Dave Cunningham wrote...

>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.

Hmmm.  Why are you using the 10K pullup resistor?  Are you using
the open-drain PIC output (RA4) ?

Some suggestions:

1) Double-check (then triple-check, if necessary) the pinout of
the transistor to make sure you've got it connected right.  If
you take a 2N3904 in the plastic TO-92 case and hold it such that
the flat side is toward you and the leads are pointing downward,
the connections will be E - B - C from left to right.

2) Are you sure your PIC pin has been properly configured as an
output?  Remember, on many PICs some of the pins are analog
inputs until configured otherwise; and clearing the associated
TRIS bits isn't enough to make them digital outputs.  Use a scope
or a DVM to monitor the PIC pin while the code toggles it HIGH
and LOW, and confirm that it's functioning right.

3) As far as I can see, the circuit should work as you've got it
drawn, though I would concur with others' comments about the 6.8K
resistor from PIC output to transistor base being possibly too
large (though this wouldn't be causing the symptoms you're
seeing) and about the 6.8K resistor from base to ground being not
very necessary.

4) If you **ARE** using RA4 as the PIC output driving this
circuit, be aware the 10K resistor is too big; it probably won't
give you enough base current to properly drive the transistor
(again, this wouldn't be the cause of the symptoms you're
seeing).

Hope this helps a bit...

Dave D.

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics