Use of opto isolators......

[macbook]

Hi,
I want to know where and why exactly do people use opto isolators?
I want to separate the grounds of the motors(running thru an L293D) and the muc... can i use an opto isolator for it? If yes, how exactly should i use it?

[Soeren]

Hi,

I want to know where and why exactly do people use opto isolators?

Google broke? 

I want to separate the grounds of the motors(running thru an L293D) and the muc... can i use an opto isolator for it? If yes, how exactly should i use it?

There's no need to, but yes, you can.

An optocoupler (look at a datasheet for one) is an (IR) LED shining on eg. a phototransistor to turn the transistor ON or not shining to turn the transistor OFF.

The controller should thus turn on the LED when the transistor should conduct.
The transistor is then used to control the other side.

Why you want to separate the grounds, I don't see, if say the motor is really noisy on the power lines, the ground of the power "side" will radiate that noise and may radiate into the low power ground.
It's better to deal with the noise by removing it as close to the source as possible instead.

[malex]

Hi,
I want to know where and why exactly do people use opto isolators?

Essentially, opto isolators are to physically (in respect to electricity) separate electronic components while still letting them interact.

A common example would be for motor control.  Since motors use a lot of current (and draw a lot during power up), the high currents can damage a micro controller if they belong to the same electrical circuit.  With an optical isolator the controller can still signal the motor circuit without being directly wired to it through light pulses (which interact with photo diodes/resistors)

[Soeren]

Hi,

Since motors use a lot of current (and draw a lot during power up), the high currents can damage a micro controller if they belong to the same electrical circuit.

Only if made horribly wrong. The high current through the motors will not go through the controller (if made correctly of course).
A bad opto coupler design can still induce noise and transients into the logics.
A transistor is adequate for separation in a properly designed circuit.
(We're not talking isolation from mains voltage for safety here).