⚠️ HIGH‑VOLTAGE WARNINGThis project operates directly from mains AC voltage (110–240 VAC) and drives a high‑power resistive heater through a solid‑state relay (SSR).
Building, modifying, or operating this project is inherently dangerous.
- Mains voltage can cause serious injury or death.
- Fire risk exists if wiring, insulation, grounding, or firmware safety logic is incorrect.
- By building or using this project, you take full responsibility for all risks.
The author provides this project for educational purposes only and assumes no liability for damages, injuries, or losses of any kind.
HEAT is a microcontroller‑based smart hot‑plate / reflow heater controller designed for electronics work such as SMD soldering, thermal experiments, and controlled heating tasks.
The system combines:
- A STM32F103C8T6 microcontroller
- A zero‑cross AC SSR for safe mains switching
- A 100 kΩ NTC thermistor for temperature feedback
- A Smart Pulse & Coast control algorithm (not classical PID)
- An SSD1306 OLED UI with buttons
- A forced‑cooling fan with safety logic
- Persistent settings storage in internal flash
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Power Input
- AC mains → AC‑DC module → 5 V → LDO → 3.3 V
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Control Core
- STM32F103C8T6 running a cooperative main loop
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Sensing
- NTC thermistor read via ADC
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Actuation
- SSR for heater control (time‑proportional)
- DC fan via MOSFET with software PWM
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User Interface
- SSD1306 OLED (I²C)
- Three buttons
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Non‑Volatile Storage
- Internal Flash (last page)
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- MCU: STM32F103C8T6
- Supply: 3.3 V
Key reasons for choosing this MCU:
- Mature HAL support
- Adequate ADC performance
- Flash storage capability
- Simple debugging (SWD)
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- AC-DC Module: HLK-PM01 (isolated offline SMPS)
- Input Voltage: 110–240 VAC
Input-side protection and conditioning (mains side):
- Fuse (1 A): protects wiring and PCB against catastrophic faults and shorts.
- MOV (varistor): clamps high-voltage transients and spikes from the mains.
- Bulk capacitor (220 nF, X2-rated): reduces conducted EMI and high-frequency noise on the AC line.
Output-side conditioning (low-voltage side):
- Electrolytic capacitor (220 µF, polarized): provides bulk energy storage, reduces ripple, and stabilizes load transients from the MCU, fan, and SSR drive.
Regulation:
- AMS1117-3.3 LDO: drops 5 V to 3.3 V for the MCU and logic domain.
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- Optotriac: MOC3063M (zero-cross)
- Power Triac: BTA16-600B
- Heater Power: up to ~400 W resistive load
- AC Fuse: 3.15 A, standard 6.3 × 32 mm (3AG) cartridge
The 3.15 A slow-blow fuse on the heater line protects against:
- Triac failure (shorted MT1–MT2)
- Heater short circuits
- Wiring or connector faults
The slow-blow characteristic tolerates inrush current while still providing reliable fault protection.
- MCU GPIO drives the LED inside the MOC3063M
- At the next AC zero crossing, the optotriac turns on
- The optotriac triggers the gate of the BTA16 triac
- The triac conducts AC current to the heater until the next zero crossing
- Firmware decides when to enable or disable the optotriac
Instead of switching at mains frequency, the firmware uses windowed control:
- Fixed window: 250 ms
- Heater ON for a percentage of that window
This approach is:
- SSR-safe
- Thermally smooth
- Simple and predictable
Falstad simulaiton:
The MOC3063M is a zero-cross optotriac, meaning it will only trigger the power triac when the AC waveform crosses approximately 0 V. Even if the MCU enables the optocoupler at an arbitrary time, the output remains inactive until a zero crossing occurs.
This behavior is critical for resistive heater loads:
- Switching at the voltage peak causes large instantaneous current steps
- These steps generate EMI, audible noise, and electrical stress
- Zero-cross turn-on ensures minimum dv/dt and di/dt at conduction start
The waveforms shown in the images demonstrate that random turn-on injects sharp current spikes, while zero-cross switching produces smooth sinusoidal current flow.
Phase-angle control deliberately turns the triac on mid-cycle. While useful for lamp dimming or motor speed control, it is unsuitable for thermal systems:
- Introduces significant EMI and harmonic distortion
- Increases thermal stress on the triac
- Requires complex timing logic synchronized to mains
- Provides no thermal benefit for a slow system like a heater
For heaters, only the average power over time matters—not instantaneous waveform shaping.
Because zero-cross optotriacs cannot modulate within a half-cycle, power is controlled using time-proportional (windowed) control:
- The firmware defines a fixed window (e.g. 250 ms)
- The heater is enabled for a percentage of that window
- Each ON event starts at a zero crossing
This results in:
- Clean switching behavior
- Stable thermal output
- SSR-safe operation
- Predictable power delivery
The heater naturally integrates the power due to its thermal mass, eliminating the need for high-frequency modulation.
- Type: 100 kΩ NTC
- Beta: 3950
- Divider: 100 kΩ series resistor
- ADC Reference: 3.3 V
The firmware:
- Averages 16 ADC samples
- Applies a low‑pass filter
- Uses the Beta equation to convert resistance → temperature
- Fan type: 5 V, 2-wire DC fan
- MOSFET: IRLML2502 (logic-level)
- Flyback diode: 1N5819
Because the fan is a 2-wire fan (no tach, no PWM pin), speed control is implemented entirely in firmware using software PWM.
The PWM logic controls:
- Airflow rate
- Cooling aggressiveness
- Noise level
The firmware enforces:
- A minimum duty cycle where the fan reliably spins
- A short 100% startup boost to overcome static friction
- Full-speed override during cooling or fault conditions
- Controller: SSD1306
- Resolution: 128×64
- Interface: I²C @ 400 kHz
Pull‑ups:
- 2.2 kΩ on SDA/SCL
- 3 momentary buttons
- Internal pull‑ups
- Active‑low
Used for:
- Start / Stop
- Navigation
- Editing values
For PCB fabrication, the following stack-up has been tested and works reliably for this project:
- Base material: FR-4 S1000H (TG150)
- Copper thickness: 1 oz
- Surface finish: HASL (lead-free)
- Board thickness: 1.6 mm
If you want the best no-compromise option, especially for improved thermal stability, mechanical rigidity, and long-term reliability, I recommend:
- Base material: S1000-2M (TG170)
- Copper thickness: 2 oz
- Surface finish: ENIG
- Solder mask: Black
- Board thickness: 2.0 mm (optional but recommended)
This configuration improves heat spreading, reduces copper resistance, and provides superior pad durability during repeated high-temperature operation.
⚠️ IMPORTANTThe heatsink attached to the triac is electrically live and is directly connected to mains voltage.
- Do not touch the heatsink while the device is powered.
Improper handling can result in serious injury or electric shock.
firmware/
├── main.c / main.h
├── control.c / control.h
├── heater.c / heater.h
├── fan.c / fan.h
├── thermistor.c / thermistor.h
├── ssd1306.c / ssd1306.h
├── ui.c / ui.h
├── storage.c / storage.h
- Single main loop
- No RTOS
- Deterministic timing via HAL_GetTick()
- ISR only used for fan PWM timing
Main loop order:
Control_Update()Heater_Update()UI_Task()
This module is the central decision-making unit of the entire system. All other subsystems react to values produced here.
- Read raw temperature from the thermistor module
- Filter temperature readings
- Detect sensor failures
- Enforce absolute safety limits
- Execute the heating state machine
- Output a heater power percentage (0–100%)
No hardware is driven directly from this module. Instead, it produces intent values that other modules act upon.
- Update period: 100 ms
- Time base:
HAL_GetTick()
Each control cycle performs the following steps in strict order:
- Read raw temperature
- Validate sensor range
- Apply low-pass filtering
- Check over-temperature cutoff
- Run the heating state machine
- Compute heater power percentage
If any safety condition fails, the module immediately:
- Sets heater power to 0%
- Forces the system into IDLE
Early versions of the firmware used a classic PID controller with extensive tuning.
Despite significant tuning effort, the best achievable result still produced ~25 °C overshoot.
Root causes:
- Heater power: ~400 W
- Plate mass: ~30 g
- Extremely fast thermal rise
- Significant thermal delay between heater and NTC
The thermistor simply could not observe the temperature rise quickly enough, causing the PID to react too late.
This led to the current solution: Smart Pulse & Coast, which is slower by design but far more precise and repeatable.
| State | Meaning |
|---|---|
| IDLE | Waiting |
| HEATING | Rising to setpoint |
| HOLDING | Maintaining temperature |
| COOLING | Forced cooldown |
| DONE | Cycle finished |
This control method explicitly accounts for thermal inertia and delay.
Instead of continuously adjusting output, the heater is driven in full-power pulses, followed by observation periods.
- Apply a controlled heat pulse
- Turn the heater fully off
- Observe how temperature continues to rise
- Decide the next pulse based on real thermal response
This prevents the controller from "chasing" delayed measurements.
Pulse width is selected based on the ratio:
current_temperature / setpoint
| Zone | Ratio | Pulse Width |
|---|---|---|
| Far | < 70% | 2000 ms |
| Mid | 70–85% | 500 ms |
| Near | > 85% | 250 ms |
During the coast phase:
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Heater is guaranteed OFF
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The firmware tracks the peak temperature
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A new pulse is allowed only after:
- A minimum wait time has passed and
- Temperature has begun to fall
This ensures the full thermal effect of each pulse is observed.
To understand why the control strategy must be conservative, it helps to look at the physics of the heater–plate system.
Even thoough the heater is listed as 400W, it was measured to have an approximate cold resistance of 340 Ω.
Using Ohm’s law:
P = V² / R
For a 230 V RMS mains supply:
P = (230 V)² / 340 Ω ≈ 155 W
This calculation is electrically correct and provides a useful order-of-magnitude estimate.
Although the formula is correct, it is not something the control system can rely on precisely, for several reasons:
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Resistance changes with temperature Heating elements increase resistance as they get hotter. The 340 Ω value is a cold measurement and rises significantly during operation.
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Mains voltage is not constant Real-world mains can vary by ±10% or more, directly affecting power.
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Thermal coupling dominates behavior The system response is governed by how fast heat flows into the plate, not by instantaneous electrical power.
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Plate mass is very small The aluminum plate mass is approximately 30 g, which means:
- Temperature rises extremely fast
- Small energy pulses produce large temperature changes
The temperature rise of the plate is governed by:
ΔT = Q / (m · c)
Where:
Q= heat energy (J)m= mass (kg)c= specific heat capacity (J/kg·K)
With:
m ≈ 0.03 kg- Aluminum
c ≈ 900 J/kg·K
Even a short, high-power pulse injects enough energy to raise the temperature dramatically before the thermistor can respond.
This is the fundamental reason why:
- PID control overshot by ~25 °C
- Slower, observation-based control is required
The Smart Pulse & Coast approach deliberately limits energy per pulse, making the system predictable even when electrical power and resistance vary.
- Implements time‑windowed SSR control
- Enforces immediate OFF on any error
Return behavior:
- No return value
- Acts directly on GPIO
- Software PWM via TIM2
- Emergency override logic
Conditions forcing 100%:
- Cooling phase
- Sensor error
- Over‑temperature
- Temperature freeze detection
- ADC averaging
- Beta equation
Thermistor_ReadTemperatureC() returns:
- Temperature in °C
- Raw (unfiltered)
Provides three screens:
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Status
- Current temp
- Target
- Heater power
- Fan power
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Settings
- Setpoint
- PID values (future‑proofing)
- Max temperature
- Hold time
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Graph
- Live temperature plot
- Setpoint overlay
- Uses last flash page (0x0800FC00)
- Stores struct with magic number
Saved parameters:
- Setpoint
- PID values
- Max temperature
- Hold time
To visualize module interactions, generate a graph using:
- Graphviz
- Doxygen with call graphs
Example idea:
control → heater
control → fan
control → ui
ui → storage
storage → control
thermistor → control
The following cable lengths reflect the actual physical implementation of the current build.
| Connection | Cable Length |
|---|---|
| Thermistor → PCB | ~160 cm |
| OLED (I²C) → PCB | ~51 mm |
| Buttons → PCB | ~130 mm |
| Fan → PCB | ~120 mm |
| AC inlet → PCB | ~75 mm |
| AC inlet Earth → PCB | ~115 mm |
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The enclosure is 3D printed from ASA.
ASA was chosen because:
- It has much better heat resistance than PLA
- It is dimensionally stable near warm electronics and heater-adjacent areas
- It offers good mechanical strength while remaining printable on consumer printers
- It is UV- and heat-resistant, making it suitable for long-term use
PLA was intentionally avoided because it softens at relatively low temperatures and would deform over time in this application.
Print settings used:
- Layer height: 0.2 mm
- Material: ASA
- Infill / walls: standard structural settings (not critical to functionality)
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At layer 155 (optional), the print can be paused to insert M4 nuts directly into the enclosure.
- Nut type: M4 hex nut
- Across flats: 6.9 mm
- Height: 3 mm
- Orientation: inserted from side to side into the printed cavities
This reinforcement:
- Significantly increases mechanical rigidity
- Improves long-term durability
- Makes the enclosure feel substantially more solid
The enclosure is designed so that:
- The heater mounting holes are placed slightly inward
- This creates mechanical tension that can hold the heater assembly even without screws
This approach:
- Works, and the device can be assembled without embedded nuts
- Is not ideal, as the structure:
- Feels less rigid
- Is more prone to loosening or disassembly over time
For best results, inserting the nuts is strongly recommended.
| Application | Fastener Used |
|---|---|
| Heater mounting | M4 × 60 mm bolts + M4 lock nuts |
| Fan mounting | M3 × 30 mm bolts + M3 lock nuts |
| Power inlet mounting | M3 × 16 mm screws + M3 lock nuts |
| Triac → heatsink connection | M3 × 10 mm bolt |
| Protective earth (ground) connection | Wire to heater bolt using 2× M4 lock nuts + 2× M4 washers |
The grounding wire needs to be stripped and bent into a hook shape so that it can later be inserted under one of the heater’s legs (the M4 bolt).
- Solder Peripherals: Before mounting the PCB, solder the wires for the thermistor, fan, OLED display, and buttons to their respective pads on the board.
- Place PCB in Enclosure: Insert the PCB into the bottom of the case. Pro Tip: If you soldered the button wires to the bottom side of the PCB (highly recommended), the tension created by the wires will allow the PCB to "snap" perfectly into position and hold itself securely without needing hot glue.
- Connect AC Inlet: Solder the wires from the AC Power Socket to the AC IN pads on the PCB.
⚠️ IMPORTANT: Perform this soldering with the PCB in a vertical position (lifted out of the case). If you try to solder these pads from above while the PCB is sitting flat inside, the heat will melt and destroy the 3D-printed enclosure walls. - Secure UI Components: OLED: Use a small amount of glue to secure the display to the front panel. Buttons: Push the tactile buttons into their housing. If you used the specific 5.5mm x 6mm buttons listed, they should fit snugly without needing glue.
- Protective Grounding: Connect the Earth/Ground wire from the AC inlet directly to one of the heater's m4 bolts. Secure it tightly with washers and lock nuts to ensure the heater plate is grounded.
- Secure the Fan Mount the fan into its dedicated slot in the enclosure and secure it with M3 screws.
- Mount the Heater Insert the four M4 × 60mm bolts into the heater block and lower it onto the enclosure. If you used the embedded nuts method during printing, simply screw the bolts into the case.
- Install Thermistor Tuck the NTC thermistor head into the small hole on the heater block.
The table below contains links to the main components used in this project.
| Component / Resource | Description | Link |
|---|---|---|
| Heater | AC resistive heater (400 W - 220V) | LINK |
| Thermistor | 100 kΩ NTC, Beta ≈ 3950 | LINK |
| OLED Display | SSD1306, 128×64, I²C | LINK |
| Fan | 5 V, 2-wire DC fan | LINK |
| Buttons | 5.5mm x 6mm | |
| 3.15 A, standard 6.3 × 32 mm (3AG) cartridge | FUSE |
This project includes an interactive Bill of Materials (iBOM) generated directly from the PCB design.
The iBOM allows you to:
- Inspect all components directly on the PCB
- Click a part to see its reference, value, and footprint
- Highlight parts on the board from the BOM list
- Better understand component placement and assembly
You can access the interactive BOM here:
License
Documentation in this repository, including this README, is licensed under
Creative Commons Attribution–NonCommercial–ShareAlike 4.0 (CC BY-NC-SA).
Hardware design files are located in /hardware and are licensed under
the same license.
Firmware source code is located in /firmware and licensed under GNU GPL v3.