Live and Learn

Several interesting things happened during the build-up to and the execution of the clinics that I gave at the NMRA national convention.

My work and travel schedule dictated that I had to postpone wiring the 3-color signals on the demonstration diorama until I was actually in Atlanta the day before my clinic.  As I started to wire the 3 additional LaunchPads that operate the signals to the 3 optical detectors, squirrely things started to happen.  Long story short (and I'm ashamed to say) that I had made an amateurish mistake in the electrical design and layout of the diorama.  I had too many signals hooked up to each optical detector.  This problem is called "fan-out" in electronics and I should have known better.  You are not likely to encounter a fan-out problem in routine working with the LaunchPad.  My problem arose because I was trying to fit too many effects into the one diorama that led to too many LaunchPads connected to each optical sensor. 

This led to another realization that I should have avoided.  The effects on the diorama were so interconnected that if anything failed it would prevent almost all of the effects from working.  This is called single-point failure and, as a long-time avionics system engineer, this is something I look for in my professional designs.  I just never applied it to my hobby pursuits.  

I could have fixed the problems; if I were at home and had access to my stock of electronics parts.  But seeing as I was in a hotel room in Atlanta, I decided that the best that I could do was not to hook up the signals.  However, single point failure was not done with me yet.

The diorama worked perfectly in the hotel Thursday and Friday in the morning before I went to do my first clinic.  After I set up in the clinic room almost nothing was working!  WTF?   After some abbreviated troubleshooting, I lifted up the diorama and saw that one of the optical sensor's LED was constantly illuminated indicating that the sensor was stuck in the "on" state.  Quickly going through a fault tree mentally, I realized that with the one sensor stuck "on", nothing but the crossing flasher would work; and I'd have to trigger that effect manually by putting my finger over one of the other sensors.  Single point failure had struck again!  So for my first clinic the diorama was a washout.

I thought that it was a simple matter of the sensitivity potentiometer being knocked out of adjustment by all of the travel.  So before my next clinic, which was that evening, I disassembled the diorama to adjust the screw, which I did.  When I powered it back up to my surprise all of the optical sensors were now stuck on!  At this point I was beginning to believe in demonic possession (especially since that had been a topic in the previous Sunday's sermon!).  Now I was really perplexed.

I wracked my brain for an answer.  In examining the diorama I turned it sideways - and all of the sensors turned off!   Then I had a flash of insight.  In a scene reminiscent of one from the movie "Aliens" I looked up towards the ceiling.  There was a hot incandescent spotlight directly overhead.  The incandescent bulb was emitting just enough infrared light to trigger the sensor.  I realized that I had never tested this sensor under incandescent light - it had always been used under the ubiquitous florescent lights.  I went to the clinic room's light switch and turned off the offending bank of lights.  Everything worked perfectly.

So the second clinic had a working diorama by the simple expedient of turning off the room light in the front of the clinic room - which also made the slides easier to see.

Fan-out, single point failure, inadequate testing - all things I should have known better. Duh!  Live and learn.