PID Temperature Controller for Reflow Soldering Hotplate using MAX6675 and Arduino

Hand-soldering SMD parts with an iron works for one-off joints, but reflow soldering — where paste melts through a controlled temperature profile — needs the board heated through specific phases, not just left on a hot burner. Overshoot the soak and the flux chars; overshoot the peak and parts tombstone or the board scorches; undershoot and the joints never wet. A dumb hotplate offers no control at all, and commercial reflow stations hide their control behind a sealed, fixed menu. This project builds the controller that closes that gap: an Arduino Uno running a PID loop, a K-type thermocouple digitized by a MAX6675 module feeding back the plate temperature, and an optoisolated solid-state relay switching the mains hotplate with time-proportioned drive. The firmware walks the plate through a soak-and-reflow profile — preheat, soak hold, ramp to reflow peak, then cooldown — with temperature, setpoint and profile phase on a 16x2 LCD. Every setpoint, phase time and PID gain is a configurable constant, so the student demonstrates closed-loop thermal control, not just relay switching.

1. The K-type thermocouple tip is clamped to the hotplate surface and wired to the MAX6675 module's screw terminals; the module digitizes the temperature at 0.25 °C resolution over SPI with cold-junction compensation handled on-chip, and the Arduino samples it about once per second.
2. The firmware compares the reading against the active setpoint — either the manual target or the current phase of the reflow profile — and computes the temperature error.
3. The PID loop turns the error into a 0–100% drive command from the configured Kp, Ki and Kd gains, with anti-windup limiting the integral term so the heater does not stay saturated after a long ramp.
4. The drive command becomes time-proportioned switching: inside each control window (design target 2 s), the SSR is on for the commanded fraction of the window, giving smooth analog-like power control from one digital output.
5. The optoisolated, zero-crossing SSR switches the 230 VAC hotplate while the 5 V logic stays galvanically isolated from mains; all high-voltage wiring sits in a fused, enclosed section of the build.
6. The profile sequencer advances the phases automatically — soak at approximately 150 °C for 60–90 s, ramp to the reflow peak of approximately 220–240 °C held 30–60 s, then the SSR releases and the plate cools — with the current phase on the LCD; an open-thermocouple reading forces the SSR off at any point.

Tech stack:

• Arduino Uno (ATmega328P) · 5 V logic
• MAX6675 module · K-type thermocouple
• Solid-state relay (optoisolated, 240 VAC)
• 16x2 character LCD (HD44780 with I2C backpack)
• Arduino IDE (C/C++ firmware)
• Single-burner electric hotplate (230 VAC mains)
• Rotary encoder · push buttons
• Terminal blocks · fused mains enclosure

• [PID closed-loop control] The firmware runs a discrete PID loop on the measured plate temperature with configurable Kp, Ki and Kd gains and anti-windup on the integral term, so the plate settles toward its setpoint with less overshoot and hunting than simple on/off control.
• [MAX6675 thermocouple interface] A K-type thermocouple is digitized by the MAX6675 module — 12-bit resolution at 0.25 °C per LSB over an SPI read-only interface with on-chip cold-junction compensation and open-thermocouple detection (datasheet values).
• [Reflow profile sequencer] The firmware walks the plate through preheat, soak, ramp-to-peak and cooldown phases with configurable time-temperature setpoints, so a full soak-and-reflow cycle runs automatically from one start command.
• [Optoisolated SSR mains switching] A zero-crossing solid-state relay switches the 230 VAC hotplate while the Arduino side stays galvanically isolated from mains — the low-voltage logic never touches the high-voltage section.
• [LCD readout and setpoint entry] A 16x2 LCD shows the live plate temperature, active setpoint, profile phase and PID drive level, while a rotary encoder and push buttons set the target temperature and profile times.
• [Thermocouple failsafe] The firmware watches the MAX6675 open-thermocouple detection bit; a disconnected or broken probe forces the SSR drive off immediately instead of running the heater blind.
• [Tunable at the bench] Every gain and phase time is a constant the student can change and re-demonstrate, and the tuning procedure shows how to raise Kp for response or Ki for steady-state error on the actual build.

• Working PID controller prototype (Arduino board, MAX6675 module, SSR, 16x2 LCD, rotary encoder and push buttons, wired on a base with an enclosed mains section)
• Arduino firmware source with PID loop, reflow profile sequencer, LCD user interface and configurable gains and phase times
• Circuit and wiring documentation (schematic-style diagrams, pin map, mains-enclosure wiring and fusing notes)
• Component list with ratings (SSR, thermocouple type, fuse, terminal blocks)
• Reflow profile table and a PID gain-tuning procedure for the student's own build
• Project report PDF (background, PID control theory, profile design, methodology, test log)
• PPT presentation for final review
• Viva Q&A preparation document (PID tuning, thermocouple interfacing, SSR isolation, mains safety)