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Opto-coupler is, as far as I know :roll: , consisting of a photodiode and a phototransistor enclosed in a small DIL package (like 555 timer IC). Transistor acts as a switch and diode acts as an activator. When you apply voltage to the diode, the transitor lets the current flow through it. Otherwise, current flow is blocked.
Optocouplers are mostly used when you need to completely isolate one part of a circut from another one, but still be able to controll it (i.e.: if you want to switch 220V AC with your computer using the parallel port). There are different types of opto-couplers available (reflexive, optointerrupters) and with different output (standard transistor output, schmitt trigger, darlington output, etc.)
Hope this helps. I'm not an electronics wiz, so don't take this for granted.
P.S. I'm using reflexive opto-couplers for IR proximity detector for my robot.
The type that have a light and a receptor,
and is intended to show interruptions to the light.
Such as motor car ignition systems.
These are usually a single item
with a light one side,
and receptor the other side.
Then theres the type that use the light to carry signals.
The most common of these are used in light fibres.
One end has light with some signal or modulation,
the other end has a receptor.
Sometimes other uses show up,
older machinery would sometimes have beams of light
across moving parts, to make sure the machine
wouldn't operate until the users hands were clear.
On a circuit board such items are to pass signals
to another part of the circuit, where electrical
isolation is required.
Transformers are not always suitable,
but a similar sort of thing,
to get signals to an isolated part of a circuit.
Optocouplers existing also with triac output for direct driving AC switcher triac or thyristor-pair. Output with FET-s: NC or NO. (NC without LED current conduct!) And AC input with two antiparalel LED(e.g. for ring detectors in DECT-s).
Also available analog-opto with monitor diode, for audio and video signal.