The Ultimate DIY Hot Plate! Powered by ESP32 and IR Sensor

I need to solder many PCBs, so I decided to build a Reflow hot plate for SMD soldering using a flat iron the esp32 microcontroller and a IR sensor. My idea is to use it as an everyday tool, so I decided to improve all the available designs online.

This hot plate is able to measure the PCBs temperature using an infrared temperature sensor rather than a temperature sensor attached to the plate, it is able to follow any soldering temperature profile storage on a SD card and, thanks to a ESP32 microcontroller, we can also upload wirelessly any temperature curve we want to follow as well as start the cycle wirelessly. In summary, we can upload and run any reflow curve we need. Imagine all possible use cases!

1. For this project we will need an old flat Iron. One of 1500 W will be more than enough.
2. A ESP32 based Microcontroller.
3. A solid state relay, able to be triggered by the 3v of the microcontroller and supporting the max power of the Iron
4. An Infrared temperature sensor, I selected the MLX90614 able to handle temperatures up to 300 Celsius degrees
5. A power regulator to power the DC circuit (basically the microcontroller) from the AC power source
6. Some panel mounting leds for state indication.
7. A push buttom, in case we want to use the hot plate without the need of a PC.
8. A SD holder, to storage our reflow curves
9. An antenna, a easy way to attach the infrared sensor above our PCB, as well as move it in any direction.
10. Additionally, we will also need a box to safely mount our hot plate . In my case I bought this cheap key holder that will make the job. 

If the box does not came with a solid cover (like mine), you will need a wood cover to mount all the components.

First we need to disassemble the iron. We will just need remove some bolts to discard the components that are not necessary.

In my case I decided to stay with the hot plate and the protecting plastic cover, this cover will be useful to limit the exposure of the heating element. Additionally, I will use the cables to make some connections, but we can always use new ones.

To avoid the thermostat activation in highest temperatures, I also disactivated the temperature control by fixing together the thermostat contacts, but we could also completely remove it.

Once we have the Iron we can start installing all the components in the box. 

We will need to make holes in the cover to mount the panel components leaving place to the iron.

The electrical connections are simple, but we have to safely install everything to ensure the durability. 

I separated the AC circuit from the DC circuit, by installing the AC in the bottom of the box and DC circuit in the top cover. Both circuits will be connected by a small pin connector.

The infrared temperature sensor has a I2C connection and the SD holder have a SPI connection. Leds and the buttom are connected to their own pins.

Additionally, I added a mechanical switch to stop the current from the microcontroller.

The algorithms takes a target temperature every 250 ms from the SD card, if the target temperature is higher than the current temperature plus a compensation temperature the relay is activated, if it is smaller, the relay is deactivated. The compensation temperature is a dynamic value that tries to avoid the peaks due to the slow temperature transmission and its inertia. Because, once the relay is activated even if we swich it off, the temperature will continue increasing for a while in many parts of the hot plate.

So, for instance, this compensation temperature will increase if the target temperature is higher than the current temperature and the relay is activated, meanwhile it will decrease if the current temperature is below the target temperature and the relay is deactivated. It seems twisted but I’m happy with the performance.

The SD management is based in a previous video about ESP32 and SD control via WiFi.

All code is available in this link.

So as you see is not perfect but it makes a very good work following curves. We can reproduce any reflow curve we upload to the SD card, which is good if we change the soldering paste, or we want to use the hot plate for other uses.