The circuit consists of several functional blocks:

• 5 volt power supply
• AC zero-cross detection using PC817
• LM358 temperature signal processing
• Temperature comparator
• CD4093 pulse generation
• MOC3021 optocoupler
• TRIAC AC power control

The AC zero-cross detector generates a 100 hertz signal after rectification. This signal is processed by the control circuit to determine the appropriate switching timing for the TRIAC.

The KYT84 changes its resistance according to temperature. The LM358 processes this feedback signal and compares it with the preset temperature.

The CD4093 then generates the control pulses for the MOC3021, which drives the TRIAC and controls the heater power.

I designed a dedicated PCB for this project and had it manufactured by JLCPCB.

The PCB has a compact layout with separate sections for the power supply, zero-cross detection, temperature sensing, and TRIAC control.

The project files, including the schematic and Gerber files, are available with the project.

When the temperature is below the set point, the heater receives power and starts heating.

As the temperature reaches the preset value, the controller reduces or turns off the heater power.

When the temperature drops again, the heater is powered back on.

This creates a simple closed-loop temperature control system.

I tested the circuit using an electric iron as the heating load.

I also used a light bulb during testing so that the output power could be easily observed.

When I heat the KYT84 sensor, its resistance increases and the controller responds by reducing or turning off the heater.

When the sensor cools down, the controller automatically turns the heater back on.

The output power can also be adjusted from 0 to 100 percent, making the circuit suitable for different types of heating loads.

This circuit has many possible applications.

My next idea is to develop it into a DIY heating plate for soldering surface-mount components.

It could also be adapted for hot plates, food warmers, heating elements, and other temperature-controlled heating applications.

This project shows that a practical temperature controller can be built using simple analog components without a microcontroller.

The combination of KYT84 + LM358 + CD4093 + MOC3021 + TRIAC provides a simple and flexible solution for controlling AC heating devices.

I hope this project gives you some ideas for your own electronics projects.

Thanks for checking out my project!

Safety: This circuit operates with dangerous AC mains voltage. Use proper isolation, fusing, insulation, enclosure, and other appropriate safety precautions when building or testing it.