Project repository: https://github.com/snivasms/SMD-Rework-Station
ESP32-based hot air **SMD rework station** designed for repair and rework of surface mount devices using thermocouple feedback, PID control and programmable airflow..
- Summary
- Warning
- Project Status
- Block Diagram
- Front Panel
- Project Overview
- System Details
- Firmware Files
- Hardware Components
- User limits
- Hardware Safety Features
- Software Safety Features
- Features
- Functional Description
- User Interface
- Assembly Notes
- Electrical Calculations
- Takeaway Lessons
- Repository Structure
- License
| Item | Description |
|---|---|
| Controller | ESP32 DevKit V1 |
| Firmware | MicroPython |
| Heater | 230 V / ~550 W |
| Fan | 24V BLDC |
| Temperature Sensor | MAX6675 K-type thermocouple |
| Display | 16×2 I2C LCD |
| Control | Rotary encoder |
Intermediate → Advanced
Requires experience with:
- mains powered equipment
- PCB assembly
- MicroPython firmware
This project operates directly from 230 V AC mains.
Improper construction may result in electric shock, fire hazard, or equipment damage.
This design is intended for experienced electronics builders only.
Users must verify electrical safety, insulation, and regulatory compliance before construction or operation.
✔ Hardware verified ✔ Firmware stable ✔ PID tuning implemented ✔ Safety interlocks operational
+----------------+
| ESP32 MCU |
+--------+-------+
|
+-----------+-----------+---------------+
| | |
MAX6675 Rotary Encoder |
Thermocouple User Input |
| |
Temperature |
|
+-----------------------------------+
|
ESP32 Outputs
|
+-------- Heater TRIAC (BT139)
|
+-------- Fan MOSFET (IRF540)
|
+-------- LCD display
+---------------+
| 230 V AC 50 Hz |
+---------------+
|
Transformer → Rectifier → 12 V → Boost → 24 V Fan
|
5 V -> ESP32 Module
This project implements a temperature controlled SMD rework station using an ESP32 DevKit V1 running MicroPython.
The system controls:
- Hot air heater
- Airflow fan
- Temperature regulation using PID
- Nozzle specific calibration
- Safety interlocks for fan, temperature, watchdog and power supply health
| Item | Description |
|---|---|
| Processor | ESP32 DevKit V1 |
| Firmware | MicroPython |
| Python Version | MicroPython v1.27.0 |
| Main Program | main.py V5.2 |
| Board | Version |
|---|---|
| Power Board | V3 |
| Processor Board | V3 |
After flashing MicroPython, the following files must be copied into the ESP32 file system:
- boot.py
- main.py
- i2c_lcd.py
- lcd_api.py
- rotary.py
- rotary_irq.py
- max6675.py
- nozzles.json
| Component | Description |
|---|---|
| Display | 16 x 2 LCD with I2C interface |
| Temperature Sensor | MAX6675 K type thermocouple |
| Input Control | Rotary encoder with push button |
The SMD handle contains:
- Heater – 220 V (~500 W)
- K type thermocouple
- Built-in BLDC fan – 24 V / 2.6 W
- Reed switch for in-stand detection
| Parameter | Range |
|---|---|
| Fan Speed | 30 % – 100 % |
| Temperature | 100 °C – 450 °C |
- Heater will not switch on if fan is not running
- Fan stall detection
- Fan unplug detection
- 5 V rail monitoring
- Hardware watchdog timer monitoring processor
- Hardware safe temperature limit at 500 °C
- Heater cutoff when handle placed in stand
- Hardware cooling if processor fails
- Heater switches on only when fan is running
- Watchdog LED indicates processor health
- Minimum fan speed limit 30 %
- Maximum fan speed limit 100 %
- Temperature limits 100 °C – 450 °C
- In-stand cool down and fan off temperature set at 70 °C
- Fan OFF after additional 30 seconds cooling
- PID based temperature control
- PWM fan speed control
- Display shows:
- Set temperature
- Actual temperature
- Fan speed
- Selected nozzle size
- Per nozzle calibration
- Recalibration option during RUN mode
- Front panel serial port for PC communication using FTDI module
- New nozzles can be added
- Unlimited calibrated nozzles
- RUN mode possible without nozzle ( Bare handle )
- Temperature and fan speed adjustable during operation
- Reused nozzle starts with default temperature 250 °C
| LED | Function |
|---|---|
| Power LED | ON when 12 V supply present |
| Temperature limit LED | ON when temperature < 500 °C |
| Watchdog LED | On when Processor resets the watchdog |
| Fan running LED | ON when fan is running normally |
| Fan connected LED | ON when fan connected and running above minimum speed |
| Processor power LED | ON when 5 V rail healthy |
| Fan ON LED | ON when Fan PWM output active |
| Pin | Signal |
|---|---|
| 1 | TCK GPIO3 |
| 2 | TDO GPIO15 |
| 3 | TMS GPIO14 |
| 4 | 3.3 V |
| 5 | TDI GPIO12 |
| 6 | GND |
12 V unregulated supply feeds SX1308 boost module generating 24 V.
MOSFET IRF540N switches the fan using PWM command from ESP32.
Cooling sequence:
- Fan runs when equipment powers up
- Heater turns OFF immediately when handle placed in stand
- Fan runs until temperature drops to 69 °C
- Fan stops 30 seconds later
Heater driven by BT139 TRIAC.
Heater activates only when:
- Fan running normally
- Fan above minimum speed
- 5 V supply healthy
- Watchdog control active
- Temperature below HW safe limit
- Processor command active
- Handle not in stand
Display shows nozzle selection menu
Encoder actions:
Rotate → scroll list of nozzles Short press → select nozzle and switches to RUN mode
Options include:
- Stored nozzles
- NEW nozzle
- NO NOZZLE
Display shows:
- Actual temperature
- Set temperature default value
- Fan speed
- Selected nozzle
Temperature adjustment: rotate encoder
Fan speed adjustment:
Press encoder → change fan speed
Press again → return to temperature control
Steps:
- Install new nozzle
- Set airflow
- Long press encoder (>3 seconds)
- Lift handle from stand
Calibration begins automatically.
Initial calibration values: Temperature : 250 °C Fan speed : user selected
PID parameters are stored when calibration completes.
-
Both PCBs are double sided without plated vias.
-
Vias connected using solder jumpers.
-
IRF540 fan MOSFET requires no heatsink.
-
BT139 mounted on heat sink with insulation washer.
-
Fan current sensing uses 3 × 1 Ω resistors in parallel.
-
Fan current signal filtered using 1 kΩ + 10 µF.
-
Bulk capacitor increased to 5000 µF.
-
Additional wiring to SX1308 input improves 12 V supply stability.
-
Star ground used near transformer secondary.
-
Fuse rating 3 A.
-
MAX6675 mounted horizontally for enclosure fit.
-
FTDI module used for firmware development.
-
Hardware tested using spare ESP32 board with no processor.
-
MOC3041 socket used for testing with a LED for testing interlocks by inserting a suitable LED in socket pin1 and 2 ,without energising BT139.
-
High voltage area coated with insulation varnish.
-
The processor board was initially etched with a timer for fan off delay ( U3). With a little modification to the fan control , during assembly, U3 was omitted.
However, the socket for U3 is still visible in the photograph. Since the modification was small and the board had already been assembled with U3,
to save time, the board was used with minor manual wiring changes.
However, the schematic and PCB files have been updated.
Heater resistance = 96 Ω ( measured using a DMM )
Power = V² / R
Power = 230² / 96
Power ≈ 551 W
Heater current ≈ 2.4 A
Triac used:
BT139 (16 A rating)
Load utilisation ≈ 15 %
555 monostable timing:
Rt = 94 kΩ
Ct = 4.7 µF
Pulse ≈ 486 ms
Minimum timing ≈ 369 ms
Watchdog reset interval used:
250 ms
Supply voltage = 12 DC
Saturation voltages of
Q3,Q4,Q5 ( power board)
and Q2,Q3,Q11 ( Processor board) = 6 x 0.2 = 1.2 V
Forward voltage drop of LED in MOC3041 = 1.2V
Total drop = 2.4 V
R4 in Power board = 1 K
MOC3041 LED current = (12-2.4)/1K
MOC3041 LED current = 9.6 mA
Three 1 Ohms resistors are connected in parallel and placed in the
low side of the IRF540N MOSFET.
Equivalent resistance (parallel) = 0.33 Ohms.
Measured fan current at 24 V DC supply = 96 mA
Current sense voltage at 24 V DC = 96 *0.33 = 31.68 mV
Voltage gain of U4A ( LM358) = 1+(R28/R23)
= 1+ (240/10)
= 25
Thermocouple output voltage = 41 uV/ °C
at 500 Deg C output voltage = 41*500 = 20.5 mV
After amplification in U4A = 20.5 x 25 = 512 mV
Input reference at Pin5 U4B = (3.3/(R30+R31)) X R31
= (3.3/(24+4.7)) x 4.7 = 540 mV
Over temperature trip will act at = (540000/25)/41 = 527 °C
-
12 V regulator is inserted between the DC output 12 V and 5 V regulator input to minimise the temperature rise of 7805 on no loads. But when the system is loaded, the 12 V output falls to about 10 V , still ok for the 7505 input.
-
Boost converter interaction was very dominant on the smoothing capacitor across fan supply. 3 capacitor failures ( 220 uF, 63V ) noted. A series inductor from the 24 V power line with a highspeed freewheeling diode across the fan DC input and a TVS diode at the output of 24 V supply solved the issue.
-
The drop in 12 V DC was seen very much when the fan is operating at the beginning, to the extent of resetting the processor module was overcome by providing a 5000uF bulk capacitor in place of 1000 uF filter capacitor. To accommodate the size of the 5000uF capacitor in the available space, it was inverted and place. The connection to the board was taken with 0.5 sqmm copper wire from capacitor.
- Transformer used: 230 V / 12-0-12 V @ 750 mA
- No load DC voltage reaches 17–18 V
- 12 V regulator added to reduce heat in 5 V regulator
- Possible improvements:
- Increase transformer to 15-0-15 V
- Increase bulk capacitor voltage rating to 35 V
- Replace transformer supply with 12 V 2 A SMPS
Further analysis required for ripple interaction with SX1308 and 5 V regulator.
SMD-Rework-Station/
firmware/
main.py
boot.py
rotary.py
rotary_irq.py
max6675.py
i2c_lcd.py
lcd_api.py
hardware/
schematics/
power_board_schematic.pdf
processor_board_schematic.pdf
pcb_layout/
power_board_B_Cu_negative.pdf
power_board_F_Cu_mirror_negative.pdf
power_board_layout.pdf
processor_board_B_Cu_negative.pdf
processor_board_F_Cu_mirror_negative.pdf
processor_board_layout.pdf
images/
Assembled_unit.jpg
power_board.jpg
processor_board.jpg
handle_on_stand.jpg
pvc_stand1.jpg
pvc_stand2.jpg
stacking_of_boards.jpg
station1.jpg
station2.jpg
with_cover.jpg
README.md
LICENSE.md
This project (firmware, hardware design files and documentation) is released under the MIT License.
Third-party drivers included in this repository retain their original MIT licenses and are credited in this documentation.
MAX6675 driver:
Copyright © original author. https://github.com/BetaRavener/micropython-hw-lib. License: MIT.
Rotary encoder driver :
Copyright © original author. https://github.com/MikeTeachman/micropython-rotary. License: MIT.
lcd_api and i2c_lcd driver :
Copyright © original author https://github.com/dhylands/python_lcd License: MIT