Driver board options:

All the following circuits may be built as either common anode or common cathode versions. Simply switch the polarity selector on the driver board to match. Common anode versions are shown. To wire common cathode versions just reverse the direction of each LED. Adjust the brightness by adding resistors to the driver board. (100 ohm minimum)

Position Light Signals Position light signals are more difficult to drive than color light signals due to the number of LEDs involved. The simplest wiring option is shown here. However this connection puts three yellow LEDs in series for each aspect. Yellow LEDs require over 2 volts each to illuminate, so it is not possible to drive this simply wired head directly from a 5 volt source. To solve this problem the 4ASD-4 board includes a voltage mirror circuit that supplies approximately 9 volts to the driver circuits. This board also multiplexes the output lines and includes the current limiters internally. This allows 4 heads to each be driven with 4 aspects using just 10 wires. Normally we drive 4 heads, with just 3 aspects each plus dark, on each output port. This frees the 4th aspect connections to optionally be used to illuminate fixed marker lights if so desired.

Absolute Position Light Signals For the late era use of red lamps as an Absolute Stop indication in some position light signals use the alternate wiring as shown. These were apparently modified from existing signals by simply changing out the lenses in the stop aspect and were wired to not illuminate the center lamp when stop was active. Adjust the 'Stop' resistor to prevent uneven brightness.

Color Position Light Signals Color position light signals only require 2 LEDs in series, so they may be directly driven from the outputs of a Tower Controller. However this can be problematic using logic level outputs because of the required voltages to illuminate series LEDs properly. To avoid potential problems with dim indications, this schematic shows them being driven with a 4ASD-4 driver card. The 4ASD-4 can drive them easily and also saves on the amount of wiring required. Color Position Lights were often used in conjunction with additional marker lights for speed indications. If your situation requires a single fixed marker indication and does not also require the lunar aspect, then it may be lighted using the marker option.

Color Light Signals As noted above, color light signals may easily be driven directly from the outputs of a Tower Controller. This schematic shows them being driven with a 4ASD-4 driver card to save on the amount of wiring required and number of ports used. Color Light Signals are often used in conjunction with additional marker lights to indicate absolute signals. If your situation requires a single fixed marker indication and does not also require the lunar aspect, then it may be lighted using the marker option.