The LM393 comparator, while low-power, still dissipates energy. At 5V and 40mA, the module consumes 0.2W. This is not much, but the FC 51 has . The small PCB, lack of a ground plane, and proximity of the potentiometer to the LM393 cause localized heating.
If you're troubleshooting a specific issue, I can help further if you tell me: Are you seeing in sunlight? Is the module itself physically heating up ? fc 51 ir sensor datasheet hot
If you are planning to use this for a project, would you like a snippet or a circuit wiring diagram to get started? The LM393 comparator
Features an onboard potentiometer to adjust the detection range based on the environment. lack of a ground plane