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