Two wire bipolar LEDs
2 wire bi-polar LEDs have also been used to build searchlight signals,
but their operation is more complex because their polarity needs
to be reversed to change from
red to green and their polarity must be rapidly switched with a variable
pulse width to achieve a proper yellow. Further there is only a saving
of one wire, the common return, over the use of the simpler to drive
three lead LEDs.
Panel indicators will usually be directly driven from the outputs
of a Tower Controller located behind the panel. For dual color
indicators use the same wiring shown here for the direct drive
two color signals.
To drive single color LEDs just use single resistors and single
LEDs. For three color panel indicators use the same circuits that
for line side signals.