7
Minimizing electrical noise effects from other sources that may cause false triggering
Switch machines, switch motors and electrical uncouplers are notorious for generating electrical noise when they are
energized. Such noise can be inadvertently coupled onto the
BA’s
photocell connections which can then lead to false
triggering of the signal circuit. The first remedy to try is to make sure that wiring for those devices is kept apart from the
photocell wiring. This may not always be practical (especially if you’re interlocking the signal head(s) with turnout position
as described above). If that’s the case, adding decoupling/filter components to the
BA
circuit will usually eliminate the false
triggering. The drawing below illustrates what needs to be done. Note that capacitors C1 and C3 are polarized so make sure
you connect them correctly; C2 has no polarity. Also, pay attention to the component location as outlined below in the parts
list. The numbers in parentheses are Radio Shack part numbers. The drawing only shows one photocell. Each photocell will
need its own “R1” and “C3” components; only one “C1” and “C2” are needed for each
BA
.
E
1
W
2
E
2
W
1
5
V
D
C
G
N
D
A
C
I
N
It may also be necessary to add filtering to the power source for the “offending” item. For example, if you are using switch
motors powered by a DC (unipolar or bipolar) power source you may need to add filter capacitors to that power source. We
suggest a 0.1ufd/50V ceramic disk capacitor (such as C2 above) in parallel with a 100ufd/35V electrolytic capacitor (similar
to C1 above, but obviously a higher value). Connect them similar to the way C1 & C2 are connected above (i.e. positive lead
to positive power output; negative lead to the negative power output or ground).
Finally, if noise problems still exist you may need to use coaxial cable for the photocell connections. Connect the shield of
such a cable to GND terminal on the
BA
and connect the center wire to the appropriate photocell terminal.
Using an infrared emitter and detector instead of the photocell
It is possible use infrared emitter and detector pairs instead of the photocells for train detection. The wiring for one photocell
input is shown below. When the infrared beam is not obstructed from the detector the output of the detector will be
approximately 0.2V. When the infrared beam is obstructed the output of the detector will essentially appear like an open
circuit to the
BA’s
photocell inputs which will be interpreted like a covered photocell.
E
1
W
2
E
2
W
1
5
V
D
C
G
N
D
A
C
I
N
