looking models is
a passion of mine.
To enhance my
BUILD A MODEL
setup, a realistic
crossing signal was
just the ticket.
The circuit in Figure 1 is the
schematic for my model railroad
crossing signal. The completed
printed circuit board (PCB) is shown
in Photo 1a and Photo 1b. The
circuit uses an infrared emitter and
detector to determine the presence
of the model train. When the
infrared beam is broken, the two red
crossing signal LEDs flash alternately.
Theory of Operation
The infrared LED (emitter) is on
one side of the track while the IR
phototransistor (detector) is on the
opposite side of the track. These
components are mounted in a line-of-sight fashion so that the infrared
light from the LED shines on the
phototransistor. The phototransistor
is shielded from stray light by
aluminum tubing (refer to Figure 2).
The infrared emitter and detector PCB connections are shown in
Figure 3. Brackets (see Figure 5) are
mounted to emitter and detector
boards with 4-40 screws and nuts.
■ FIGURE 1