One of the main differences between a mechanical relay and a photorelay is the output switching mechanism. A mechanical relay uses a set of metal contacts to switch the output while a photorelay uses a light activated MOSFET power transistor. The advantage of a photorelay is that it has no moving parts to wear out which gives it superior reliability over long periods of time. They are also smaller, lighter, have higher switching speeds and consume very little power. In addition, there are no physical contacts which makes them completely silent.