PAGE 070 OF 150 · DRIVING WITH SWITCHES & TRANSISTORS
Optocoupler Basics
An LED and light sensor pass signals without electrical contact.
LED SYMBOLTriangle points from anode to cathode; the arrows show light leaving the part.
What it does
An optocoupler holds an infrared LED and a light-sensitive transistor in one package. Light crosses a gap, so the input and output circuits have no electrical connection.
Using one
- Drive the input LED through a resistor, as for any LED.
- The output transistor acts like a switch with a limited current rating.
- Current transfer ratio (CTR) tells how much output current you get for a given input current; common parts such as the PC817 vary widely.
Optocouplers are chosen for safety isolation and noise immunity, and are slower than a normal transistor.
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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