DIY Soldering Station

Project for Pre-Fab Academy

Did I make an entire soldering station myself? really?. No, I just made the controller, the iron has to be obtained separately. Though this controller is made to be used with Weller PES51 pencil , the idea and the circuit can be used for any pencil with minor modification in the circuit and firmware, be it Weller, Hakko, Goot etc. And most importantly, I was not alone, I had help from my friend and fellow trainee (at Pre Fab Academy), Yadu, he did most the micro-controller interfacing and coding, like LCD interfacing. Though I started it as a solo project, later we decided to to do it together.
Okay now let's read about the development.



First of all thanks to whoever selected and sent the the equipments to the FabLab here. He/she was the reason for this project, why? In India we have 230v and almost all the equipments here works on 110v. Most of these machine (including a 2KW electric furnace) require a transformer to convert the 230v to 100v. Though there is a 'big' sticker pasted on almost all equipments saying that it need 110v AC, some 'illiterate' 'idiot' will come and plug something to the 230v supply, and BHOOOM, he just contributed something to this pile of crap.

Most of the these items are a result of carelessness

So one day in the Fab Academy we had to make the FabISP and one of the two soldering station is not working. My friend Yadu told me that he saw someone plugging it into the 230v wall socket. So we were almost sure that it got fried, so me, Yadu and our trainer Mr Luciano tried opened the controller unit expecting a blown fuse. To our surprise this $100 equipment didn't have fuse inside. On inspecting further we found that the transformer primary winding of the transformer and the voltage regulator inside the circuit board is fried. The micro-controller was not getting any power even if we supply 24v to the board directly, using a lab power supply.

Inside-out: The disassembled station. This image is taken long after we started the project so all the wires are disconnected from the board.
Now we are left with only one soldering station, I am not waiting for anyone and they are slow. So I decided to use the lab power supply to power the pencil directly, so that I can just finish soldering my circuit and I don't have to wait for anyone. That was the motivation, I immediately remembered Mr Luciano talking about the project we were supposed to do as a part of the academy. I just got mine.


The pencil had 5 wires coming out of it, I knew that the pencil had a heating element, a temperature sensor and the pencil's rubber grip is grounded for ESD protection.

The 5 pin connector.
5 wires
The 5 wires coming out of the front panel connector. Notice the green wire with a screw on connector, thats definitely the ground wire, for ESD protection.
I had to know which wire is which. 'Googled' for some time, but couldn't find the answer I was looking for, seems that this model has many revisions. Now I need to find out what each of the wire is for. But I got some other information retarding the sensor used, very likely to be a K-type thermocouple.

5 wires, so the guess would be, 2 for heater, 2 for sensor, and 1 for ESD(grounding). On inspecting the connections, I find that the green were is connected to the earth wire of the mains via the body of the transformer, so that wire clearly is the ESD. One down 5 more to go.

Then I inspected the circuit board and followed the traces hoping identify the wires.

logic top mine
The top side of the logic board from our station.
logic bot mine
The bottom.

I was so excited about the project, so much that I forgot to take a any pictures before removing the pot and the wires from the front-panel. You can also see that I broke some of the traces for testing purposes. I had completely that I was supposed to document this project. So I'm posting two pictures from this page for reference. This guy took good quality pictures, thanks you Triden.
You can see that the two thick traces from the pin one and two of component with the heat sink. I will be mentioning about these soon.
logic top not mine
Top side of Triden's board.
logic bot not mine
And the bottom.

See that big component with a heat sink? It's FET or a power transistor (very likely a MOSFET) which controls the power flow to the heater. Component is marked BTA12, and I found this data-sheet . I was wrong! it's not a FET, not a transistor; It is a Triac, a Silicon Bidirectional Thyristor. The control circuit/micrcontroller will be controlling this component via a PWM (Pulse Width Modulation) signals to the heater power. So?, follow the traces from the Triac, see the two thick traces from the Triac?, from pin 1 and pin 2?, these wires must be going to the heater. Following the trace further, I found that one these traces goes to the transformer secondary via a shunt resistor. And the other goes to one of the RED wires from the front-panel. So the other RED wire must be the going to the second wire of the transformer secondary, right? Yes it does and the circuit is complete. So the RED wires are going to the heater. The clues were, thick traces to carry high current (about 2A), direct connection to the 24v AC output of the transformer (no digital component/sensor will be powered by the unregulated AC supply).
Lets be sure about it, the pencil is rated for 50W at 24v, means it draws about 2A at 24v. So the heating element must have a resistance lower than 12 ohms, lets see.
I set 10 V at a maximum current limit of 2A with the power supply and connected the supply to the RED wires. The pencil drew only 0.94 A, which means the resistance is about 10/0.94 = 10.6 ohm. Just like we expected.

With just two wires remaining it is obvious that these are going to be the sensor leads. Now what type of a sensor is this?
If the sensor is resistive (the resistance changes with temperature), PTC, NTC, PT100 etc it must have a high resistance, ranging from 50ohm for an RTD to 10s of Kohms for a thermistor. . And if the sensor is a thermocouple (produces a potential difference proportional to the temperature), it must have a very low resistance. When measured I found a very low resistance, almost short circuit so it must be a thermocouple and my 'googling' also supports this guess.

Lets check the thermocouple, the information from Wikipedia and NIST (Nationla Institute of Standards and Technology) data tells that about 4 mV potential difference can be expected across the terminals at about (100℃ + room temperaure). (100℃ + room temperaure) because the thermocouple can only measure the temperature relatively. It is made of a junction between two metals and the potential drop produced by the junction depends on the temperature difference between the metals. In case of the soldering iron one of the metal is at the tip temperature and the other will be at room temperature. So the tip temperature is measured relative to the room temperature.
This tells us that even at the maximum temperature of the iron, the potential difference by the thermocouple will be under 20 mV.

Stage one: the makeshift soldering station

Okay, I don't have the time to make the control unit now, I need to make the FabISP.(don't runaway I'm going to make it, keep reading) So I set the power supply to a maximum of 24v and connected to the heating element (RED wires). The thermocouple is connected to the oscilloscope to monitor the 'temperature' (actually the potential differance). The DSO is set to read the average DC voltage across the thermocouple.
This one is a really expensive soldering station, but it will do for the day. Okay, now, picture time....

The makeshift soldering station.
The wiring: RED wires are connected to the power-supply and the yellow and orange wires to the DSO.
me and station
Thats me with he setup.
me yadu and station
Selfie by Yadu, while I'm using the setup.

To make use of this setup we need to hold the pencil at a safe temperature and we don't know what value of the DSO reading corresponds to that temperature. The fix? I connected the pencil to the working controller and let it heat up to the set value, disconnect and immediately connect to the socket of our make-shift soldering station, whatever the DSO displays corresponds to the temperature we need the pencil to be at. Tweaking the current limit dial in the power supply, I found the current required to keep the pencil at this temperature, 0.9A to 1.0A.
The setup is working and I could solder my FabISP. I also used this setup to de-solder the board later, and use the components to make another board, FabISP Key. But the signal is too weak compared to the noise. I have to amplify the signal.

Stage 2: Op-Amp

I will be making the controller using one of the atmega micro-controller available in the lab. Most likely a Atmega168p or Atmega328p. Its pretty obvious that we have to scale (amplify) the 1-20mv to 0-5v for measuring the thermocouple potential via the ADC. This process obviously requires an amplifier, so time to make one. But one problem, the components in the FabLab are only available SMT packages. So prototyping is going to be challenging.

I made a simple non-inverting amplifier using the op-amp AD-8615, few filter capacitors and the potentiometer from the original Weller logic board. And this little creature was born.

The schematics of my first om-amp prototype along with some of my notes.
signal before
The DSO screen with the reading from thermocouple.
op-amp on the way
Prototyping the op-amp.
The prototype is ready; this is a nice setup isn't it?.
prototype zoom
Zoom out.. This is not nice, its a mess, but it works.

Stage 3: Op-Amp 'onboard'

Its time to take the project to the next level, time to make a proper PCB for the amplifier. I did a lot of research and came across an application note from Microchip, link here and here. The app-note describes a differential amplifier using two op-amps to amplify the signal from a thermocouple to 0-5 V range. So I used this design as a reference for my design. The board I designed has an amplifier setup described in the app-note, a 10K NTC temperature sensor for measuring the absolute room temperature and the MOSFET, RFD16N05L (16A, 50V, 60W continues, logic level N-channel-MOSFET). All five wires from the pencil goes to this board. The board has two outputs for the ADC, basically the absolute and tip temperatures. The board also requires two levels of voltage, 24v, 2A supply for the heater and 5v for everything else. I also made a large copper pad which act as heat sink for the MOSFET. The design uses a double sided PCB and the bottom layer is used only as a ground plane.

The schematic of the amplifier board.
sch brd
The top layer of the board (bottom is just a ground plane, no traces. The outer rectangle is drawn in milling layer and this will be used to cutout the board after milling.

Eagle files

Schematic. Board.

From this design we milled the board using the Roland MDX-20 mill in our lab. The image of the board (top, pads, vias and milling layers) is exported as monochrome .png and edited using Gimp to make the following images. These image were used to mill and cut the board. The fab modules are used to control the machine. Once the image is loaded and correct settings are chosen the software predicts the tool path so that the white area in the image is preserved.
The file used for milling the traces using the Roland mill.
cut and drill
The file used for drilling and cutting the board.

One thing to keep in mind if you are using the image files to mill the board, find out what the actual dimension of is from the .brd file in the eagle. Use this dimension to scale your image.

Few more pictures and videos.
An interesting animation of the last stages of the design, actually the routing.
failed board
A failed board, vias came out, because vias where too small. Time to redesign, with bigger vias.
during milling
The final board is getting milled.
after milling
The board milled and cut, ready to be populated.

Even after the redesign there was a silly mistake, at some places the traces where too close that the milling bit didn't pass through, so they were still connected, I had to use a knife to fix it before soldering.
fixing short
The patch work: used the knife to fix the short. There were two shorts, one for each of the op-amps.
populated board top
The board populated with all components (except few wires and the NTC sensor section at bottom left).
populated board bottom
Bottom side of the board, you can see the 'DIY' interconnects. Well, whats with the texture you may ask; I used sand-blaster it, why not?. It had lot of fingerprints and other dirt on it, so 'I did what I have to do'. (or!!! what I wanted to do).

DSO amplifier board
Amplifier board is working: the noisy yellow channel is the direct reading, before the amplifier.

During this time my friend Yadu, was experimenting with interfacing the LCD to the FabDuino he made (I too made a similar board, not FabDuino, will talk about it later). He had some troubles prototyping the LCD interface; you can see the pictures in the next section and you will know what I'm talking about. In the beginning of the Pre-Fab Academy one of our trainer Mr. Luciano told us that every one of us should do an individual project, later because of the shortage of time and equipments, he said that we can do group projects, thats when Yadu joined the project, actually he was helping before too, but now it's officially OUR project. So he took over the micro-controller section, he started by interfacing the LCD and later he started working on the ADC interface and PWM control.
The FabLab or any lab, needs some basic stuff in their inventory, most importantly jumper wires, a lot of them, and all types(M-M, M-F, F-F).
Anyway it's time to merge our projects.

Stage 4: Putting the pieces together

Lets give our project a display and a 'brain'. Yadu had the LCD interfaced with the FabDuino on a breadboard. We connected the amplifier section to the micr-controller and tried to read the temperature. We got the ADC values displaying, that was easier done than said. But converting the ADC values to the temperature was not easy. We had to calibrate, for that we needed some source of known temperature or a thermometer. We had none. So I came up with this crazy setup to get at-least one calibration point, the boiling point of water.

pencil in water
The pencil in hot water; the pencil is also being powered to maintain keep the water boiling, this should keep the tip temperature constant.
The heater will be kept powered, and the tip will be kept immersed in water. This will result in a maximum tip temperature of about 100℃, the boiling point of water. The water will never allow the tip temperature to rise above 100 boiling point, till then all the heat will be used to rise the temperature of the water and the tip itself. But once water starts boiling all the energy will be absorbed by the water to boil, no heat will be used to rise the temperature until all the water is boiled off.

Okay, nice, but how do we use this for calibration ?
Well, we have the tip at a constant and 'known' temperature, let us assume that this is 100℃. Now we get the corresponding ADC reading through the display, so we know that this ADC readout corresponds to 100℃, to be precise (100-room temperature), because as we discussed before, the thermocouple measures relative temperature. There should be some way to get the room temperature, a thermometer, but what if you don't have one?. I used my smartphones's temperature sensor after keeping the phone in sleep for some time to let it cool down to almost the room temperature.

So, once we have the ADC reading and the room temperature, we can calculate the conversion factor for converting the ADC value to the temperature.
conversion.factor = (100-room.temperature) / ADC.value .
tip.temperature = ADC.value * conversion.factor + offset
where offset is the room temperature
I accept, this is a very bad calibration and assumes linear relationship between temperature difference across the thermocouple junction and the potential difference it produces. But we didn't have any choice. We have to add the NTC and start using it ASAP, but I fear even this may need a calibration.
Anyway this method will give an approximate tip temperature reading.

after callibration
After callibration; forgot to add the offset, thats why the boiling temperature of water is at 72℃.
pencil heated
The pencil at about 400℃ and display showing the value.

            // initialize the library with the numbers of the interface pins

            LiquidCrystal lcd(7, 6, 5, 4, 3, 2);
            int rawin = 0;
            float  temp =0.0;

            void setup() {
              // set up the LCD's number of columns and rows:


            void loop() {
              // set the cursor to (0,0):

              lcd.setCursor(1, 0);
              // print from 0 to 9:
              lcd.print("Temp in Celsius");

              char a[32]= "Hello dude, this is Fab Academy";
              for (int i=0; i<32; i++) {
                for (int j=i; j<32; j++) {
              rawin = analogRead(A0);
              temp = rawin * 0.22 + 20.0;

We used the 16X2 character LCD (LCM-S01602DTR/M). For the LCD interface the connections are..

            * LCD RS pin to digital pin 7
            * LCD Enable pin to digital pin 6
            * LCD D4 pin to digital pin 5
            * LCD D5 pin to digital pin 4
            * LCD D6 pin to digital pin 3
            * LCD D7 pin to digital pin 2
            * LCD R/W pin to ground
            * LCD pins 1 & 2 to +5V and ground
            * LCD VO pin (pin 3) to ground

Also use the ADC0/Pin A0 for the input.

Stage 4: Get control

We are going to use the present tip temperature to control the tip temperature (what? well, thats how it is.) by controlling the power flow. This is done by switching the MOSFET on and off to control the duty cycle and thus the average energy flow (power). The duty cycle control is achieved using PWM (Pulse Width Modulation).

At first, we will be using a simple control, we set a temperature (at first in the program itself, not through the interface), then we switch on the heater if the set temperature (S.T) is greater than present temperature (P.T). And we switch of the heater if P.T > S.T. This method is not very safe as the tip temperature tends to oscillate around or below the S.T. This is because of the delay between the heater control signal and the actual temperature change. We control the heater by switching on and off, the MOSFET, depending on the P.T. When we switch on the heater element gets the full power and heats up very fast, depending on the placement of the actual temperature sensor we can only read this new temperature after a delay, this causes the temperature to overshoot slightly. This in-turn may produce an oscillating behavior. One fix would be to reduce the power to the heater at any given instant. Thus reducing the overshoot. The power-flow can be controlled by adjusting the PWM duty cycle.

During this time we actually made a three button control board to use in future, to set the temperature and control other functions. We plan to use the top and bottom buttons for temperature up/down and the middle button for menu election. On pressing any of the button corresponding interrupt codes will be executed to make changes accordingly.

control board
The three button control board.
control board schematic
The three button control board schematic.

Eagle files

Schematic. Board.