
Some Applications
531
acitivates an LED by light; the circuit in Figure 15.14b activates the LED by
darkness. The resistor R
3
limits the current owing through the LED.
It can be more effective to activate another load, rather than a simple LED.
The load can be even another circuit just “activated” by the light/darkness. For
example, a lighting system can be switched on at nightfall. To this aim a relay can
be helpful.
A relay is an electromechanical switch controlled by a current. It operates by
means of an inductor that, when energized, generates a magnetic eld acting on a
metalic arm, which makes up a physical contact, so as to close or open ...