I recently had a "Stahl Tools SSVT Variable Temp Soldering Iron Station" (cheapo) go bad on me. I wanted a digital soldering iron station and decided to use the housing and a few of the parts to make one that I could plug a purchased digital iron into. Looking around, I could get irons anywhere from $8-$20 USD. After looking around at a few other projects, I came up with my own rendition. TechBuilder has a good video and writeup found here: https://www.instructables.com/DIY-Digital-Soldering-Station/
I've breadboarded the circuit to check everything out. Things look good, but I'm seeing something weird with the thermocouple resistance on the voltage divider going into the OpAmp circuit. When disconnected to anything else, I am seeing about 1-2ohms on the TC resistance when at room temperature. Using a IR temp gun (best thing I have to get an idea, I don't have any other high temp TCs), when disconnected from the voltage divider, I see about 20 ohms at 400 C. Using a 250 Ohm resistor connected to 5V and the one leg of the TC and the other leg of the TC to ground, I would expect to see 0.37 VDC in the middle of the divider at 400C. I am seeing about .051 VDC. At room temp connected with the 250 Ohm divider circuit, I should see .029 VDC but I am metering .02 VDC. When I check the resistance of both the 250 Ohm and the TC, they are behaving properly, but hooked together with 5V to ground, I get different results. The results are still linear, meaning that as the iron heats up the measured voltage on the divider predictably corresponds with the temperature observed. Maybe the less than stellar connections of the breadboard are causing issues? Either way, I think I can make this circuit work, I will just have to change the amount of gain on the op amp circuit to get good resolution in the Nano's ADC. I am going to fabricate my own PCB next and see what I see.
As a positive, the other parts of the circuit are working as expected. The MOSFET drive and MOSFET are switching the iron on and off as expected and I have the display hooked up and working. The potentiometer should work fine for setpoint. I will probably even use this breadboarded circuit to solder up the PCB for the project.
The circuit board will break down into these main parts:
-24V to 5V Power for Nano and other ICs
I am using a buck converter module for this. The output is settable with an onboard pot. The switch will be placed inline before the input to this module.
-24V Switch circuit to supply power to the iron
I will be using a IRF540N MOSFET to handle the current/voltage. As a buffer between the Nano and the FET, I decided on a MCP14E7 MOSFET driver. This is probably overkill, but I had some on hand. We aren't switching this FET quickly and the FET is only handling 2A. You could probably use a transistor or another FET to interface to the Nano, but I wouldn't control the FET directly from the Nano. #GreatScott! has a great video on driving MOSFETS .
-Voltage divider and amplifier circuit to bring the thermocouple resistance feedback into the Nano's ADC
For this I am using an LM358 OpAmp. The iron's resistance is around 1 Ohm at room temperature and about 20 Ohms at 400C. Because this resistance is so small, I need an amplifier circuit to get better resolution coming into the Nano's ADC. The LM358 has two amps in one chip, so I will probably use a voltage divider and the other amp to monitor incoming power voltage.
-Potentiometer feedback for setpoint
Most potentiometers will work here to create a voltage divider for the Nano's analog input to create a way to control a setpoint temperature. I would use something larger than 2K Ohm. I am reusing the 500K Ohm pot that is from the old soldering station.
-OLED display connection and I2C communication
Simple 4 wire connection to the display, SDA and SCL from the Nano, power and ground. I'm also including pullup resistors for the SDA and SCL lines, but I need to meter the OLED and see if they are already present on the display board.
I disassembled the old iron and salvaged a few parts that I could use, mainly the potentiometer and power switch. Looking at the space inside, I should have a 45mmx40mm area for the main board and plenty of room for the panel that will house the pot, switch, display, 5V regulator board and iron connector. I'd like to have the DC power connector in the back to reuse the existing hole and have the USB port of the nano facing an edge so I can reprogram without disassembling, but we'll see how the board lays out.