If the relay is attached parallel to a switch in some external device (such as a remote control), closing the relay is like closing the switch. Bipolar Junction Transistors (BJT) BJTs are a very ...

This isn’t just in the realm of theory. Many relay computing devices were built in the last century. There are several modern examples, too, although they are mostly for show, not practical devices.

The circuit presented here (Fig. 1) takes the same power-reduction principle to the next logical level by eliminating the series voltage-dropping resistor and replacing it with efficient switch-mode.