Modern 4 aspect signalling warns the engine driver that he is approaching a junction where there is a feather (route indicator) lit to show the points are set for the diverging route. The driver is warned by first reaching a double yellow flashing signal and then reaching a single yellow flashing signal. Our two control boards the IRDASC-Flashing-Double-Yellow and the IRDASC-Flashing-Yellow replicates this for model signals.
When the mainline route is set (feather not lit) then the IRDASC-Flashing-Double-Yellow and the IRDASC-Flashing-Yellow operate as a standard 4aspect IRDASC-4.
IRDASC is the acronym for Infra Red Detection And Signal Control. These boards have built in infra red detection and the electronics to control a 4 aspect colour light signal automatically in conjunction with the positions of trains and points. Each MAS Sequencer, MAS Sequencer RI, IRDASC-4, IRDASC-4RI, IRDASC-5 or IRDASC-5RI has an S (send) terminal this is wired to the R (Receive terminal of the signal controller before it). This chain of S to R connections can have as many links and branches as you require. Its purpose is to tell the previous signal the aspect of the next signal so it can set the correct aspect. For example if it is yellow then the previous signal will show double yellow.
The circuit board controlling the feather may be a MAS-Sequencer-RI or an IRDASC-4-RI. In either case it needs to know which way the point is set to determine when to light the feather. It does this by using a contact on the point motor to switch a connection to its OI terminal. In a similar way the IRDASC-Flashing-Yellow and IRDASC-Flashing-Double-Yellow are connected to the point motors contact so they can determine whether or not to flash.
The circuit boards fit beneath the baseboard with the infra red detection fitted into a hole between the sleepers. They are powered from preferably 12 volts DC but will also work with 12 to 16 volts AC. The infra red detection is positioned just after the signal where you wish the signal to change to red. On full size railways this is where the train reaches a track circuit and it seems this is a short distance from the signal of around half a coach length.