Well, there I go again.  Earlier I posted a column celebrating the completion of all the hardware and wiring for my optical detection system (well, it’s not really complete….I still have to build the panels that will display the occupancy indicator LEDs).

Problem is, I just assumed all readers would know what I was talking about.  Not!  I’ve been informed that I should quit assuming.

So, I’ll try to explain in 3000 words or less, just what an optical detection system is.  Now keep in mind that my explanation here is strictly in the context in which I am employing said detection system.  And I will avoid getting into the directions that the electrons are traveling and the gory details of the circuitry that makes this all happen.

I have six staging tracks, all of which are hidden from normal viewing.  In order to ascertain where a train is while running on this track, I decided to employ a detection system.  There are several ways to detect a train, and I chose to employ optical detection.  The system utilizes photo-transistors (PTs) which can sense whether or not they are seeing light.  When they do, they will turn on, that is, they act as a switch and will close a circuit.  I have installed my PTs centered in the track between a pair of ties, with their tops just above the roadbed.  The PTs are wired to a circuit board located under the layout.

As a light source, I’m using infrared LEDs (IR-LEDs).  To avoid seeing light beneath the layout, and to make the circuit less susceptible to ambient lighting, I chose to use components that are sensitive to light in the infrared range.  This isn’t visible to the naked eye, and in fact, that’s what most TV remote controls use.  The PTs and the IR-LEDs selected are matched to their light spectrum and work in harmony together.  The IR-LEDs are mounted on the “towers” I spoke of in the last post, and are pointed down at the PTs.  They are simply wired to a 12 volt DC bus (a pair of wires).

The circuit board I mentioned above is the “brain” of the system.  The inputs to the board are the PTs.  The outputs are to plain old red LEDs which are used as indicators, installed on a simple panel with a track diagram.  Each indicator LED is placed on the track diagram at the location where a PT sensor is located on the actual track.

With the power on and the IR-LEDs shining brightly, the PTs see the light and the circuit board determines there is nothing out there as no PT is “closed”.  Therefore, no power is supplied to the indicator LEDs on the panel.  But when a train comes along, it blocks the light of an IR-LED shining on a PT below, and that PT turns on, which in turn lets the circuit board know that something is at that location.  It in turn lights up the appropriate panel indicator LED and you now have a visual indication of where the train is.

Simple, huh?

Actually, it really is.  I purchased the circuit boards already built up from a fellow hobbyist up in Canada.  All I had to do was install the PTs and IR-LEDs, and wire them up.

Your test will arrive in the mail later in the week.

-Jack