is PID algorithm, that allows to smoothly supply the power to the iron and keep the required temperature very accurate.

Key Features

The key features of this controller are:

  • Each tip can be calibrated separately by 3 reference temperature points to increase the accuracy.
  • The PID algorithm is implemented to keep the desired temperature of the soldering iron. The iron raises the temperature in about 12-15 seconds and keeps it inside 1 degree of Celsius. The controller keeps the temperature in case of heavy usage because the PID algorithm is very sensitive and can quickly increase the supplied power.
  • The controller supports two working modes: keep the temperature and keep the power supplied.
  • The controller is silent. The Timer1 working at 31250 Hz is used to generate the PWM signal.
  • The controller implements accelerated rotary encoder. When the encoder is rotated slowly, the temperature setting is changed by 1 degree. When the encoder rotated quickly, the temperature setting is changed by 5 degrees.
  • The preset temperature is stored in arduino EEPROM after the soldering iron was used in case the temperature settings were changed.
  • The controller saves in EEPROM units used for the displayed temperature (Celsius or Fahrenheit), preset temperature and calibration data.
  • Automatic switch-off in case of idle operation is implemented in the controller.

Step 1: The Connector

First problem was the connector, built on the soldering iron. It was so rarely used, I could not find the socket for it, so I used another pair of connectors that suit the characteristics of the soldering iron. This is aviation plug GX12-5 with 5 pins. The wires of the iron handle are: black - minus, green - plus (24v), red - should be ground. Actually, I did not attached anywhere the red wire. The main two wires are the black one which is connected to the ground of the controller and the green one which is connected to the plus pin.

Step 2: Connect the Sensors

Second problem was one that the iron has only two pins to connect to the controller. The iron has the thermocouple inside that is connected consequently with the heating element. The thermocouple generates a small amount of electricity, about 2-9 millivolts depending on the iron temperature. The black wire is minus, the green wire is plus, do not change them. To register this electricity accurate operating amplifier IC must be used. But, the iron should be powered through the same two wires. No one operating amplifier likes 24 volts on the input pins! Fortunately, some qualified people have solved this problem already. Here is the soldering iron controller for the hakko t12 iron. The schematics have brilliant idea to use the zener diode (vd6) to limit voltage on the input pin of the operating amplifier. As you can see on the picture above, In this controller 3.3 v zener diode is implemented.

The controller has two parameters to check: the iron temperature via first amplifier on pins 1,2,3 and the current that is going through the soldering iron via the second amplifier on pins 5,6,7. First amplifier should be tuned by 500k potentiometer at least for the first time. This variable resistor should be tuned the following way: when the iron is 450 degree of Celsius, the output voltage should become near 5 volts (reading of A0 pin in the arduino near 850-900). To simplify the calibrate procedure the tune mode is implemented inside the controller. The tune mode can be run from the settings menu. The tune procedure usually runs only once: it is implemented to help adjusting the variable register so the controller could correctly check the iron temperature.

The second amplifier is used to check the soldering iron is connected to the controller. The controller switch on power to the iron and checks the current is going through it. The 1uF and 100n capacitors increase the readings stability. The current is checked permanently when the iron is powered on and is checked every second by the small power every second when the iron is switched off.

Step 3: Make It Silent

Arduino Controller

To supply the power to the iron, the P-channel mosfet transistor is implemented. Arduino controller can manage this mosfet throughout the PWM signal. The Arduino software implements the function analogWrite() to manage PWM pins. Unfortunately, the frequency of the standard PWM signal is 490 Hz. If this function would be implemented, the controller would generate the noise while the power supplied to the iron. But who needs such a noise? To make the controller silent, Timer1 running at 31250 Hz is implemented.


Each mosfet has some capacitance on the gate. This capacitance is really matters when we need to switch the mosfet quickly. Here you can find the detailed description of the problem along with the schematics that allows manage the mosfet at the high frequency. The schematics of the heater is shown in the picture. To increase the speed of mosfet, the two-transistors driver is implemented on bipolar transistors. You can use another transistor pair. The fr304 diode does not allow the backward current when the mosfet is closed. You can change this diode with another high frequency impulse diode.

Step 4: Assemble the Controller

You can divide the controller into two pieces: hight-voltage heater controller and low-voltage sensor controller. The high-voltage heater consists of the mosfet and its driver and fr304 diode. The low-voltage part can be placed on the PCB that plugged directly to the lcd screen (just behind the screen).

Do not use the DC-DC converter or non-isolated power supply to get 5 volts for the controller. These devices can have small distortion on the ground and there can be negative voltage on the input pin of the amplifier. In that case, it is not possible to check the iron temperature. You can buy another isolated power supply or build the one by yourself from old transformer power adapter and 7805 voltage regulator.

Finally, burn the sketch to the controller.

Step 5: The Controller Menu

As I mention before, the controller has several modes:

  • standby mode
  • main working mode (keep the temperature)
  • power mode (manually regulate the power)
  • setup mode
  • tip calibration mode
  • tip selection mode
  • tune mode (tune the potentiometer)

When the controller is just powered on, the standby mode is activated. In this mode the soldering iron is powered off and the main display shows the following information:

  • The preset temperature in the left side of the upper line (in units selected - Celsius or Fahrenheit);
  • The 'OFF' message in the right side of the upper line indicating that the iron is not powered;
  • The current temperature of the iron in the left side of the second linee;

When the iron become cold after it was used, the 'cold' message is displayed on the second line. The preset temperature can be adjusted by rotating the encoder handle while the iron is powered off.

Sometimes the active tip name is displayed in the upper-left corner of the standby screen instead of the preset temperature.

Tip Selection

Tip selection mode is activated from standby mode when the iron tip (or iron handle) disconnected from the controller. When you remove the tip from the handle, the controller allows you to select new tip from the list menu of supported tips. This list can be adjusted in the sketch manually by editing the source code. As you can see, by default the sketch supports 5 different tips. You can change this number and you can supply new names of your tips (each tip name could have up to 4 letter in its name).

Just select new tip name from the menu list before plug the tip in the handle.

Iron Control

To power on the soldering iron, push the encoder handle lightly. The controller will be switched on and the main working mode activated. Now the controller keeps the iron temperature near the preset temperature. When the iron is heavily used, the temperature can deviate from the preset one. In the main mode the display shows the following information:

  • The preset temperature in the left side of the upper line (in units selected - Celsius or Fahrenheit);
  • The 'ON' message in the right side of the upper line indicating that the iron is powered on and is heating. When the iron reaches the preset temperature, the 'rdy' (ready) message will be displayed. The 'wrk' message indicates that the iron is in use;
  • The current temperature of the iron is displayed in the left side of the second line;
  • The power supplied to the iron (in percents) is displayed in the right side of the screen;

By rotating the encoder it is possible to change the preset temperature. The message 'ON' will be showed again till the iron reaches the new preset temperature. To return to the standby mode, press the encoder handle lightly.

In the main working mode, the controller checks for the current through the iron. If the current would not detected, the controller switches off the power and the message 'Failed' would displayed on the screen. Light press the encoder button to return to the standby mode.

Power Mode

The power mode can be switched on by long press the encoder while it is in the main mode. In the power mode, you can manually adjust the power supplied to the soldering iron by rotating encoder. In the power mode the screen shows the power supplied to the iron in the internal units (0-255) and the iron temperature in human readable units. Light press of the encoder toggles on-off power in the power mode. Long press of rotary encoder handle will return the controller from power mode to the main mode.

Setup Mode

To get to the setup mode, long press the encoder in standby mode. In the setup mode the configuration parameters can be adjusted. There are 6 menu entries in this mode:

  • Automatic switch-off timeout is minites [3-30]. This feature can be disabled. To work properly you may need to tune the PID algirithm parameters manually.
  • Temperature units (Celsius or Fahrenheit).
  • Tip calibration ('tip cfg').
  • The iron potentiometer calibration ('tune').
  • Save the changes.
  • Cancel the changes.

Rotate the handle to select the menu item. To change the selected item, press lightly the encoder handle. After adjusting the parameter, press the handle again to return to the setup menu. Long press of the encoder handle can be used to return back to standby mode and save the parameters in the EEPROM. Or the parameters can be saved, by selecting the 'save' menu item.

Step 6: Tune the Controller

You need an external thermometer to calibrate the controller.

As it was said before, the controller has to be tuned at least once. The potentiometer 500k should be adjusted accordingly. When the iron reaches the temperature of 450 centigrade, the readings of the A0 analog pin should be 850-900. To simplify calibration procedure, the tune mode is implemented in the controller.

When the tune mode activated the iron is powered-off. Lightly press the encoder in the tune mode to toggle the iron on or off. Rotate the rotary encoder to switch the iron on or to change the supplied power. The controller displays the temperature readings in internal units (readings from A0 pin) in this mode. Rotating the encoder adjust the power to keep the 450 degrees of centigrade (use external thermometer). At the beginning you can increase the power to the maximum value to speed up the iron heating, then decrease the power to keep the temperature at the 450 degrees. You need to adjust the power to keep the iron temperature as near to 450 degrees as possible.

Now rotate the variable resistor handle to get the sensor readings near 900. Ensure that this readings is not the maximum one, rotate the variable register and get 930-950, then return back to 900. It is important because the controller should be able to measure the temperature greater than maximum value. When the variable register adjusted, press and hold the encoder for about 2 seconds to return to the setup menu. The controller calibration successfully completed.

Step 7: Calibrate the Tip

You need external thermometer to calibrate the tip.

The Hakko T12 tips have different parameters and have to be calibrated separately to keep the temperature more accurately. You can calibrate the currently selected tip. To change the tip, return to the standby mode, remove the tip from the handle and select new tip name from menu list rotating the encoder. Asterisk ('*') in the lower-right corner of the display indicates that this tip has not calibrated yet.

Each tip calibrated by three reference temperatures: 200, 300 and 400 degrees of Celsius. To calibrate the current tip, select 'tip cfg' menu item from the setup menu. The controller will be switched to the calibration mode. In this mode first select the desired reference temperature (200, 300 or 400 degrees of Celsius) then lightly press the encoder. The iron starts to heat and the controller starts to keep the iron temperature near the selected value. As soon the iron reaches the selected temperature, the buzzer will beep. This indicates that the controller is ready to adjust the temperature at his reference point. Use external thermometer to evaluate actual temperature of the iron, then rotate the encoder to adjust the reference point to the controller. Then press the encoder lightly. The reference point will be saved. Select all or some temperature reference points to adjust. To finish the tip calibration, press the encoder button for about 2 seconds.

As was mentioned before, the tip list can be adjusted in the software sketch. You can add mode tips to the list and select new names of your tips. Tip name could be up to 4 symbols length.

If the tip is not calibrated, the calibration points are avaluated as the average value of all another tips that are calibrated. If there is no one tip calibrated in the controller, the default calibration points are used.

Step 8: Automatic Power-off Feature

The automatic switch-off feature has been implemented in the controller. This was a great challenge because the iron handle has no shock sensor or other way to verify correctly the iron is in use now or just is laying down on the table. The main idea was to use the value of supplied power to the iron. In the idle state, the controller decreased the power to the minimum value to keep the required temperature. Unfortunately, the math dispersions of the temperature and the supplied power are not constant in the idle state and can periodically change. Tuning the PID algorithm parameters, I have stabilized and minimized both dispersions. The controller in the idle state now keeps the temperature dispersion as low as possible. The criteria of the iron usage is the power has been incremented slightly from the smallest value registered. This algorithm has been tested for a while, and I suppose can be used for automatic power-off feature.

This solution is not very reliable, so you can disable this feature if something will be going wrong. Also, the sketch contains the class for debug and tune the PID parameters manually.

You can implement another iron handle in your version of the controller, the handle can have the shock sensor or similar to surely distinguish the idle state.




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