[App_rpt-users] Slight mod to DMK GPIO driver circuit when used for a fan controller

Bryan D. Boyle bdboyle at bdboyle.com
Wed Feb 1 09:07:30 EST 2012

In hacking (thanks, Kyle) through getting both the script working (hey, 
had to modify it for my own install, which is a good thing...:)) and 
addressing the proper node/dongle number...and getting the parts to 
build the circuit (at Radio Shack, believe it or not...) as shown at the 
DMK site so I could actually drive my cooling fans (with 3 repeaters, a 
100w amp, and batwing spectra transmitter that gets Real Toasty even at 
12w out, the amount of constant noise is a bit much, even in the 
basement...not to mention the electricity consumption), it's all up and 

However, in talking with Scott KB2EAR, he brought up the issue of the 
possibility (not remote..) of the controller or a dongle hanging or 
going brain dead in between the time that the radio keys up and the fan 
controller script turns on the relay...with the possibility that the 
radio would key (along with the amp...) but the fans would never come up.

So...belt and suspenders time.  I have an extra box of shalco 
thermoswitches...NO contacts, close at 115c, reopen at 95c or 
thereabouts.  Since the relay (I like electromechanical stuff, ok? 
Nothing like the sound of contactors closing all around the shack when 
you press one button...brings me back to the days of running BIG 
transmitters and the plate contactor closing when you pressed 'PLATE ON' 
and saw the plate voltage meter swing to 18KV @ 2.6A) that's being 
driven by the DMK circuit is actually controlling the B+ to the fan 
string...putting one of the thermo switches across the armature and NO 
contact of the relay...well, if the computer hangs, or the relay fails, 
and the transmitter and amps are up...when they get warm enough, the 
secondary mechanical (rather than logical) circuit takes over and STILL 
turns the fan on.  I'd rather fail, if that's the scenario, with cooling 
flow across the heat sinks than not.

Obviously, this implies that the logical circuit is driven from the same 
power supply as the fans (mine are...) to eliminate that issue.

Anyway...wired that up as an addition to the relay circuit; actually, 
it's just a pair of wires coming back from the shalco and paralleled 
across the leads from the relay.

Just throwing that out there for y'all's consideration, if you're 
building the circuit from DMK to do something like this.

(Still have to mount the DMK circuit on perfboard in a small project box 
and clean up the wiring...right now, it's just hanging by its wires in 
front of the repeater shelving unit (plastic...one of those Sears 
5-shelf organizers...).

Final cost, by the by, of the DMK circuit, using bits and pieces from 
the Shack, was under $10 USD, btw.  And the biggest cost was the big 
piece of perfboard (which I cut down, so I have lots extra to use for 
other projects) and the small DPDT relay.  Bought the 10-pack of 
transistors, 10-pack of diodes, and couple sleeves of the 4.7K 
resistors, so, have some extras to build other interfaces...


In this world, you must be oh so smart or oh so pleasant.
Well, for years I was smart. I recommend pleasant.
You may quote me.

Sent from my MacBook Pro.

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