Thanks for your input. Finally there are two different circuit proposals:


SIGNAL                      +-------+---------o Separate +5V from welder PS
FROM                        |       +--+
PIC   220 ohm               | Relay |  |
 o----/\/\/\-------+        |      (  ---
                   | 4N25   |      (   ^
                   V ---> |/       (  /_\ 1N4001
                   - ---> |         |  |
                   |      |\        +--+
                   |        V       |
                   |        |     |/
                   |        +-----|   BD139
                   |        |     |\
                   |        |       V
                   |        \       |
                   |        /       |
                   |     1K \       |
                   |        /       |
                   |        |       |
                   |        |       |
                  ---      ---     ---

using a 1K resistor between 4N25 emitter and BD139 base if needed.

And this other one:



                                     O +V (unregulated) being 12Vac
                                     |
                                     |
                                 ----*
                                 |   |
                                 |   C Diode is usually 1N4001
                                ---  C
                                 A   C relay coil
                                 |   C
                                 |   C
                                 |   |
                                  ---*
                                     |
                          1K       |/
               PIC o-*- /\/\/\-*---|   gain>50
                     |         |   |\
                    +++       ---    V
                22K | |        A     |
                    +++        |     |
                     |         |     |
                    ---       ---   ---

A couple questions on this second circuit:

1.- Is the second diode a 1N4001?
2.- Would a 2N2222A a good choice for the NPN transistor?




The values of the DOCUMENTED relays ("n.a." was "not available", Paul) from
FINDER I'll be using are:

1.- For the big one, FINDER ref. # 40.52:

        Coil: 12Vdc, 220ohm, 55mA.

2.- For the other 3 of them, FINDER ref. # 30.22:
        Coil: 12Vdc, 360 ohm, 33mA.



Once again, Thanks for your time and patience.

Jaume.


----------------------
  Jaume Aragay Badia
   aragay@email.com
----------------------