Direct drive options:

Searchlight Signals

The simplest and most cost efficient option for signals is to use three wire dual color LEDs to create searchlight style signals. The three wire LED design allows both the red and the green portion of the LED to be illuminated independently. If the LED has both color chips located close to each other and the brilliance of the two colors are well matched then our eyes will see yellow when they are both illuminated at the same time. This visual multiplexing allows us to drive 4 heads, each with 3 aspects plus dark, from each output port.

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

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 are used for line side signals.

Two Color Signals

Direct drive of dual LEDs is possible for users with simple two color signals. In this case 8 dual color signals may be controlled directly from each output port. The color changes from red to green as the output lines change between high and low. These LEDs must be individual LEDs, not bi polar LEDs.

Color Light Signals

Color light signals of various styles may also be directly driven from the outputs of a Tower Controller. However there is some loss of efficiency because it takes three or four lines to directly drive these same signals. (three or four aspects may be encoded using just two lines) Also the usual three aspect signals do not map well onto the 8 line ports, yielding only 2-2/3 heads per output port. This means that connections to one head may span two ports. Another disadvantage to this method is that it takes 1 wire per light from the TC-64 out to the signal heads. There is no opportunity for multiplexing the lines to cut down on the total amount of wiring required. On large layouts the extra wiring cost alone can justify the use of driver cards.