A solid state relay (SSR) is an industrial component used for interfacing low voltage control signals to high current mains voltage loads. The SSR may be designed to switch either AC or DC to the load. It serves the same function as an electromechanical relay, but has no moving parts. You can control SSRs easily for example with Arduino:
They sell them on eBay at a ridiculously low price, so it’s time to see what’s actually inside one of these eBay units. Teardown of an eBay 25A Solid State Relay. (SSR) video shows what is inside cheap device that is marketed with name FOTEK:
UL has a warning about these fake relays:
UL warns of solid state relay with counterfeit UL Recognition Mark (Release 13PN-52)
Note that the Model SSR-25 DA relay with the counterfeit UL Mark is similar to Model SSR-25 DA that is authorized to bear the UL Mark. The relay that is authorized to bear the UL Mark is marked “Rated: 25A max.” and “Taiwan Made” on the label, these markings do not appear on the counterfeit product.