and an interrupter optical detection system can function satis-
factorily on a model railroad.
Ambient light:
A second type of optical detection system I’ll
call the ambient light sensor. In this system a light detector is
placed between two ties and is facing upwards. The light detec-
tor is kept on by ambient room light. When a train passes over
it and cuts off the ambient light, the train is detected. This
arrangement is illustrated in [2].
Other than a small hole between two ties at intervals down the
track, there is nothing to interfere with your scenery. The ambient
light sensor does suffer from the inter-car drop out limitation; but
that can be addressed with an electronic or software time delay.
3
3. The Reflective Optical Train Detector.
Optical Train Detection - 4
So, is the ambient light sensor is the perfect optical train detec-
tor? No, not really. The ambient light sensor has its own quirk.
If you want to have night operations, and you turn off the
room lights and/or the lights over the railroad, it’s very likely
that the ambient light system will cease to function because
there’s now insufficient light to activate the detector in the
first place, and all, or almost all, of the sensors will indicate a
detection all of the time. It has been suggested that a second-
ary, point-source light system be placed above each detector
for just this reason. But this has its own problems.
For one, if your system uses visible light, the secondary lighting
systemwill have to use visible light too. So during night operations
there would be these spots of light at intervals down the track.
MRH-May 2014
