> But i already saw, and assembled myself a circuit without the
> transistor, with the 4N25 being connected directly to the relay and
> the diode. Is it right or wrong? What are the implications as both
> seem to work well.

Depends on the relay coil resistance and how much power the
chip will have to dissipate. Have a look here

http://dspace.dial.pipex.com/isocom/4n25.htm

to see if you're breaking any specs by driving the coil directly
with the opto. And by using an extra transistor you don't have to
drive the LED so hard

Higher voltage, eg 24V, versions (of the same model relay) usually
have a higher coil resistance. A 24V coil **may** energise at 12V,
meaning the opto-only option would be more within spec, although
the relay won't hold as fast at 1/2 voltage (but some relays are pretty
good down to just a few volts). It is possible to apply simple voltage
doubling with a cap to energise the coil at close to its rated voltage,
and when that cap's drained, the 1/2V left should hold it fairly firmly

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