Home › Lessons › Transistors and thyristors › Light-activated switch with relay

Light-activated switch with relay

Transistors and thyristors · Beginner · about 10 minutes · part of Electronics Technician

An LDR and a pot form a divider whose middle rises as it gets dark. A Darlington pair turns that into enough current to pull in a relay, which switches the lamp. The pot sets the light level where it switches.

Light-activated switch with relay: the circuit on the simulator's bench

Open this lesson in the labFree, in your browser, no account needed.

How it works

The relay coil draws (12 − V_CE(sat)) ÷ R_coil ≈ (12 − 0.8) ÷ 400 = 28 mA when on. A Darlington needs only about 30 µA of base current for that, so the divider is hardly loaded.

The diode across the coil catches the inductive kick when the relay releases.

Try it on the bench

  1. Set the LDR to 5 lux (dusk): the relay pulls in and the lamp lights.
  2. Turn the threshold pot: it switches at a different light level.

Parts on the bench

In the course

Electronics Technician, module “Transistor circuits”.

More on transistors and thyristors