It depends on the current the relay coil draws. You might get by with a reed relay,
but if you are driving a typical mechanical relay, the external transistor will
give you the drive you need.
Rick

Rafael Vidal Aroca wrote:

>         Hi guys, there is a question i'm always in doubt. Let see if anyone can
> make it clearer.
>
>         I always followed the traditional rules as the diagram below taken from
> http://www.hut.fi/Misc/Electronics/circuits/parallel_output.html
>
>                                                    V+  (12V)
>                                                   |
>                                      +------------+
>                                      |            +------+
> Parallel                            |            |      |
> Port                                |    D1     ---     |
>                                      |   1N4001  / \    Relay coil
>            R1    1  ----------- 5    |          /---\    |
>   D(x) ----1k------| Opto-     |-----+            |      |
>                    | Isolator  |                  +------+
>   GND -------------|           |-+                |
>                  2  ----------- 4|                |
>                     CNY 17 or    |   R2        | /
>                     4N25         |   4.7K    B |/  C   T1
>                                  +--\/\/\/\/---|      BC547A or 2N2222A
>                                                |\  E
>                                                | V
>                                                  |
>                                            external circuit ground
>
>         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.
>
>         Thanks.
>
> []s Rafael.
>
> --

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