320 mA, which can easily handle the 20 mA load of a pilot
valve solenoid. Note that the 24 VDC power supply ground
is also isolated from the PIC’s 5 VDC power supply ground.
The only disadvantage — if you want to call it that — to this
circuit is that the HSR312’s LED places a small current load
on the PIC’s I/O pin. This disadvantage is offset by the total
isolation provided by the HSR312.
A standard PIC microcontroller I/O pin can source and
sink up to 25 mA. As long as you keep the aggregate I/O
pin source/sink current below 200 mA, the PIC will do its
job reliably. The pilot solenoid driver circuit drawn up in
Schematic 1C is designed so as to not add any substantial
current load to the PIC’s I/O subsystem. An NUD3105
integrated relay, inductive load driver (which is rated to
support 5 VDC 500 mA inductive loads) is acting as a
high-current buffer for the PIC I/O pin responsible for
driving the HSR312’s internal LED. This circuit provides the
same amount of isolation as the 1B circuit and places a
reduced load on the PIC’s I/O subsystem. An added
advantage to using the NUD3105 buffer is that you can
add a visible LED to monitor the logical state of the PIC’s
I/O pin without placing any additional load on the I/O pin.
Where’s the Code??
I hate to disappoint you, but there is no complicated
code associated with driving the manifold’s valve
complement. All you need to know how to do is write a
logical high and logical low to a PIC I/O pin. That means you
can drive the valve’s pilot solenoids we’ve discussed with a
PIC starter kit and a handful of easy-to-find components.
For the PIC microcontroller challenge, start with a look at
my Doing Big Things with Little Micros article that appeared
in SERVO 08.2009. You can adapt many of the concepts
offered in that article to driving the pilot solenoids.
Remember you’re working with compressed air. Don’t
exceed 90 PSI through the manifold. Polyurethane tubing
connected to a valve port that is left to flap in the breeze
will oscillate wildly as high pressure air flows through it.
Trust me. The tubing will smack you silly if you’re testing
under pressure with the manifold in your lap. Also
remember that you are working with pilot valves that need
a minimum amount of pressure to operate. So, don’t blow
yourself silly by trying to replace a pressurized air supply
with your lungs. Trust me on this. The valves won’t switch.
I had a bunch of fun doing this article and playing with
the SMC manifold. I think you’ll have just as much fun with
this stuff, too. If you have any questions about using valves
and manifolds in your projects, send me an email and
I’ll be glad to help. SV
■ Contact Fred Eady at email@example.com.
SERVO 11.2009 47