PAGE 069 OF 150 · DRIVING WITH SWITCHES & TRANSISTORS
Relay Basics for Lighting Control
A relay isolates a control circuit from a load.
LED SYMBOLTriangle points from anode to cathode; the arrows show light leaving the part.
What a relay is
A relay is an electromagnet that moves a mechanical switch. It lets a small circuit control a separate, higher-power one, with the two sides electrically isolated.
Contacts
- COM: common terminal.
- NO: normally open, connected when the coil is on.
- NC: normally closed, connected when the coil is off.
Driving the coil
Use a transistor to supply coil current and add a diode across the coil, cathode to supply, to absorb the voltage spike when it switches off.
Contacts have voltage and current ratings. Mains wiring should only be done by someone qualified.
REMEMBER
A transistor lets a tiny signal control a much larger LED current.Try it yourself
Sketch the circuit on paper first, calculate the values, then build it on a solderless breadboard using a low-voltage supply. Measure with a multimeter before you make anything permanent.
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