DIY Temperature Controller Using KYT84 and TRIAC

Details

In this video, I build and test a DIY temperature controller circuit using the KYT84 temperature sensor, LM358, CD4093, MOC3021, and TRIAC. PCB from JLCPCB

This circuit can automatically control the temperature of heating devices such as an electric iron, hot plate, and other AC heaters. The output power can be adjusted from 0 to 100 percent, while the temperature feedback allows the circuit to turn the heater on and off automatically to maintain the desired temperature.

The circuit uses an AC zero-cross detection circuit, temperature sensing, comparator control, and an optocoupler to control the TRIAC.

I also test the circuit with an electric iron and a light bulb to demonstrate how the temperature feedback and power control work in real time.

In the future, I plan to develop this circuit into a heating plate for soldering surface-mount components.

Files

Project.rar

Schematic, PCB layout, PDF file, Gerber file for this project

RAR Archive - 2.33 MB - 08/16/2026 at 14:41

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Build Instructions

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1
Circuit Design and Completed Video
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.
2

PCB Design

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.
3

Temperature Feedback

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.