I have built a simple watering system using a PIC 1684. Works well on the
test bench but am having problems running live.

I am using a 24V power pack (1A). The 24v is to power the solenoids to
open/close the water valve. I then cut this 24v down to 12v via a 7812
to power the 12 relays that I use to activate the solenoids. I then
use a 7805 to get 5v to power the control circuitry.

My problem is that I seem to be getting spikes (back EMF) into my
control circuitry that is reseting my PIC. I have put a diode across
each solenoid to limit the spike but this doesnt seem to be enough.
I put the diodes inside the box I built. Should I put something
right across the solenoid itself. I was thinking a filter cap. What
sort of cap and what value should I use.



        12v                24v
        |                   |
        |                   |
RA1-4  |\ ULM2803           |
  -----|  \---------+       |
       | /          |       |
       |/           O  -----+
                    O  \
              Relay O   ------+-------------//-------------+
                    |         |                            O
                    |        ---        Long Cable         O solenoid
                    |        /\ Diode   +---//-------------+
                    |        --         |
                    |         |         |
                    |         |         |
            GND ----+---------+---------+


                    12v
                    |
24V     +--------+  |   +--------+    5v to pic etc
--------|  7812  |--+---|  7805  |-------
        +--------+      +--------+    |
             |               |        |
             |               |        |+
             |               |       --- 200uf
             |               |       ---
             |               |        |
             |               |        |
             |               |        |
     GND ----+---------------+--------+

--
Daryl Sayers
Corinthian Engineering                         Ph: (02) 9906 7866
Suite 19, 401 Pacific Hwy                     Fax: (02) 9906 1556
Artarmon, NSW, 2064                         email: daryl@ci.com.au
Australia                                     www: http://www.cordoors.com.au

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