I'd like to preface this question by saying that while I can program
fine my electronics knowledge goes little beyond the absolute basics.

I want to use a pic to switch an inductive load on/off and it was
suggested (on usenet) that the following should work

                                  VCC (5VDC)

                                   |
                                   |
                              .----o
                              |    |   5V Coil
                       1N4002 -   _|_   Relay
                              ^  |_/_|-
                              |    |
                              |    |
                              |----o
                                   |
                                   |
                  6.8K             |
                  ___            |/
          o------|___|---o-------|   2N3904
     PIC Output          |       |>
                        .-.        |
                        | |        |
                   6.8K | |        |
                        '-'        |
                         |         |
                        GND       GND


I've assembled the circuit (with the input being a PIC pin pulled up to
5V via a 10K resistor), however the relay switches (on) as soon as power
is supplied to the circuit and the output state of the PIC pin has no
effect on the relay state.

As I've said at the start, my electronics knowledge is basic in the
extreme but I've been googling a bit to try to understand how the above
circuit should work (& why it doesn't). I've found a similar (to my
mind) relay switching circuit using a BC377 rather than 2N3904 and
missing the 6K8 resistor between ground & the transistor base. Following
that I removed the ground resistor & replaced the second resistor with a
2K6 one (using calculations provided with the BC377 circuit), but this
doesn't work either!

Could anyone explain how the above circuit works and how the values of
the resistors are calculated? Should it in fact be functioning as in
when connected to a pic as I've described?

Any suggestions of suitable foundation electronics books would also be
appreciated!
--
Dave Cunningham                                  PGP KEY ID: 0xA78636DC

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